U.S. patent application number 16/097771 was filed with the patent office on 2019-05-30 for hot stamped steel.
This patent application is currently assigned to Nippon Steel & Sumitomo Metal Corporation. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Koji AKIOKA, Akihiro SENGOKU, Hiroshi TAKEBAYASHI.
Application Number | 20190160507 16/097771 |
Document ID | / |
Family ID | 60266417 |
Filed Date | 2019-05-30 |
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United States Patent
Application |
20190160507 |
Kind Code |
A1 |
SENGOKU; Akihiro ; et
al. |
May 30, 2019 |
HOT STAMPED STEEL
Abstract
A hot stamped steel according to one embodiment of the present
invention includes a base material and a plated layer, wherein 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 which compositions and thicknesses of each layer are
controlled.
Inventors: |
SENGOKU; Akihiro; (Tokyo,
JP) ; TAKEBAYASHI; Hiroshi; (Tokyo, JP) ;
AKIOKA; Koji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
Nippon Steel & Sumitomo Metal
Corporation
Tokyo
JP
|
Family ID: |
60266417 |
Appl. No.: |
16/097771 |
Filed: |
May 10, 2016 |
PCT Filed: |
May 10, 2016 |
PCT NO: |
PCT/JP2016/063856 |
371 Date: |
October 30, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 2/12 20130101; C23C
28/322 20130101; C23C 28/321 20130101; C23C 28/345 20130101; C22C
38/06 20130101; B21D 22/022 20130101; C21D 8/005 20130101; C22C
38/28 20130101; C22C 38/001 20130101; C22C 38/00 20130101; C22C
38/02 20130101; C22C 38/58 20130101; C21D 6/008 20130101; C22C
38/32 20130101; C23C 2/06 20130101; C22C 38/002 20130101; C22C
18/04 20130101; C21D 6/002 20130101; C21D 6/005 20130101; C23C 2/28
20130101; C22C 38/04 20130101 |
International
Class: |
B21D 22/02 20060101
B21D022/02; C22C 18/04 20060101 C22C018/04; C22C 38/32 20060101
C22C038/32; C22C 38/28 20060101 C22C038/28; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/00 20060101
C21D008/00; C21D 6/00 20060101 C21D006/00 |
Claims
1. A hot stamped steel comprising: a base material; and a plated
layer, wherein 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 .alpha.Fe,
Fe.sub.3Al, 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 a 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 .mu.m or more, in
the intermediate layer, a structure includes 99 area % or more in
total of Fe(Al, Zn).sub.2 and Fe.sub.2(Al, Zn).sub.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 average layer thickness is 5.0 .mu.m or
more, and in the oxide layer, an average layer thickness is 0.1
.mu.m to 3.0 .mu.m.
2. The hot stamped steel according to claim 1, wherein the average
layer thickness is 1.0 .mu.m to 10.0 .mu.m in the interface
layer.
3. The hot stamped steel according to claim 1, wherein a total
weight per unit area of Al and Zn in the plated layer is 20
g/m.sup.2 or more and 100 g/m.sup.2 or less.
4. The hot stamped steel according to claim 1, wherein the plated
layer further includes 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).sub.2 and the Fe.sub.2(Al, Zn).sub.5 are
substituted into Fe(Al, Si).
5. The hot stamped steel according to claim 2, wherein a total
weight per unit area of Al and Zn in the plated layer is 20
g/m.sup.2 or more and 100 g/m.sup.2 or less.
6. The hot stamped steel according to claim 2, wherein the plated
layer further includes 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).sub.2 and the Fe.sub.2(Al, Zn).sub.5 are
substituted into Fe(Al, Si).
7. The hot stamped steel according to claim 3, wherein the plated
layer further includes 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).sub.2 and the Fe.sub.2(Al, Zn).sub.5 are
substituted into Fe(Al, Si).
8. The hot stamped steel according to claim 5, wherein the plated
layer further includes 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).sub.2 and the Fe.sub.2(Al, Zn).sub.5 are
substituted into Fe(Al, Si).
Description
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 Ac.sub.3
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 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.
PRIOR ART DOCUMENT
Patent Document
[0008] [Patent Document 1] Japanese Unexamined Patent Application,
First Publication No. 2003-73774
[0009] [Patent Document 2] Japanese Unexamined Patent Application,
First Publication No. 2003-49256
[0010] [Patent Document 3] Japanese Unexamined Patent Application,
First Publication No. 2005-113233
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0011] 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
[0012] The gist of the present invention is as described below.
[0013] (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 .alpha.Fe,
Fe.sub.3Al, 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 .mu.m or more; in
the intermediate layer, a structure includes 99 area % or more in
total of Fe(Al, Zn).sub.2 and Fe.sub.2(Al, Zn).sub.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 average layer thickness is 5.0 .mu.m or
more; and in the oxide layer, an average layer thickness is 0.1
.mu.m to 3.0 .mu.m.
[0014] (2) In the hot stamped steel according to (1), the average
layer thickness may be 1.0 .mu.m to 10.0 .mu.m in the interface
layer.
[0015] (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/m.sup.2 or more and 100 g/m.sup.2 or less.
[0016] (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).sub.2 and the
Fe.sub.2(Al, Zn).sub.5 may be substituted into Fe(Al, Si).
Effects of the Invention
[0017] 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
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] FIG. 7 is a cross-sectional view of a vicinity of a surface
of a hot stamped steel according to the present embodiment.
[0025] 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
[0026] 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".
[0027] 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 .alpha.Fe, Fe.sub.3Al, and 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 .mu.m or more, a structure of the intermediate
layer includes 99 area % or more in total of Fe(Al, Zn).sub.2 and
Fe.sub.2(Al, Zn).sub.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 .mu.m or more, and an average layer thickness of the oxide
layer is 0.1 .mu.m or more and 3.0 .mu.m 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).
[0028] <Hot Stamped Steel>
[0029] 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.
[0030] [Composition of Base Material]
[0031] 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 %".
[0032] (C: Preferably 0.05% to 0.40%)
[0033] 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 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%.
[0034] (Si: Preferably 0.5% or Less)
[0035] 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%.
[0036] (Mn: Preferably 0.5% to 2.5%)
[0037] 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%.
[0038] (P: Preferably 0.03% or Less)
[0039] 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 may be set to 0.03% or less. The P
content in the base material is preferably as small as
possible.
[0040] (S: Preferably 0.01% or Less)
[0041] 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.
[0042] (sol. Al: Preferably 0.10% or Less)
[0043] 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 Ac.sub.3 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%.
[0044] (N: Preferably 0.01% or Less)
[0045] 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.
[0046] 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.
[0047] (B: Preferably 0% to 0.0050%)
[0048] 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%.
[0049] (Ti: Preferably 0% to 0.10%)
[0050] 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%.
[0051] 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.
[0052] (Cr: Preferably 0% to 0.5%)
[0053] 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 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%.
[0054] (Mo: Preferably 0% to 0.50%)
[0055] 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%.
[0056] 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.
[0057] (Nb: Preferably 0% to 0.10%)
[0058] 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%.
[0059] (Ni: Preferably 0% to 1.0%)
[0060] 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%.
[0061] 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 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.
[0062] [Plated layer]
[0063] 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.
[0064] [Interface Layer]
[0065] The interface layer is formed adjacent to the base material.
A majority of the structure of the interface layer is configured of
.alpha.Fe, Fe.sub.3Al, 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 .alpha.Fe, Fe.sub.3Al, 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.
[0066] 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 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.
[0067] 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 .alpha.Fe phase having the smallest Al content,
Fe.sub.3Al 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.
[0068] In a case in which the average layer thickness of the
interface layer is less than 1.0 .mu.m, 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 .mu.m
or more. In a case in which the average layer thickness of the
interface layer is set to 2.0 .mu.m 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 .mu.m, 6.0 .mu.m, or 7.0 .mu.m. 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 .mu.m 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 .mu.m and more
preferably 10.0 .mu.m, 9.0 .mu.m, or 8.0 .mu.m.
[0069] [Intermediate Layer]
[0070] 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).sub.2 and Fe.sub.2(Al, Zn).sub.5. Fe(Al, Zn).sub.2 is a phase
in which some of Al in FeAl.sub.2 that is a kind of a Fe--Al
intermetallic compound is substituted into Zn, and Fe.sub.2(Al,
Zn).sub.5 is a phase in which some of Al in Fe.sub.2Al.sub.5 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).sub.2 and
Fe.sub.2(Al, Zn).sub.5, the influence of the above-described
inclusion can be ignored.
[0071] 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.
[0072] 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 50 mass % or 45 mass %, and, in this
case, it is possible to more reliably improve the phosphatability
of the oxide layer.
[0073] 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.
[0074] 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 .mu.m 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 .mu.m or less.
[0075] [Oxide layer]
[0076] 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 is necessary to set the average layer thickness of the oxide
layer to 0.1 .mu.m 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 .mu.m or less.
Meanwhile, in a case in which the average layer thickness of the
oxide layer is set to 2.0 .mu.m or less, the performance of the
corrosion resistance, weldability, or the like of the compact is
exhibited at a high level, which is preferable.
[0077] The states of the interface layer, the intermediate layer,
and the oxide layer can be specified by the following means.
[0078] 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.
[0079] The metallographic structures of the interface layer and the
intermediate layer can be obtained by analyzing the crystal
structure using TEM or the like.
[0080] 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.
[0081] 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.
[0082] 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 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.
[0083] 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.
[0084] 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/m.sup.2 or more and 100 g/m.sup.2 or less. When the
total of the Al content and the Zn content in the plated layer is
set to 20 g/m.sup.2 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/m.sup.2
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/m.sup.2. A preferred upper limit value of the total of the Al
content and the Zn content in the plated layer is 90 g/m.sup.2.
[0085] 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, a method in which the plated layer is melted
using hydrochloric acid not including any inhibitor at a liquid
temperature of 40.degree. C. to 50.degree. 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.
[0086] 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 %.
[0087] 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).sub.2 and Fe2(Al, Zn).sub.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).sub.2 and Fe.sub.2(Al, Zn).sub.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).sub.2 and Fe.sub.2(Al, Zn).sub.5, and
the amount of Fe(Al, Si) reaches 0 area % to 50 area %.
[0088] Meanwhile, in a case in which the Si content is small, Si
forms a solid solution in of Fe(Al, Zn).sub.2 and Fe.sub.2(Al,
Zn).sub.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
.alpha.Fe, Fe.sub.3Al, and FeAl.
[0089] <Method for Manufacturing Hot Stamped Steel>
[0090] 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.
[0091] [Base Material-Manufacturing Step]
[0092] 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
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.
[0093] [Plating Treatment Step]
[0094] 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.
[0095] 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.
[0096] 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.
[0097] 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 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
%.
[0098] 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.
[0099] 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/m.sup.2 or more and 100 g/m.sup.2 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/m.sup.2 or more and 100 g/m.sup.2 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.
[0100] 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 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.
[0101] [Hot Stamping Step]
[0102] 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.
[0103] 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.degree. C. to 750.degree. C. (the alloying
temperature range) and held in this temperature range for 10
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
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.
[0104] 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.degree. 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.degree. 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.
[0105] 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.
[0106] 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.
[0107] 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
AC.sub.3 temperature to 950.degree. C., and then hot working is
carried out. At this time, it is necessary to limit the time during
which the temperature of the plated steel for hot stamping is held
in the temperature range of the AC.sub.3 temperature to 950.degree.
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.
[0108] 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.
[0109] 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.
[0110] 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.
[0111] 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.
[Antirust Oil Film-Forming Step]
[0112] 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.
[0113] [Blanking Work Step]
[0114] 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.
[0115] Hitherto, the embodiment of the present invention has been
described, but the above-described embodiment is simply an example
of the present invention.
[0116] 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.
EXAMPLES
[0117] 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
[0118] 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.
[0119] 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.degree. 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.degree. C., and then the
plated steel was removed from the second heating furnace so that
the steel temperature was in a range of the Ac.sub.3 temperature to
950.degree. 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.degree. 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.degree.
C. to a martensite transformation-starting point (approximately
410.degree. C.) reached 50.degree. 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.
[0120] 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.
[0121] 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.
[0122] 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.degree. C./second or faster until the martensite
transformation-starting point (approximately 410.degree. 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.
[0123] 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.
[0124] 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
[0125] 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 Ac.sub.3 temperature of the base material
was approximately 810.degree. C.
TABLE-US-00001 TABLE 1 Chemical composition of base material (unit:
mass %, remainder: Fe and impurities) C 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
[0126] 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 Ac.sub.3 temperature to 950.degree. 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.
[0127] 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 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.
[0128] 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.degree. C./second or faster until
approximately the martensite transformation-starting point
(410.degree. 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.degree. 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.degree. C. for 20
minutes. The average of the thicknesses of the coatings after the
electrodeposition coating was 10 .mu.m in all of invention examples
and comparative examples.
TABLE-US-00002 TABLE 2 Hot stamping conditions Plating treatment
conditions Total weight Alloying Alloying Holding Composition (mass
%) of Al and Zn temperature time time Al Zn Si (g/m.sup.2)
(.degree. C.) (seconds) (seconds) Invention 1 55 45 0 60 700 120 30
Example 2 55 45 0 60 700 120 30 3 55 45 0 60 700 120 30 4 55 45 0
60 700 120 30 5 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 9 60 40 0 60 500 300 30
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 15 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 Comparative 101 25 75 0 60 700 120 30
Example 102 75 25 0 60 700 120 30 103 55 45 0 60 800 120 45 104 55
45 0 60 400 120 30 105 55 45 0 60 700 500 30 106 55 45 0 60 500 5
15 107 55 45 0 60 800 60 120
[0129] The configurations of the invention examples and the
comparative examples obtained by the above-described means were
confirmed using a method described below.
[0130] 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 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.
[0131] 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% HCl 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.
[0132] 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.
TABLE-US-00003 TABLE 3 Average Interface layer Intermediate layer
Oxide Si in entire Total Average composition Average composition
layer plated weight of (mass %) Structure (mass %) Structure
Thickness Thickness layer Al and Zn Average Al Average Zn Judgment
Thickness Average Al Average Zn Judgment (.mu.m) (.mu.m) (mass %)
(g/m.sup.2) 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 5 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 8 15 3 OK 15 40 25 OK 30 2 0 77 9
15 3 OK 10 30 25 OK 20 2 0 57 10 15 3 OK 10 50 25 OK 20 2 0 57 11
15 3 OK 10 40 10 OK 20 2 0 57 12 15 3 OK 10 40 40 OK 20 2 0 57 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
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
[0133] 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.
[0134] 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.
[0135] 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/m.sup.2 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/m.sup.2 were evaluated as
being poor (BAD) in terms of the phosphatability.
[0136] 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.degree. C. for 500
hours. After the immersion, polyester tape was attached to the
entire surface of a 60 mm.times.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).times.100
[0137] Al represents the area (60 mm.times.120 mm=7,200 mm.sup.2)
of the test region, and A2 represents the area (mm.sup.2) 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.
[0138] 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 m.OMEGA. or less were evaluated as being favorable
(GOOD) in terms of the weldability.
[0139] 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.
TABLE-US-00004 TABLE 4 Fatigue Coating properties Phosphatability
adhesion Weldability 1 GOOD GOOD GOOD GOOD 2 GOOD GOOD GOOD GOOD 3
GOOD GOOD GOOD GOOD 4 GOOD GOOD GOOD GOOD 5 GOOD GOOD GOOD GOOD 6
GOOD GOOD GOOD GOOD 7 GOOD GOOD GOOD GOOD 8 GOOD GOOD GOOD GOOD 9
GOOD GOOD GOOD GOOD 10 GOOD GOOD GOOD GOOD 11 GOOD GOOD GOOD GOOD
12 GOOD GOOD GOOD GOOD 13 GOOD GOOD GOOD GOOD 14 GOOD GOOD GOOD
GOOD 15 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
EVALUATION RESULTS
[0140] 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.
[0141] 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.
[0142] 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.
[0143] 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.
[0144] 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.
[0145] 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.
[0146] 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.
[0147] 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.
[0148] 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
[0149] 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
[0150] 1 HOT STAMPED STEEL
[0151] 10 BASE MATERIAL
[0152] 20 PLATED LAYER
[0153] 21 INTERFACE LAYER
[0154] 22 INTERMEDIATE LAYER
[0155] 23 OXIDE LAYER
[0156] 30 Al-Zn-BASED PLATED LAYER
[0157] 40 Zn-BASED PLATED LAYER
[0158] 50 Al-BASED PLATED LAYER
[0159] 60 CHEMICAL CONVERSION CRYSTAL
[0160] 70 TRANSPARENT REGION
* * * * *