Note: Descriptions are shown in the official language in which they were submitted.
CA 03020663 2018-10-11
[Document Type] Specification
[Title of the Invention] HOT STAMPED STEEL
[Technical Field of the Invention]
[0001]
The present invention relates to a hot stamped steel.
[Related Art]
[0002]
There are cases in which structural members (compacts) that are used for cars
and the like are manufactured by hot stamping (hot pressing) in order to
increase both the
strength and the dimensional accuracy. In the manufacturing of a compact by
hot
stamping, steel for hot stamping is heated to the Ac3 temperature or higher,
and the steel
for hot stamping is rapidly cooled in a die while being pressed. That is, in
the
manufacturing, pressing and quenching are carried out at the same time. Hot
stamping
enables the manufacturing of compacts having a high dimensional accuracy and a
high
strength.
[0003]
Meanwhile, a compact manufactured by hot stamping has been worked at a high
temperature, and thus iron scales are formed on the surface. Therefore, a
technique in
which a plated steel sheet is used as a steel sheet for hot stamping, thereby
suppressing
the formation of iron scales and, furthermore, improving the corrosion
resistance of
compacts has been proposed (refer to Patent Documents 1 to 3). For example,
Patent
Document 1 discloses a plated steel sheet for hot pressing on which a Zn-
plated layer is
formed, and Patent Document 2 discloses a plated steel sheet for a car member
on which
an Al-plated layer is formed. Furthermore, Patent Document 3 discloses a
galvanized
steel sheet for hot pressing in which a variety of elements such as Mn are
added to a
plated layer of the Zn-plated steel sheet. However, these plated steel sheets
have
problems described below.
[0004]
In the technique of Patent Document 1, Zn remains on the surface of the
compact
after hot stamping, and thus a strong sacrificial anticorrosion action can be
expected.
However, in the technique of Patent Document 1, a plated steel sheet is hot-
pressed in a
state in which Zn is molten, and thus there is a concern that molten Zn may
intrude into a
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base material of the plated steel sheet during hot pressing, and cracks may be
generated
inside the base material. These cracks are referred to as liquid metal
embrittlement
(hereinafter, in some cases, referred to as "LME"). Due to LME, the fatigue
properties
of compacts deteriorate.
[0005]
Meanwhile, currently, in order to avoid the generation of LME, it is necessary
to
appropriately control the heating conditions during the working of a plated
steel sheet.
Specifically, a method or the like in which the plated steel sheet is heated
until all of the
molten Zn diffuses into the base material of the plated steel sheet and forms
a Fe-Zn solid
solution is employed. However, in order to carry out these methods, the plated
steel
sheet needs to be heated for a long period of time, and consequently, there is
a problem of
the degradation of the productivity.
[0006]
In the technique of Patent Document 2, Al having a high melting point than Zn
is
used as a component of the plated layer, and thus the concern of the intrusion
of molten
metal into a base material of a plated steel as in Patent Document 1 is small.
Therefore,
according to the technique of Patent Document 2, excellent LME resistance can
be
obtained, and furthermore, it is expected that a compact having excellent
fatigue
properties can be obtained after hot stamping. However, for compacts on which
an Al-
plated layer is formed, there is a problem in that it is difficult to form a
phosphate film
during a phosphating treatment that is carried out before the painting of a
member for a
car. In other words, the compact by the technique of Patent Document 2 has a
problem
in that the phosphatability cannot be sufficiently obtained.
[0007]
In the technique of Patent Document 3, the outermost layer (oxidized film) of
a
hot stamped steel is reformed, thereby improving weldability. However, in the
technique of Patent Document 3, there is also a concern that LME may be
generated and
the fatigue properties of a hot stamped steel may not be sufficiently
obtained. In
addition, in the technique of Patent Document 3, there is another concern that
an element
that is added to a plated layer may degrade the phosphatability.
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[Prior Art Document]
[Patent Document]
[0008]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2003-73774
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2003-49256
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2005-113233
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
The present invention has been made in consideration of the above-described
circumstances, and an object of the present invention is to provide a hot
stamped steel
being excellent in terms of fatigue properties, a phosphatability, coating
adhesion, and
weldability.
[Means for Solving the Problem]
[0010]
The gist of the present invention is as described below.
[0011]
(1) According to an aspect of the present invention, there is provided a hot
stamped steel including a base material and a plated layer, in which the
plated layer
includes an interface layer, an intermediate layer, and an oxide layer in
order from a base
material side to a surface side; in the interface layer, a structure includes
99 area% or
more in total of aFe, Fe3A1, and FeAl, an average Al content is in a range of
8.0 mass% or
more and 32.5 mass% or less, an average Zn content is limited to more than an
Zn content
of the base material and 5 mass% or less, a remainder of a chemical
composition includes
Fe and impurities, and an average layer thickness is 1.0 [ail or more; in the
intermediate
layer, a structure includes 99 area% or more in total of Fe(Al, Zn)2 and
Fe2(Al, Zn)5, an
average Al content is 30 mass% to 50 mass%, an average Zn content is 10 mass%
to 40
mass%, a remainder of a chemical composition includes Fe and the impurities,
and an
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average layer thickness is 5.0 1.1m or more; and in the oxide layer, an
average layer
thickness is 0.11,tm to 3.0 [tm.
(2) In the hot stamped steel according to (I), the average layer thickness may
be
1.0 [tm to 10.0 gm in the interface layer.
(3) In the hot stamped steel according to (1) or (2), a total weight per unit
area of
Al and Zn in the plated layer may be 20 g/m2 or more and 100 g/m2 or less.
(4) In the hot stamped steel according to any one of (1) to (3), the plated
layer
may further include more than 0 mass% and 10.0 mass% or less of Si on average,
and, in
the intermediate layer, 0 area% to 50 area% of the Fe(Al, Zn)2 and the Fe2(Al,
Zn)5 may
be substituted into Fe(Al, Si).
[Effects of the Invention]
[0012]
In the hot stamped steel according to the present invention, improvements were
made respectively to the alloy form of the plated layer, the Al content and
the Zn content
in specific layers of the plated layer, and the layer thickness of an oxide
formed as the
outermost layer of the plated layer. As a result, according to the hot stamped
steel
according to the present invention, it is possible to achieve all of the
improvement in the
fatigue properties of the compact based on the suppression for generating of
LME, the
improvement in the phosphatability of the compact and the consequent
improvement in
the coating adhesion, and the improvement in the weldability of the compact.
[Brief Description of the Drawings]
[0013]
FIG. 1 is an example of a cross-sectional SEM image showing a worked portion
of a compact obtained by immediately performing hot-V-bent on an Al-Zn-based
plated
steel after heating under the conditions of Example 1.
FIG. 2 is an example of a cross-sectional SEM image showing a worked portion
of a compact obtained by immediately performing hot-V-bent on a Zn-based
plated steel
after heating under the conditions of Example 1.
FIG. 3 is an example of a cross-sectional SEM image showing a worked portion
of a compact obtained by immediately performing hot-V-bent on an Al-based
plated steel
after heating under the conditions of Example 1.
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FIG. 4 is an example of a SEM image (secondary electron image) showing a
surface of a compact in a case in which an Al-Zn-based plated steel is heated
under the
conditions of Example 1, immediately worked and rapidly cooled in a flat sheet
die
including a water-cooling jacket, and then subjected to a phosphating
treatment.
FIG. 5 is an example of a SEM image (secondary electron image) showing a
surface of a compact in a case in which a Zn-based plated steel is heated
under the
conditions of Example 1, immediately worked and rapidly cooled in the flat
sheet die
including the water-cooling jacket, and then subjected to a phosphating
treatment.
FIG. 6 is an example of a SEM image (secondary electron image) showing a
surface of a compact in a case in which an Al-based plated steel is heated
under the
conditions of Example 1, immediately worked and rapidly cooled in the flat
sheet die
including the water-cooling jacket, and then subjected to a phosphating
treatment.
FIG. 7 is a cross-sectional view of a vicinity of a surface of a hot stamped
steel
according to the present embodiment.
FIG. 8 is a schematic view of an Al concentration and a Zn concentration in
the
vicinity of the surface of the hot stamped steel according to the present
embodiment.
[Embodiments of the Invention]
[0014]
Hereinafter, an embodiment of a hot stamped steel according to the present
invention will be described in detail. Meanwhile, the unit "%" regarding the
chemical
composition of a hot stamped steel according to the present embodiment refers
to
"mass%" unless particularly otherwise described. In addition, in the present
embodiment, the hot stamped steel refers to a compact obtained by carrying out
hot
stamping (hot pressing) on a plated steel for hot stamping. Hereinafter, there
will be
cases in which the hot stamped steel is simply referred to as the "compact"
and the plated
steel for hot stamping is simply referred to as the "steel" or the "plated
steel".
[0015]
The present inventors studied the fatigue properties (LME resistance) and the
phosphating treatment properties of hot stamped steels (an Al-Zn-based plated
steel, a Zn-
based plated steel, and an Al-based plated steel). As a result, the present
inventors found
that, in a case in which a plated layer of a hot stamped steel includes an
interface layer, an
intermediate layer, and an oxide layer in order from a base material side to a
surface side,
a structure of the interface layer includes 99 area% or more in total of aFe,
Fe3A1, and
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FeAl, an Al content is in a range of 8.0 mass% or more and 32.5 mass% or less
and
decreases toward the base material, an average Zn content is limited to 5
mass% or less, a
remainder of a chemical composition of the interface layer includes Fe and
impurities,
and an average layer thickness is 1.0 Jim or more, a structure of the
intermediate layer
includes 99 area% or more in total of Fe(Al, Zn)2 and Fe2(Al, Zn)5, an average
Al content
is 30 mass% to 50 mass%, an average Zn content is 10 mass% to 40 mass%, a
remainder
of a chemical composition of the intermediate layer includes Fe and the
impurities, and an
average layer thickness is 5.0 i.tm or more, and an average layer thickness of
the oxide
layer is 0.1 [tm or more and 3.0 [tm or less, both the fatigue properties and
the
phosphatability of the hot stamped steel are favorable. Meanwhile, in the
present
specification, the average layer thickness refers to the average value of the
maximum
thickness and the minimum thickness of a subject layer (film).
[0016]
<Hot stamped steel>
Hereinafter, the hot stamped steel according to the present embodiment will be
described. A hot stamped steel 1 according to the present embodiment includes
a base
material 10 and a plated layer 20 as shown in FIG. 7.
[0017]
[Composition of Base Material]
Hereinafter, a preferred composition of the base material in the hot stamped
steel
according to the present embodiment will be described. The improvement of the
LME
resistance and the phosphatability, which is the object of the hot stamped
steel according
to the present embodiment, is realized using the configuration of the plated
layer.
Therefore, the base material in the hot stamped steel according to the present
embodiment
is not particularly limited. However, in a case in which the composition of
the base
material is in a range described below, a compact having preferred mechanical
properties
in addition to the LME resistance and the phosphatability can be obtained.
Hereinafter,
the unit "%" of the amounts of alloying elements included in the base material
refers to
"mass%".
[0018]
(C: Preferably 0.05% to 0.40%)
In a case in which 0.05% or more of carbon (C) is included in the base
material,
the strength of the hot stamped steel can be increased. On the other hand, in
a case in
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which the C content in the base material is more than 0.40%, there are cases
in which the
toughness of the base material in the compact lacks. Therefore, the C content
in the base
material may be set to 0.05% to 0.40%. A more preferred lower limit value of
the C
content in the base material is 0.10%, and a still more preferred lower limit
value is
0.13%. A more preferred upper limit value of the C content in the base
material is
0.35%.
[0019]
(Si: Preferably 0.5% or Less)
Silicon (Si) has an effect of deoxidizing steel. However, when the Si content
is
increased, the wettability of steel to platings is degraded, and there is a
likelihood of an
ordinary plating treatment being impossible. Therefore, the Si content in the
base
material may be set to 0.5% or less. A more preferred upper limit value of the
Si content
in the base material is 0.3%, and a still more preferred upper limit value of
the Si content
in the base material is 0.2%. A more preferred lower limit value of the Si
content in the
base material can be determined depending on a required deoxidation level and
is, for
example, 0.05%.
[0020]
(Mn: Preferably 0.5% to 2.5%)
In a case in which more than 0.5% of manganese (Mn) is included in the base
material, the hardenability of the base material of steel before hot stamping
is enhanced,
and the strength of the base material in the compact after hot stamping is
increased. On
the other hand, in a case in which the Mn content in the base material exceeds
2.5%, this
effect is saturated. Therefore, the Mn content in the base material may be set
to 0.5% to
2.5%. A more preferred lower limit value of the Mn content in the base
material is
0.6%, and a still more preferred lower limit value is 0.7%. A more preferred
upper limit
value of the Mn content in the base material is 2.4%, and a still more
preferred lower
limit value is 2.3%.
[0021]
(P: Preferably 0.03% or Less)
Phosphorus (P) is an impurity that is included in steel. P included in the
base
material, in some cases, is segregated at crystal grain boundaries in the base
material and
thus degrades the toughness of the base material in the compact and degrades
the delayed
fracture resistance of the base material. Therefore, the P content in the base
material
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may be set to 0.03% or less. The P content in the base material is preferably
as small as
possible.
[0022]
(S: Preferably 0.01% or Less)
Sulfur (S) is an impurity that is included in steel. S included in the base
material, in some cases, forms a sulfide and thus degrades the toughness of
the base
material in the compact and degrades the delayed fracture resistance of the
base material.
Therefore, the S content in the base material may be set to 0.01% or less. The
S content
in the base material is preferably as small as possible.
[0023]
(so!. Al: Preferably 0.10% or Less)
In a case in which a term "Al content" is used regarding the base material in
the
compact according to the present embodiment, this term refers to the amount of
sol. Al
(acid-soluble Al) in the base material. Aluminum (Al) is generally used for
the purpose
of deoxidizing steel. However, in a case in which the Al content is large, the
Ac3
temperature of steel before hot stamping is increased, and a heating
temperature
necessary for the quenching of steel during hot stamping is increased, which
is not
desirable in terms of manufacturing by hot stamping. Therefore, the Al content
in the
base material may be set to 0.10% or less. A more preferred upper limit value
of the Al
content in the base material is 0.05%. A more preferred lower limit value of
the Al
content in the base material is 0.01%.
[0024]
(N: Preferably 0.01% or Less)
Nitrogen (N) is an impurity that is included in steel. N included in the base
material, in some cases, forms a nitride and thus degrades the toughness of
the base
material in the compact. Furthermore, in a case in which B is included in the
base
material in order to improve the hardenability of steel before hot stamping, N
included in
the base material, in some cases, bonds to B and thus decreases the amount of
a solid
solution B and degrades a hardenability-improving effect of B. Therefore, the
N content
in the base material may be set to 0.01% or less. The N content in the base
material is
preferably as small as possible.
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[0025]
The base material in the hot stamped steel of the present embodiment may
further include one or more selected from the group consisting of B and Ti.
[0026]
(B: Preferably 0% to 0.0050%)
B has an action of enhancing the hardenability of steel and is thus capable of
increasing the strength of the base material in the compact after hot
stamping. However,
when the B content in the base material is excessive, this effect is
saturated. Therefore,
the B content in the base material may be set to 0% to 0.0050%. A more
preferred lower
limit value of the B content in the base material is 0.0001%.
[0027]
(Ti: Preferably 0% to 0.10%)
Ti included in the base material bonds to N included in the base material and
thus forms a nitride. In a case in which Ti and N bond to each other as
described above,
the bonding between B in the base material and N in the base material is
suppressed, and
thus the degradation of the hardenability of the base material by the
formation of BN can
be suppressed. Furthermore, Ti included in the base material decreases the
austenite
grain sizes during heating in hot stamping due to austenite pinning effect and
thus also
has an effect of enhancing the toughness and the like of the compact. However,
when
the Ti content in the base material is excessive, the above-described effect
is saturated,
and furthermore, there is a concern that a Ti nitride may be excessively
precipitated and
thus the toughness of the base material in the compact may degrade. Therefore,
the Ti
content in the base material may be set to 0% to 0.10%. A preferred lower
limit value of
the Ti content in the base material is 0.01%.
[0028]
The base material configuring the hot stamped steel of the present embodiment
may further include one or more selected from the group consisting of Cr and
Mo.
[0029]
(Cr: Preferably 0% to 0.5%)
Cr included in the base material enhances the hardenability of the base
material
of steel before hot stamping. However, when the Cr content in the base
material is
excessive, a Cr carbide is formed. This Cr carbide is not easily dissolved
during heating
in hot stamping, hinders the progress of austenizing, and, in some cases,
degrades the
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hardenability. Therefore, the Cr content in the base material may be set to 0%
to 0.5%.
A more preferred lower limit value of the Cr content in the base material is
0.1%.
[0030]
(Mo: Preferably 0% to 0.50%)
Mo included in the base material enhances the hardenability of the base
material
of steel before hot stamping. However, when the Mo content in the base
material is
excessive, the above-described effect is saturated. Therefore, the Mo content
in the base
material may be set to 0% to 0.50%. A more preferred lower limit value of the
Mo
content in the base material is 0.05%.
[0031]
The base material configuring the hot stamped steel of the present embodiment
may further include one or more selected from the group consisting of Nb and
Ni.
[0032]
(Nb: Preferably 0% to 0.10%)
Nb included in the base material forms a carbide and thus miniaturizes crystal
grains in the base material during hot stamping and enhances the toughness of
the
compact. However, when the Nb content in the base material is excessive, the
above-
described effect is saturated. Furthermore, when the Nb content in the base
material is
excessive, there is a case in which the hardenability of the base material is
degraded.
Therefore, the Nb content may be set to 0% to 0.10%. A more preferred lower
limit
value of the Nb content in the base material is 0.02%.
[0033]
(Ni: Preferably 0% to 1.0%)
Ni included in the base material enhances the toughness of the base material
in
the compact. Ni in the base material also suppresses embrittlement attributed
to the
presence of molten Zn during heating in hot stamping. However, when the Ni
content in
the base material is excessive, these effects are saturated. Therefore, the Ni
content in
the base material may be set to 0% to 1.0%. A more preferred lower limit value
of the
Ni content in the base material is 0.1%.
[0034]
A remainder of the chemical composition of the base material configuring the
hot stamped steel of the present embodiment includes Fe and impurities. In the
present
specification, an impurity refers to a substance that can be included in
mineral or scraps
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as a raw material or a substance that can be mixed into the base material due
to the
manufacturing environment or the like during the industrial manufacturing of
the
compact.
[0035]
[Plated layer]
Next, the plated layer 20 in the hot stamped steel 1 according to the present
embodiment will be described. The plated layer 20 in the compact 1 includes an
interface layer 21, an intermediate layer 22, and an oxide layer 23 in order
from a base
material 10 side of the compact 1 to a surface side of the compact 1 as shown
in FIG. 7.
[0036]
[Interface Layer]
The interface layer is formed adjacent to the base material. A majority of the
structure of the interface layer is configured of aFe, Fe3A1, and FeAl. That
is, the
interface layer in the hot stamped steel according to the present embodiment
is mainly
configured of a Fe-Al alloy phase having a small Al content. Meanwhile, there
is also a
case in which a small amount of an inclusion or the like attributed to an
impurity mixed
into the interface layer during the formation of a plating is included in the
interface layer.
However, the inventors confirmed that, in a case in which the interface layer
is observed
in a cross section of the plated layer in the hot stamped steel, when the
structure includes
99 area% or more in total of aFe, Fe3A1, and FeAl, the influence of the above-
described
inclusion can be ignored. In order to control the structure of the interface
layer as
described above, it is necessary to set to average Al content in the interface
layer to 8.0
mass% or more and 32.5 mass% or less. Meanwhile, the Al content in the
interface
layer is not uniform as described below, and the Al content in the interface
layer decreases
toward the base material.
[0037]
In the interface layer, Zn is present in a state of forming a solid solution
in the
above-described Fe-Al alloy phase. However, according to what the inventors
found, in
the interface layer in the compact according to the present embodiment, Zn
barely forms a
solid solution, and the average Zn content in the interface layer is 5 mass%
or less. The
presence of the interface layer enables the suppression of liquid metal
embrittlement
(LME). Meanwhile, there is a case in which the Zn content in the interface
layer is also
not uniform, but LME is suppressed as long as the average Zn content in the
interface
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layer is 5 mass% or less, and thus the interface layer may include a region
including more
than 5 mass% of Zn. The Zn content in the interface layer is minimized in the
interface
between the interface layer and the base material. Therefore, the minimum
value of the
Zn content in the interface layer exceeds the Zn content in the base material.
[0038]
The configuration of the interface layer is schematically shown in FIG. 8. As
described above, the Al content in the interface layer 21 is not uniform. The
Al content
in the interface between the base material 10 and the interface layer 21 is
the same as the
Al content in the base material 10. As the portion moves away from the
interface
between the base material 10 and the interface layer 21, the Al content
increases, and the
structure changes to aFe phase having the smallest Al content, Fe3A1 phase
having the
second smallest Al content, and FeAl phase having the third smallest Al
content in order.
The Zn content in the interface between the base material 10 and the interface
layer 21 is
the same as the Zn content in the base material 10. The Zn content also
increases away
from the interface between the base material 10 and the interface layer 21,
but the Zn
content is suppressed at a low level, and the Zn content does not exceed 5
mass% on the
average throughout the interface layer 21.
[0039]
In a case in which the average layer thickness of the interface layer is less
than
1.0 pim, the LME suppression effect cannot be sufficiently obtained.
Therefore, it is
necessary to set the average layer thickness of the interface layer is to 1.0
pim or more.
In a case in which the average layer thickness of the interface layer is set
to 2.0 pan or
more, the above-described effect is exhibited at a higher level. The lower
limit value of
the average layer thickness of the interface layer is more preferably 5.0 ptm,
6.0 Jim, or
7.0 ptm. It is not necessary to regulate the upper limit value of the average
layer
thickness of the interface layer, but there is a case in which the interface
layer having an
average layer thickness of more than 15.0 gm degrades the performance such as
the
corrosion resistance, which is not preferable. Therefore, the upper limit
value of the
average layer thickness of the interface layer is preferably 15.0 pm and more
preferably
10.0 ptm, 9.0 ptm, or 8.0 pm.
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[0040]
[Intermediate Layer]
The intermediate layer 22 is a layer including Fe, Al, and Zn and is formed on
the interface layer 21. A majority of the structure of the intermediate layer
is configured
of Fe(Al, Zn)2 and Fe2(A1, Zn)5. Fe(Al, Zn)2 is a phase in which some of Al in
FeAl2
that is a kind of a Fe-Al intermetallic compound is substituted into Zn, and
Fe2(Al, Zn)5 is
a phase in which some of Al in Fe2A15 that is a kind of a Fe-Al intermetallic
compound is
substituted into Zn. Meanwhile, there is also a case in which a small amount
of an
inclusion or the like attributed to an impurity mixed into the intermediate
layer during the
formation of a plating is included in the intermediate layer. However, the
inventors
confirmed that, in a case in which the intermediate layer is observed in a
cross section of
the plated layer in the hot stamped steel, when the structure includes 99
area% or more in
total of Fe(Al, Zn)2 and Fe2(Al, Zn)5, the influence of the above-described
inclusion can
be ignored.
[0041]
In the intermediate layer, the Al content and the Zn content are almost
uniform.
The chemical composition of the intermediate layer includes, by unit mass%,
30% or
more and 50% or less of Al on the average and 10% or more and 40% or less of
Zn on the
average. In addition, the average Al content in the intermediate layer is
above the
average Al content in the interface layer.
[0042]
In a case in which the configuration of the interface layer is controlled as
described above, thereby suppressing LME in the interface layer and imparting
excellent
fatigue properties to the compact, the average Al content in the intermediate
layer reaches
30 mass% or more. In addition, when the oxide layer is mainly configured of a
Zn
oxide, the average Al content in the intermediate layer reaches 50 mass% or
less in a case
in which an excellent phosphatability is imparted to the compact. That is, in
a case in
which the average Al content in the intermediate layer is outside a range of
30 mass% to
50 mass%, there is an extremely high likelihood of the configuration of the
interface layer
or the oxide layer becoming inappropriate. The lower limit value of the
average Al
content in the interface layer is preferably 32 mass% or 35 mass%, and, in
this case, it is
possible to more reliably develop the LME suppression effect of the interface
layer. In
addition, a preferred upper limit value of the average Al content in the
interface layer is
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50 mass% or 45 mass%, and, in this case, it is possible to more reliably
improve the
phosphatability of the oxide layer.
[0043]
In a case in which the oxide layer in the compact is mainly configured of a Zn
oxide, and an excellent phosphatability is imparted to the compact, the
average Zn
content in the intermediate layer reaches 10 mass% or more. In addition, in a
case in
which LME is suppressed in the interface layer, and excellent fatigue
properties are
imparted to the compact, the average Zn content in the intermediate layer
reaches 30
mass% or less. That is, in a case in which the average Zn content in the
intermediate
layer is outside a range of 10 mass% to 40 mass%, there is an extremely high
likelihood
of the configuration of the interface layer or the oxide layer becoming
inappropriate. A
preferred lower limit value of the average Zn content in the intermediate
layer is 12
mass% or 13 mass%, and, in this case, it is possible to more reliably improve
the
phosphatability of the oxide layer. A preferred upper limit value of the
average Zn
content in the intermediate layer is 28 mass% or 25 mass%, and, in this case,
it is possible
to more reliably develop the LME suppression effect of the interface layer.
[0044]
The thickness of the intermediate layer does not have any direct influences on
the phosphatability and the LME resistance of the compact. However, in a case
in which
the thickness of the intermediate layer is small, the performance of the
corrosion
resistance of the compact is degraded, and thus the thickness of the
intermediate layer is
desirably set to 5.0 pm or more. In addition, when the thickness of the
intermediate
layer becomes excessively large, there is a concern that the manufacturing
costs may be
increased and, furthermore, the HS heating time may be extended. Therefore,
the
thickness of the intermediate layer is desirably 30.0 m or less.
[0045]
[Oxide layer]
Furthermore, on the compact surface side of the intermediate layer, the oxide
layer 23 including a Zn oxide as a main component is formed as the outermost
layer of
the compact. The oxide layer 23 is generated due to the oxidation of a plating
of the
plated steel for hot stamping in a heating process during the manufacturing of
the hot
stamped steel. This oxide layer improves the phosphatability of the hot
stamped steel.
In order to obtain an effect of improving the phosphatability and the coating
adhesion, it
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is necessary to set the average layer thickness of the oxide layer to 0.1 pm
or more.
However, when the oxide layer is too thick, the corrosion resistance,
weldability, and the
like of the compact are adversely affected, and thus the average layer
thickness of the
oxide layer is set to 3.0 m or less. Meanwhile, in a case in which the
average layer
thickness of the oxide layer is set to 2.0 pm or less, the performance of the
corrosion
resistance, weldability, or the like of the compact is exhibited at a high
level, which is
preferable.
[0046]
The states of the interface layer, the intermediate layer, and the oxide layer
can
be specified by the following means.
The Al content in the interface layer can be obtained by cutting the compact
perpendicularly to the surface, polishing the cross section, and analyzing the
distribution
of the Al content in a region including the interface layer in the cross
section using an
analyzer such as EPMA. The average Zn content in the interface layer, the
average Al
content and the average Zn content in the intermediate layer, and the average
Si content in
the plated layer can be obtained on the basis of concentration distributions
obtained using
the above-described method.
The metallographic structures of the interface layer and the intermediate
layer
can be obtained by analyzing the crystal structure using TEM or the like.
The thicknesses of the interface layer, the intermediate layer, and the oxide
layer
can be obtained by capturing an enlarged photograph of the above-described
cross section
using an electronic microscope and image-analyzing this enlarged photograph.
Meanwhile, the configuration of the plated layer in the compact according to
the
present embodiment is substantially not uniform along a direction parallel to
the surface
of the compact. Particularly, the thicknesses of the interface layer, the
intermediate
layer, and the oxide layer often differ in a worked region and a non-worked
region.
Therefore, the above-described analyses need to be carried out in a non-worked
region of
the compact. A compact in which the state of the plated layer in a non-worked
region is
in the above-described range is considered as the compact according to the
present
embodiment.
[0047]
In the hot stamped steel according to the present embodiment having the
configuration described above, improvements are made to the alloy forms of the
interface
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layer and the intermediate layer configuring the plated layer, the Al content
and the Zn
content in the interface layer and the intermediate layer, and the thicknesses
of the
interface layer, the intermediate layer, and the oxide layer. As a result,
according to the
hot stamped steel according to the present embodiment, it is possible to
satisfy both the
improvement of the fatigue properties of the compact based on the suppression
of the
occurrence of LME and the improvement of the phosphatability.
[0048]
Hitherto, the present embodiment has been described, but the present invention
is not limited to the above-described embodiment, and a variety of
modifications can be
made within the scope of the gist of the invention.
[0049]
For example, the plated layer is preferably formed so that the total of the Al
content and the Zn content in the plated layer reaches 20 g/m2 or more and 100
g/m2 or
less. When the total of the Al content and the Zn content in the plated layer
is set to 20
g/m2 or more, the above-described effects (the fatigue properties and the
phosphatability)
of the interface layer, the intermediate layer, and the oxide layer can be
further enhanced.
Meanwhile, when the total amount is set to 100 g/m2 or less, it is possible to
reduce the
manufacturing costs by suppressing the cost for raw materials of the compact,
and
furthermore, the weldability of the hot stamped steel can be enhanced.
Meanwhile, a
preferred lower limit value of the total of the Al content and the Zn content
in the plated
layer is 30 g/m2. A preferred upper limit value of the total of the Al content
and the Zn
content in the plated layer is 90 g/m2.
[0050]
The total of the Al content and the Zn content included in the plated layer
can be
measured by melting the hot stamped steel in hydrochloric acid and carrying
out
inductively coupled plasma-atomic emission spectrometry (ICP-AES) on the
molten
liquid. The Al content and the Zn content can be separately obtained using
this method.
In the melting of the plated steel before heating for hot stamping, it is
common to add an
inhibitor that suppresses the melting of Fe in the base material to
hydrochloric acid in
order to melt only the plated layer. However, the plated layer in the hot
stamped steel
includes Fe, and thus, in the above-described method, the plated layer in the
hot stamped
steel is not sufficiently melted or the melting rate is extremely slow.
Therefore, when
the Al content and the Zn content in the plating in the compact are obtained
by ICP-AES,
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a method in which the plated layer is melted using hydrochloric acid not
including any
inhibitor at a liquid temperature of 40 C to 50 C is appropriate. In addition,
in order to
confirm the absence of the plating component such as Al or Zn after the
melting, it is
desirable to carry out a composition analysis on the surface of the hot
stamped steel after
the melting by EPMA. The above-described analysis needs to be carried out on a
non-
worked region of the compact.
[0051]
Furthermore, the plated layer preferably further includes more than 0 mass% to
10.0 mass% of Si on the average. When the average Si content in the plated
layer is set
to more than 0 mass%, it is possible to enhance the adhesion between the base
material
and the plated layer. On the other hand, when the average Si content is set to
10.0
mass% or less, it is possible to prevent the degradation in the performance of
the
corrosion resistance, weldability, and the like of the hot stamped steel. A
more preferred
lower limit value of the average Si content in the plated layer is 0.1 mass%
or 0.3 mass%.
A more preferred upper limit value of the average Si content in the plated
layer is 8.0
mass%. However, even in a case in which the plated layer does not include Si,
the hot
stamped steel according to the present embodiment has excellent properties,
and thus the
lower limit value of the average Si content in the plated layer is 0 mass%.
[0052]
In a case in which the plated layer includes more than 0 mass% to 10.0 mass%
of
Si on the average, the configuration of phases in the intermediate layer is
changed. In a
case in which the plated layer does not include Si as described above, the
intermediate
layer includes 99 area% or more in total of Fe(Al, Zn)2 and Fe2(Al, Zn)5,
however, in a
case in which the plated layer includes more than 0 mass% to 10.0 mass% of Si
on the
average, some of Fe(Al, Zn)2 and Fe2(Al, Zn)5 are substituted into Fe(Al, Si).
Fe(Al, Si)
refers to a phase in which some of Al in FeAl is substituted to Si. In a case
in which the
hot stamped steel according to the present embodiment is manufactured so that
the
average Si content in the plated layer reaches 10.0 mass%, the amount of
Fe(Al, Si) in the
intermediate layer reaches approximately 50 area%. Therefore, in a case in
which the
plated layer includes more than 0 mass% to 10.0 mass% of Si on the average,
the
intermediate layer includes 99 area% or more in total of Fe(Al, Zn)2 and
Fe2(A1, Zn)5, and
the amount of Fe(Al, Si) reaches 0 area% to 50 area%.
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CA 03020663 2018-10-11
[0053]
Meanwhile, in a case in which the Si content is small, Si forms a solid
solution in
of Fe(Al, Zn)2 and Fe2(Al, Zn)5, and the configuration of the intermediate
layer does not
change. According to the present inventors' investigation, it is assumed that,
in a case in
which the average Si content in the plated layer is 0 mass% to 0.1 mass%,
Fe(Al, Si) is
not generated in the intermediate layer. In addition, according to the present
inventors'
investigation, it is assumed that, even in a case in which the plated layer
includes more
than 0 mass% to 10.0 mass% of Si on the average, the phase constitution of the
interface
layer does not change. Therefore, even in a case in which the plated layer
includes more
than 0 mass% to 10.0 mass% of Si on the average, the interface layer includes
99 area%
or more in total of ccFe, Fe3A1, and FeAl.
[0054]
<Method for Manufacturing Hot stamped steel>
Next, a method for manufacturing the hot stamped steel according to the
present
embodiment will be described. The method for manufacturing the hot stamped
steel
according to the present embodiment includes a step of manufacturing the
plated steel for
hot stamping and a step of carrying out hot stamping on the plated steel for
hot stamping.
The step of manufacturing the plated steel for hot stamping includes a step of
manufacturing a base material of the plated steel for hot stamping and a step
of forming
an Al-Zn-plated layer on the base material of the plated steel for hot
stamping. The
method for manufacturing the hot stamped steel according to the present
embodiment
includes a step of forming an antirust oil film and a blanking work step as
necessary.
Hereinafter, the respective step will be described in detail.
[0055]
[Base Material-Manufacturing Step]
The plated steel which is a material of the hot stamped steel includes a base
material and a plated layer. In the base material-manufacturing step, the base
material of
the plated steel for hot stamping is manufactured. For example, molten steel
having the
same chemical composition as the chemical composition of the base material of
the hot
stamped steel according to the present embodiment exemplified above is
manufactured,
and a slab is manufactured by a casting method using this molten steel.
Alternatively, an
ingot may be manufactured by an ingot-making method using molten steel
manufactured
as described above. Next, the slab or the ingot is hot-rolled, thereby
obtaining the base
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material (hot-rolled sheet) of the plated steel for hot stamping. Meanwhile,
if necessary,
a cold-rolled sheet obtained by carrying out a pickling treatment on the hot-
rolled sheet
and carrying out cold rolling on the hot-rolled sheet which has been subjected
to the
pickling treatment may be used as the base material of the plated steel for
hot stamping.
[0056]
[Plating treatment Step]
In the plating treatment step, an Al-Zn-plated layer is formed on the base
material of the plated steel for hot stamping, thereby manufacturing the
plated steel for
hot stamping.
[0057]
In the plating treatment step, the Al content in a plating bath is set to 40
mass%
to 70 mass%, and the Zn content is set to 30 mass% to 60 mass%. The plating of
the
plated steel for hot stamping is formed using a plating bath having the above-
described
composition, and hot stamping is carried out on the plated steel for hot
stamping under
conditions described below, whereby the configuration of the plated layer of
the hot
stamped steel can be made as described above.
[0058]
Meanwhile, the Al content (Al concentration) and the Zn content (Zn
concentration) in the plating bath are substantially the same as the Al
content (Al
concentration) and the Zn content (Zn concentration) in the plated layer of
the plated steel
for hot stamping, but the average Al content (Al concentration) and the
average Zn
content (Zn concentration) in the plated layer in the hot stamped steel are
smaller than the
average Al content (Al concentration) and the average Zn content (Zn
concentration) in
the plating bath. This is because Al and Zn in the plated layer and Fe in the
base
material form an alloy during hot stamping and thus the Fe concentration in
the plated
layer is increased.
[0059]
Hereinafter, the plated layer of the plated steel for hot stamping will be
referred
to as the non-alloyed plated layer in some cases. The average Al content and
the
average Zn content in the non-alloyed plated layer can be measured by melting
the non-
alloyed plated layer in acid corrosion inhibitor-added hydrochloric acid, and
then
analyzing using inductively coupled plasma-atomic emission spectrometry. In
addition,
in order to enhance the adhesion between the base material of the plated steel
for hot
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CA 03020663 2018-10-11
stamping and the non-alloyed plated layer, it is preferable to further add 0.1
mass% to
15.0 mass% of Si to the non-alloyed plated layer of the plated steel for hot
stamping.
The Si content in the non-alloyed plated layer is decreased since Fe in the
plated layer
diffuses during the alloying of the base material and the plating. Therefore,
in a case in
which the Si content in the non-alloyed plated layer is set to 0 mass% to 15
mass%, the Si
content in the alloyed plated layer is reached 0 mass% to 10 mass%.
[0060]
A method for forming the non-alloyed plated layer may be a hot-dip plating
treatment or any other treatment such as a thermal-spraying plating treatment
or a
deposition plating treatment as long as the average Al content and the average
Zn content
in the non-alloyed plated layer are controlled as described below. For
example, in the
case of forming the non-alloyed plated layer by a hot-dip plating treatment, a
plating
treatment step includes a step of immersing a base material of the plated
steel for hot
stamping in a hot-dip plating bath including Al, Zn, and impurities and
further randomly
including Si and a step of lifting the base material of the plated steel for
hot stamping to
which plated metal is attached from the plating bath. In the case of forming
the non-
alloyed plated layer by a different treatment, it is necessary to carry out a
plating
treatment according to an ordinary method so that the chemical composition of
a non-
alloyed plated layer to be obtained is in the above-described range.
[0061]
Meanwhile, as described above, in the hot stamped steel, the plated layer is
preferably formed on the base material with the total weight per unit area of
Al and Zn in
the plated layer being 20 g/m2 or more and 100 g/m2 or less. In order to
ensure this total
weight per unit area, in the present step, it is important to set the total
weight per unit area
of Al and Zn in the plated layer to 20 g/m2 or more and 100 g/m2 or less in
the lifting of
the base material of the plated steel for hot stamping from the plating bath.
Meanwhile,
the total weight per unit area of Al and Zn included in the plated layer
slightly decreases
during alloying due to oxidation and evaporation. In addition, in the present
step, the
total weight can be ensured by appropriately adjusting the lifting rate of the
steel from the
plating bath or the flow rate of gas during wiping.
[0062]
The plated steel for hot stamping manufactured using the above-described
method includes the base material and the non-alloyed plated layer, and the
non-alloyed
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CA 03020663 2018-10-11
plated layer includes 40.0 mass% to 70.0 mass% of Al, 30.0 mass% to 60.0 mass%
of Zn,
and 0 mass% to 15.0 mass% of Si. When hot stamping is carried out on this
plated steel
for hot stamping under conditions described below, the hot stamped steel
according to the
present embodiment is obtained. Hereinafter, hot stamping conditions will be
described
in detail.
[0063]
[Hot Stamping Step]
In the hot stamping step, hot stamping is carried out on the above-described
plated steel for hot stamping. Ordinary hot stamping is carried out by heating
steel up to
a hot stamping temperature range (hot working temperature range),
subsequently, hot-
working the steel, and furthermore, cooling the steel. According to ordinary
hot
stamping techniques, it is preferable to make the heating rate of steel as
fast as possible in
order to shorten the manufacturing time. In addition, heating steel up to the
hot
stamping temperature range sufficiently alloys the plated layer, and thus, in
ordinary hot
stamping techniques, the control of the heating conditions of steel is not
considered to be
important. However, in the hot stamping step for manufacturing the hot stamped
steel
according to the present embodiment, (1) the plated steel for hot stamping is
heated up to
an alloying temperature range, (2) the temperature of the plated steel for hot
stamping is
held in the alloying temperature range, (3) the plated steel for hot stamping
is heated up to
the hot stamping temperature range, and (4) the plated steel for hot stamping
is hot-
worked and cooled. The present inventors found that, in order to obtain the
plated layer
having the above-described configuration, it is essential to hold the heating
of the steel in
the alloying temperature range for a short period of time and then resume the
heating
during the heating of the plated steel for hot stamping up to the hot stamping
temperature
range.
[0064]
In the hot stamping step, first, the plated steel for hot stamping is charged
into a
heating furnace (a gas furnace, an electric furnace, an infrared furnace, or
the like). In
the heating furnace, the plated steel for hot stamping is heated up to a
temperature range
of 500 C to 750 C (the alloying temperature range) and held in this
temperature range for
seconds to 450 seconds. Due to the holding of the temperature, Fe in the base
material diffuses into the plated layer, and alloying proceeds. Due to this
alloying, the
non-alloyed plated layer changes to a layer including an interface layer, an
intermediate
- 21 -
CA 03020663 2018-10-11
layer, and an oxide layer from the base material side toward the surface side
of the
compact. Meanwhile, the above-described holding time refers to a period of
time during
which the temperature of the plated steel for hot stamping is in the alloying
temperature
range. The temperature of the plated steel for hot stamping may change in the
alloying
temperature range during the holding of the temperature as long as the above-
described
holding time condition is satisfied.
[0065]
In a case in which the temperature of the plated steel for hot stamping is
held
below the alloying temperature range (that is, lower than 500 C), the alloying
rate of the
plated layer is extremely slow, and the heating time significantly extends,
which is not
preferable from the viewpoint of the productivity. In a case in which the
temperature of
the plated steel for hot stamping is held above the alloying temperature
range, that is,
higher than 750 C, the growth of an oxide on the surface layer of the plated
layer is
excessively accelerated in this holding process, and the weldability of a
compact to be
obtained after HS degrades.
[0066]
In a case in which the time during which the temperature of the plated steel
for
hot stamping is held in the alloying temperature range is shorter than 10
seconds, the
alloying of the plated layer is not completed, and thus a plated layer having
the interface
layer, the intermediate layer, and the oxide layer described above cannot be
obtained. In
a case in which the time during which the temperature of the plated steel for
hot stamping
is held in the alloying temperature range is longer than 450 seconds, the
amount of the
oxide grown becomes excessive, which leads to the degradation of the
productivity.
[0067]
The heating conditions during the heating the plated steel for hot stamping up
to
the above-described alloying temperature range are not particularly limited.
However,
from the viewpoint of the productivity, the heating time is desirably short.
[0068]
In the hot stamping included in the method for manufacturing the hot stamped
steel according to the present embodiment, the temperature of the plated steel
for hot
stamping is held in the alloying temperature range as described above, then,
the plated
steel for hot stamping is heated up to a temperature range of the Ac3
temperature to
950 C, and then hot working is carried out. At this time, it is necessary to
limit the time
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during which the temperature of the plated steel for hot stamping is held in
the
temperature range of the Ac3 temperature to 950 C (oxidation temperature
range) to 60
seconds or shorter. When the temperature of the plated steel for hot stamping
is held in
the oxidation temperature range, the oxide layer on the surface layer of the
plated layer
grows. In a case in which the time during which the temperature of the plated
steel for
hot stamping is in the oxidation temperature range is longer than 60 seconds,
there is a
concern that an oxide film may excessively grow and thus the weldability of
the compact
may be degraded. Meanwhile, the generation rate of the oxide film is extremely
fast,
and thus the lower limit value of the time during which the temperature of the
plated steel
for hot stamping is in the oxidation temperature range is longer than 0
seconds.
However, in a case in which the plated steel for hot stamping is heated in a
non-oxidative
atmosphere such as a 100% nitrogen atmosphere, the oxide layer is not formed,
and thus
the plated steel for hot stamping needs to be heated in an oxidative
atmosphere such as
the atmosphere.
[0069]
As long as the time during which the temperature of the plated steel for hot
stamping is in the oxidation temperature range is 60 seconds or shorter, the
conditions
such as the heating rate and the peak heating temperature are not particularly
limited, and
a variety of conditions under which hot stamping can be carried out can be
selected.
[0070]
Next, the plated steel for hot stamping removed from the heating furnace is
press-formed using a die. In the present step, the steel is quenched at the
same time as
the press-forming. In the die, a cooling medium (for example, water) is
circulated, and
the die accelerates the release of heat from the plated steel for hot
stamping, thereby
quenching the plated steel. With the above-described steps, the hot stamped
steel can be
manufactured.
[0071]
Meanwhile, in the above description, the plated steel for hot stamping was
heated using the heating furnace. However, the plated steel for hot stamping
may be
heated by energization heating. Even in this case, the steel is heated for a
predetermined
period of time by energization heating, and the steel is press-formed using
the die.
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CA 03020663 2018-10-11
[0072]
Hitherto, essential steps of the method for manufacturing the hot stamped
steel
of the present embodiment have been described; however, hereinafter, random
selective
steps of the manufacturing method will be described.
[0073]
[Antirust Oil Film-Forming Step]
The antirust oil film-forming step is a step of forming an antirust oil film
by
applying an antirust oil to the surface of the plated steel for hot stamping
after the plating
treatment step and before the hot stamping step and may be randomly included
in the
manufacturing method. In a case in which the time taken to carry out hot
stamping from
the manufacturing of the plated steel for hot stamping is long, there is a
concern that the
surface of the plated steel for hot stamping may be oxidized. However, the
surface of
the plated steel for hot stamping on which an antirust oil film is formed by
the antirust oil
film-forming step is not easily oxidized, and thus the antirust oil film-
forming step is
capable of suppressing the formation of scales on the compact. Meanwhile, as a
method
for forming the antirust oil film, any well-known technique can be used.
[0074]
[Blanking Work Step]
The present step is a step of forming the steel in a specific shape by
carrying out
a shearing work and/or a punching work on the plated steel for hot stamping
after the
antirust oil film-forming step and before the hot stamping step. The sheared
surface of
the steel which has been subjected to the blanking work is easily oxidized.
However,
when the antirust oil film has been formed in advance on the steel surface,
the antirust oil
also spreads on the sheared surface to a certain extent. Therefore, the
oxidation of the
steel after the blanking work can be suppressed.
[0075]
Hitherto, the embodiment of the present invention has been described, but the
above-described embodiment is simply an example of the present invention.
Therefore,
the present invention is not limited to the above-described embodiment and can
be
appropriately modified in design within the scope of the gist of the present
invention.
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CA 03020663 2018-10-11
[Examples]
[0076]
Hereinafter, the effects of the present invention will be specifically
described
using invention examples. Meanwhile, the present invention is not limited to
conditions
used in the following invention examples.
Example 1
[0077]
The present inventors formed an Al-Zn-based plated layer, a Zn-based plated
layer, and an Al-based plated layer on a base material 10 respectively. The Al-
Zn-based
plated layer included 55.0 mass% of Al and 45.0 mass% of Zn, the Zn-based
plated layer
substantially included only Zn, and the Al-based plated layer substantially
included only
Al.
[0078]
Next, a steel on which each of the plated layers was formed (a plated steel
configured of the base material and the plated layer) was charged into a first
heating
furnace, heated up to 700 C, and held in this temperature range for 120
seconds. After
that, the plated steel was immediately charged into a second heating furnace
and heated
up to 900 C, and then the plated steel was removed from the second heating
furnace so
that the steel temperature was in a range of the Ac3 temperature to 950 C for
30 seconds.
Immediately after the plated steel was removed from the second heating
furnace, a hot V-
bending test was carried out on the plated steel using a hand pressing
machine. The time
taken from the removal of the steel from the furnace to the beginning of the
work on the
steel was approximately five seconds, and the bending work was carried out at
a steel
temperature of approximately 800 C. V-bending was carried out so that the
outer
diameter of a bent portion increased by approximately 15% from that before the
V-
bending. After that, the steel was cooled, thereby quenching the steel. The
cooling
was carried out so that the cooling rate from approximately 800 C to a
martensite
transformation-starting point (approximately 410 C) reached 50 C/second or
faster.
Finally, a SEM image of the bent outside portion of the worked portion of the
compact
after the completion of the cooling was captured, and the fatigue properties
(LME
resistance) of the compact were evaluated on the basis of the presence or
absence of the
occurrence of LME.
- 25 -
CA 03020663 2018-10-11
[0079]
FIGS. 1 to 3 are cross-sectional photographs of the worked portions of the
compacts manufactured from the Al-Zn-based plated steel, the Zn-based plated
steel, and
the Al-based plated steel. In the compact of FIG. 1, an alloyed Al-Zn-based
plated layer
30 was formed on the base material 1, in the compact of FIG. 2, an alloyed Zn-
based
plated layer 40 was formed on the base material 1 and an alloyed Al-based
plated layer 50
was formed on the base material 1 in the compact of FIG. 3. Meanwhile, the
worked
portion of the observed compact was a portion on which a tensile work was
carried out
and an outside portion of the V-bending worked portion with respect to the
bending center
in which the occurrence of LME was concerned.
[0080]
According to FIGS. 1 to 3, it is found that, in the compact having the alloyed
Zn-
based plated layer 40, cracks extended up to the inside of the base material
10; however,
in the compact having the alloyed Al-Zn-based plated layer 30 and the compact
having
the alloyed Al-based plated layer 50, no cracks extended to the inside of the
base material
10.
[0081]
Furthermore, the steel that had been heated and held in the specific
temperature
range as described above was removed from the furnace, the steel was formed
using a flat
sheet die including a water-cooling jacket and then quenched so that the
cooling rate
reached 50 C/second or faster until the martensite transformation-starting
point
(approximately 410 C) even in a portion with a slow cooling rate. After that,
the surface
of the compact was conditioned, and a phosphating treatment was carried out on
the
compact. Finally, a SEM image of the surface of the compact was captured, and
the
phosphatability was evaluated on the basis of the degree of a phosphate film
formed.
[0082]
FIGS. 4 to 6 are examples of SEM images (secondary electron images) showing
the surfaces of the compacts in a case in which the Al-Zn-based plated steel,
the Zn-based
plated steel, and the Al-based plated steel removed from the second heating
furnace were
worked and rapidly cooled in the flat sheet die including the water-cooling
jacket and
then subjected to a phosphating treatment.
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CA 03020663 2018-10-11
[0083]
According to FIGS. 4 to 6, it is found that, in the Al-Zn-based plating and
the
Zn-based plating, chemical conversion crystals 60 (phosphate films) were
formed on the
entire surface; however, in the Al-based plating, regions in which no chemical
conversion
crystals were formed, that is, transparent regions 70 were present on some of
the surface.
Example 2
[0084]
First, a slab was manufactured by a continuous casting method using molten
steel having a chemical composition shown in Table 1. Next, the slab was hot-
rolled so
as to manufacture a hot-rolled material, and the hot-rolled material was
further pickled
and then cold-rolled, thereby manufacturing a cold-rolled steel. In addition,
this cold-
rolled steel was used as a base material (sheet thickness: 1.4 mm) that was
used to
manufacture a hot stamped steel. The Ac3 temperature of the base material was
approximately 810 C.
[0085]
[Table 1]
Chemical composition of base material (unit: mass%, remainder: Fe and
impurities)
Si Mn P S sol. Al N B Ti Cr
0.2 0.2 1.3 0.01 0.005 0.02 0.002 0.002 0.02 0.2
[0086]
Next, plating was formed on the base material manufactured as described above
using a plating bath having a composition shown in Table 2, thereby obtaining
the steel
for hot stamping. The adhesion amount of the plating was controlled so that
the total
weight of Al and Zn reached a value shown in Table 2. This steel was heated up
to an
alloying temperature shown in Table 2, and the temperature was held for an
alloying time
shown in Table 2. After that, the steel was charged into a heating furnace and
heated up
to a range of the Ac3 temperature to 950 C, and then the steel was removed
from the
heating furnace so that the temperature of the steel was held in this
temperature range for
a holding time shown in Table 2.
[0087]
Next, in order to carry out a hot V-bending test, the following step was
carried
out. Hot V-bending work was immediately carried out on the steel removed from
the
heating furnace using a hand pressing machine. The time taken from the removal
of the
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CA 03020663 2018-10-11
steel from the heating furnace to the beginning of the work on the steel was
set to five
seconds. In addition, as the shape of the die, a shape which extended an
outside portion
having a bending radius by the V-bending work by approximately 15% at the end
of the
bending work was used.
[0088]
In addition, in order to carry out a phosphatability evaluation test and a
coating
adhesion evaluation test, the following step was carried out. Hot stamping was
immediately carried out on the steel removed from the heating furnace using
the flat sheet
die including the water-cooling jacket, and then accelerated cooling was
carried out.
The cooling rate was set to reach a cooling rate of 50 C/second or faster
until
approximately the martensite transformation-starting point (410 C).
Furthermore, for
the respective hot stamped steels, the surfaces were conditioned at room
temperature for
20 seconds using a surface conditioning treatment agent (trade name: PREPALENE-
X)
manufactured by Nihon Parkerizing Co., Ltd. Next, a phosphating treatment was
carried
out on the respective hot stamped steels using a phosphating treatment liquid
(trade name:
PAUL BOND 3020) manufactured by Nihon Parkerizing Co., Ltd. In the phosphating
treatment, the temperature of a treatment liquid was set to 43 C, and the hot
stamped
steels were immersed in the treatment liquid for 120 seconds. After the above-
described
phosphating treatment was carried out, the respective hot stamped steels were
electrodeposition-coated with a cationic electrodeposition coating
manufactured by
NIPPONPAINT Co., Ltd. by slope energization at a voltage of 160 V and,
furthermore,
baking-coated at a baking temperature of 170 C for 20 minutes. The average of
the
thicknesses of the coatings after the electrodeposition coating was 10 Ilm in
all of
invention examples and comparative examples.
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CA 03020663 2018-10-11
[0089]
[Table 2] ______________________________________________
Plating treatment
conditions Total Hot stamping conditions
weight of ____________________________________________________________
Composition (mass%) Al and Zn Alloying Alloying
Holding
/
Al Zn Si (gm2 temperature time time
)
(7C)
(seconds) (seconds)
_
1 55 45 0 60 700 120 30
2 55 45 0 60 700 120 30
3 55 45 0 60 700 120 30
4 55 45 0 60 700 120 30
55 45 0 60 700 120 30
6 45 40 15 60 700 120 30
7 55 45 0 40 700 120 30
8 55 45 0 80 700 120 30
Invention 9 60 40 0 60 500 300 30
Example 10 40 60 0 60 500 300 30
11 65 35 0 60 700 120 30
12 40 60 0 60 700 120 30
13 55 45 0 60 700 120 30
14 45 40 15 60 700 120 30
55 45 0 20 700 120 30
16 55 45 0 100 700 120 30
17 55 45 0 60 700 120 5
18 55 45 0 60 750 90 60
101 25 75 0 60 700 120 30
102 75 25 0 60 700 _ 120 30
103 55 45 0 60 800 120 45
Comparative
104 55 45 0 60 400 120 30
Example
105 55 45 0 60 700 _ 500 30
106 55 45 0 60 500 5 15
107 55 45 0 60 800 60 120
[0090]
The configurations of the invention examples and the comparative examples
obtained by the above-described means were confirmed using a method described
below.
[0091]
The states of interface layers, intermediate layers, and oxide layers in the
invention example and the comparative examples were specified by the following
means.
The average Al content and the average Zn content in the interface layer, the
average Al
content and the average Zn content in the intermediate layer, and the average
Si content in
the plated layer were obtained by cutting the compact perpendicularly to the
surface of
the compact, polishing a cross section, and analyzing this cross section using
an analyzer
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CA 03020663 2018-10-11
such as EPMA. The metallographic structures of the interface layer and the
intermediate
layer were obtained by analyzing the crystal structure using TEM or the like.
Examples
in which the metallographic structure satisfied the regulation of the present
invention
were indicated as "OK", and examples in which the crystallographic structure
did not
satisfy the regulation were indicated as "NG". The thicknesses of the
interface layer, the
intermediate layer, and the oxide layer were obtained by capturing an enlarged
photograph of the above-described cross section using an electronic microscope
and
image-analyzing this enlarged photograph. The above-described analyses were
carried
out on a non-worked region of the compact.
[0092]
The total weight of Al and Zn in the plated layer in the invention examples
and
the comparative examples was measured by high-frequency inductively coupled
plasma-
atomic emission spectrometry (ICP-OES). That is, a sample was taken from the
non-
worked portion (a place which was not V-bent) in each of the invention
examples and the
comparative examples, and the plated layer was melted in an aqueous solution
of 10%
HC1 and analyzed. The energy of plasma was imparted to each solution,
component
elements were excited, and the locations and intensities of emitted light rays
(spectrum
rays) being emitted were measured, thereby identifying the respective elements
and
measuring the amounts thereof.
[0093]
The configurations of the invention examples and the comparative examples
confirmed by the above-described means are shown in Table 3. A remainder of
the
average composition of the interface layer and the intermediate layer shown in
Table 3
was Fe and impurities.
- 30 -
[0094]
[Table 3]
Interface layer
Intermediate layer Oxide layer Average Si in
Total weight
Average composition Average composition
entire plated
Structure Structure Thickness
Thickness of Al and Zn
(mass%) . Thickness (mass%)
layer
(1-1)
Average Al Average Zn Judgment Average Al Average Zn Judgment 111)
(Jim) (mass%) (g/m2
1 10 3 OK 10 40 25 OK
20 2 0 57
2 25 3 OK 10 40 25 OK
20 2 0 57
3 15 1 OK 10 40 25 OK
20 2 0 57
4 15 5 OK 10 40 25 OK
20 2 0 57
15 3 OK 10 40 25 OK 20 2
0 57
6 15 3 OK 10 40 25 OK
20 2 9 57
7 15 3 OK 5 40 25 OK
10 2 0 37 Q
8 15 3 OK 15 40 25 OK
30 2 0 77 0
'
(....) 9 15 3 OK 10 30 25 OK
20 2 0 57 E
0
,
0
15 3 OK 10 50 25 OK 20 2
0 57
.
r.,
11 15 3 OK 10 40 10 OK
20 2 0 57 0
,
0
,
12 15 3 OK 10 40 40 OK
20 2 0 57 ,
0
,
13 15 3 OK 10 40 25 OK
20 2 0 57 ,
,
14 15 3 OK 10 40 25 OK
20 2 9 57
15 3 OK 10 40 25 OK 5 2
0 17
16 15 3 OK 10 40 25 OK
30 2 0 97
17 15 3 OK 10 40 25 OK
20 0.5 0 59
18 15 3 OK 10 40 25 OK
20 3 0 55
101 15 10 OK 10 20 45 NG
20 2 0 52
102 25 0.5 OK 10 40 9 NG
25 0.3 0 58
103 13 3 OK 10 30 10 OK
20 5 0 52
104 15 7 OK 5 50 30 NG
15 1 0 58
105 13 3 OK 10 30 10 OK
20 5 0 52
106 15 8 OK 5 50 35 NG
20 0.05 0 60
107 10 2 OK 20 30 10 OK
20 7 0 45
CA 03020663 2018-10-11
[0095]
Furthermore, the fatigue properties (LME resistance), the phosphating
treatment properties, the coating adhesion, and the weldability of the
invention
examples and the comparative examples obtained by the above-described means
were
confirmed by methods described below.
[0096]
The fatigue properties of the examples and the comparative examples were
evaluated by the following means. The presence and absence of the occurrence
of
liquid metal embrittlement (LME) was observed by observing a reflection
electron
image of a cross section of the V-bending worked portion in the steel
thickness direction
of each of the examples and the comparative examples using a scanning electron
microscope (SEM) and a reflection electron detector. In addition, samples in
which no
cracks were generated and samples in which cracks were generated, but ended in
the
plated layer were evaluated as being favorable (GOOD) in terms of the fatigue
properties. On the other hand, samples in which cracks extended up to the base
material beyond the plated layer were evaluated as being poor (BAD) in terms
of the
fatigue properties.
[0097]
The phosphating treatment properties of the examples and the comparative
examples were evaluated by the following means. A phosphate film formed on
each of
the phosphating-treated samples was melted and removed using a heavy ammonium
chromate solution, and the weight difference of the steel before and after the
removal of
the film was measured and considered as the adhesion amount of the phosphate
film.
In addition, samples having an adhesion amount of 2.0 g/m2 or more were
evaluated as
being favorable (GOOD) in terms of the phosphatability. On the other hand,
samples
having an adhesion amount of less than 2.0 g/m2 were evaluated as being poor
(BAD) in
terms of the phosphatability.
[0098]
The coating adhesion of the examples and the comparative examples were
evaluated by the following means. Each of the electrodeposition-coated samples
was
immersed in an aqueous solution of 5% NaCl having a temperature of 50 C for
500 hours. After the immersion, polyester tape was attached to the entire
surface of a
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CA 03020663 2018-10-11
60 mm x 120 mm test region and then peeled off. The area of a region from
which the
coated film had been peeled off by pulling the tape was obtained, and the
coated film
peeling percentage (%) was obtained on the basis of the following expression.
Coated film peeling percentage = (A2/A1) x 100
Al represents the area (60 mm x 120 mm = 7,200 mm2) of the test region, and
A2 represents the area (mm2) of the region from which the coated film was
peeled off.
Samples having a coated film peeling percentage of less than 5.0% were
evaluated as
being favorable (GOOD) in terms of the coating adhesion. On the other hand,
samples
having a coated film peeling percentage of 5.0% or more were evaluated as
being poor
(BAD) in terms of the coating adhesion.
[0099]
The weldability of the examples and the comparative examples were evaluated
using a surface resistance value. The surface resistance value of the sample
was
computed from a voltage value obtained when a current of 2A was made to flow
through the sample using a pressurization-type direct-current inverter power
supply at a
welding pressure of 250 kgf. Samples having a surface resistance value of 20
mO or
less were evaluated as being favorable (GOOD) in terms of the weldability.
[0100]
The fatigue properties (LME resistance), the phosphating treatment properties,
the coating adhesion, and the weldability of the invention examples and the
comparative
examples confirmed by the above-described means are shown in Table 4.
[0101]
[Table 4]
Fatigue Coating
Phosphatability Weldability
properties adhesion
,
1 GOOD GOOD GOOD GOOD
2 GOOD GOOD GOOD GOOD
3 GOOD GOOD GOOD GOOD
4 GOOD GOOD GOOD GOOD
GOOD GOOD GOOD GOOD
6 GOOD GOOD GOOD GOOD
7 GOOD GOOD GOOD GOOD
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CA 03020663 2018-10-11
Fatigue Coating
Phosphatability Weldability
properties adhesion
8 GOOD GOOD GOOD GOOD
9 GOOD GOOD GOOD GOOD
GOOD GOOD GOOD GOOD
11 GOOD GOOD GOOD GOOD
12 GOOD GOOD GOOD GOOD
13 GOOD GOOD GOOD GOOD
14 GOOD GOOD GOOD GOOD
GOOD GOOD GOOD GOOD
16 GOOD GOOD GOOD GOOD
17 GOOD GOOD GOOD GOOD
18 GOOD GOOD GOOD GOOD _
,
101 BAD GOOD GOOD GOOD
102 GOOD BAD BAD GOOD
103 GOOD GOOD GOOD BAD
104 BAD GOOD GOOD GOOD
105 GOOD GOOD GOOD BAD
106 BAD BAD BAD GOOD
107 GOOD GOOD GOOD BAD
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CA 03020663 2018-10-11
[0102]
[Evaluation Results]
As shown in Table 3, it is found that, in all of the hot stamped steels of the
invention examples in which improvements were made to the alloy form and the
composition of the plated layer and improvements were made to the thickness of
an
oxide that was formed as the outermost layer of the plated layer, both the
improvement
of the fatigue properties of the compact based on the suppression of the
occurrence of
LME and the improvement of the phosphatability of the compact were achieved.
[0103]
In contrast, it is found that, in all of the hot stamped steels of the
comparative
examples in which improvements were not made to the alloy form, the
composition, and
the like of the plated layer, all of the fatigue properties, the
phosphatability, and the
weldability were not sufficiently improved.
[0104]
Comparative Example 101 was manufactured using a plating bath including an
insufficient Al content, and thus it was not possible to prevent LME.
Therefore, the
fatigue properties of Comparative Example 101 was poor.
Comparative Example 102 was manufactured using a plating bath including an
insufficient Zn content, and thus the structure of the intermediate layer
became
inappropriate due to the lack of Zn. Therefore, in Comparative Example 102,
the
phosphatability was impaired, and the coating adhesion was poor.
In Comparative Example 103, the alloying temperature during the hot stamping
was too high, and thus the thickness of the oxide layer became excessive, and
the
weldability was poor.
In Comparative Example 104, the alloying temperature during the hot stamping
was too low, and thus the plated layer was not sufficiently alloyed, a Zn-rich
phase was
generated, and it was not possible to prevent LME. Therefore, the fatigue
properties of
Comparative Example 104 was poor.
In Comparative Example 105, the alloying time during the hot stamping was
too long, and thus the thickness of the oxide layer became excessive, and the
weldability was poor.
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In Comparative Example 106, the alloying time during the hot stamping was
too short, and thus the heating for alloying became insufficient. Therefore,
in
Comparative Example 106, LME occurred, and the fatigue properties degraded.
Furthermore, in Comparative Example 106, heating was not sufficient, and thus
the
amount of the oxide was small, and the phosphatability and the coating
adhesion lacked.
In Comparative Example 107, the alloying temperature and the holding time
during the hot stamping were excessive, and thus the thickness of the oxide
layer
became excessive, and the weldability was poor.
[Industrial Applicability]
[0105]
According to the present invention, in the hot stamped steel in which the
plated
layer is formed on the surface of the base material, both the fatigue
properties and the
phosphatability are sufficiently exhibited. Therefore, the present invention
is hopeful
in the field of structural members and the like which are used in cars and the
like.
[Brief Description of the Reference Symbols]
[0106]
1 HOT STAMPED STEEL
BASE MATERIAL
PLATED LAYER
21 INTERFACE LAYER
22 INTERMEDIATE LAYER
23 OXIDE LAYER
Al-Zn-BASED PLATED LAYER
Zn-BASED PLATED LAYER
Al-BASED PLATED LAYER
CHEMICAL CONVERSION CRYSTAL
TRANSPARENT REGION
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