FIELD
[0001]
The present invention relates to a hot stamped body.
10 BACKGROUND
[0002]
As a technique for press-forming a material which is difficult to shape, such as high
strength steel sheet, hot stamping (hot pressing) is known. Hot stamping is a hot shaping
technique which shapes a material supplied for shaping after heating it. In this technique, the
15 material is shaped after heating, therefore at the time of shaping, the steel material is soft and has
good shapeability. Therefore, even a high strength steel material can be precisely formed into a
complicated shape. Further, the press die simultaneously performs the shaping and hardening,
therefore it is known that after shaping, the steel material has sufficient strength.
[0003]
20 PTL 1 describes a plated steel sheet for hot pressing characterized by having an Al-Znbased
alloy plating layer containing Al: 20 to 95 mass%, Ca+Mg: 0.01 to 10 mass%, and Si on
the steel sheet surface. Further, PTL 1 describes that such a plated steel sheet can prevent the
plating from adhering to the die at the time of hot pressing, since oxides of Ca or Mg are formed
on the surface of the Al-Zn-based alloy plating layer.
25 [0004]
In relation to an Al-Zn-based alloy plating, PTL 2 describes an alloy plated steel material
characterized by containing, by mass%, Al: 2 to 75%, Fe: 2 to 75%, and a balance of 2% or more
of Zn and unavoidable impurities in the plating layer. Further, PTL 2 teaches that, from the
viewpoint of improvement of the corrosion resistance, it is effective to further include Mg: 0.02
30 to 10%, Ca: 0.01 to 2%, Si: 0.02 to 3%, etc., in the plating layer.
[0005]
Further, in relation to an Al-Zn-based alloy plating, PTL 3 describes a plated steel material
comprising a steel material and a plating layer arranged on the surface of the steel material and
containing a Zn-Al-Mg alloy layer, wherein the Zn-Al-Mg alloy layer has a Zn phase, the Zn
35 phase contains an Mg-Sn intermetallic compound phase, and the plating layer contains, by
mass%, Zn: more than 65.0%, Al: more than 5.0% to less than 25%, Mg: more than 3.0% to less
2
than 12.5%, Ca: 0% to 3.00%, Si: 0% to less than 2.5%, etc.
[0006]
Similarly, PTL 4 describes a plated steel material comprising a steel material and a plating
layer arranged on a surface of the steel material and containing a Zn-Al-Mg alloy layer, wherein,
in a cross-section of the Zn-Al-Mg alloy layer, 5 an area ratio of an MgZn2 phase is 45 to 75%, an
area ratio of a total of the MgZn2 phase and Al phase is 70% or more, an area ratio of a Zn-Al-
MgZn2 ternary eutectic structure is 0 to 5%, and the plating layer contains, by mass%, Zn: more
than 44.90% to less than 79.90%, Al: more than 15% to less than 35%, Mg: more than 5% to less
than 20%, Ca: 0.1% to less than 3.0%, Si: 0% to 1.0%, etc.
10
[CITATIONS LIST]
[PATENT LITERATURE]
[0007]
[PTL 1] Japanese Unexamined Patent Publication No. 2012-112010
15 [PTL 2] Japanese Unexamined Patent Publication No. 2009-120948
[PTL 3] WO 2018/139619
[PTL 4] WO 2018/139620
SUMMARY
20 [TECHNICAL PROBLEM]
[0008]
If, for example, using a Zn-based plated steel material in hot stamping, the material is
worked in a state where the Zn is molten, therefore the molten Zn will sometimes penetrate into
the steel and cause cracking inside the steel material. Such a phenomenon is called “liquid metal
25 embrittlement (LME)”. It is known that the fatigue characteristics of a steel material fall due to
the LME.
[0009]
On the other hand, if using a plated steel material containing Al as a constituent of the
plating layer in hot stamping, it is known that, for example, the hydrogen generated at the time of
30 heating in the hot stamping will sometimes penetrate the steel material and cause hydrogen
embrittlement cracking.
[0010]
However, in conventional Al-Zn-based plated steel materials used in hot stamping, there
has not necessarily been sufficient study from the viewpoint of suppressing LME and hydrogen
35 embrittlement cracking. As a result, in a hot stamped body obtained from such a plated steel
material, there was still room for improvement relating to the LME resistance and hydrogen
3
penetration resistance.
[0011]
Further, it is known that the Si and other elements for which addition to the plating layer is
taught in PTL 2 are extremely easily oxidized and that, in particular if the content becomes
greater, oxides are formed on the sur 5 face of the steel material and sometimes obstruct adhesion
of the chemically converted film. In such a case, sometimes a drop is caused in the corrosion
resistance after coating the plated steel material.
[0012]
Therefore, an object of the present invention is to provide a hot stamped body improved in
10 the LME resistance and hydrogen penetration resistance and, further, excellent in the chemical
convertibility.
[SOLUTION TO PROBLEM]
[0013]
15 The present invention to achieve the above object is as follows:
(1) A hot stamped body comprising a steel base material and a plating layer formed on a
surface of the steel base material, wherein the plating layer has a chemical composition
comprising, by mass%,
Al: 15.00 to 55.00%,
20 Mg: 4.50 to 12.00%,
Si: 0.05 to 3.00%,
Ca: 0.05 to 3.00%,
Fe: 20.00 to 65.00%,
Sb: 0 to 0.50%,
25 Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
Ti: 0 to 1.00%,
Sr: 0 to 0.50%,
30 Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Mn: 0 to 1.00%, and
balance: Zn and impurities,
the plating layer comprises an interfacial layer positioned at an interface with the steel base
35 material and containing Fe and Al and a main layer positioned on the interfacial layer,
the main layer comprises, by area ratio, 10.0 to 90.0% of an Mg-Zn containing phase, 5.0 to
4
less than 30.0% of an Fe-Al containing phase, and 2.0 to 25.0% of an Al-Si containing oxide
phase,
the Mg-Zn containing phase comprises at least one selected from the group consisting of an
MgZn phase, Mg2 Zn3 phase, and MgZn2 phase, and
the Fe-Al containing phase comprises at least one 5 of an FeAl phase and Fe-Al-Zn phase.
(2) The hot stamped body according to the above (1), wherein the chemical composition
of the plating layer comprises, by mass%,
Al: 25.00 to 35.00% and
Mg: 6.00 to 10.00%.
10 (3) The hot stamped body according to the above (1) or (2), wherein the Mg-Zn containing
phase comprises an MgZn phase, and an area ratio of the MgZn phase in the main layer is 30.0%
or more.
(4) The hot stamped body according to any one of the above (1) to (3), wherein the Mg-Zn
containing phase comprises an MgZn phase and Mg2 Zn3 phase, and an area ratio of a total of
15 the MgZn phase and Mg2 Zn3 phase in the main layer is 40.0 to 85.0%.
(5) The hot stamped body according to any one of the above (1) to (4), wherein the Fe-Al
containing phase comprises an FeAl phase and an area ratio of the FeAl phase in the main layer
is 5.0 to 20.0%.
20 [ADVANTAGEOUS EFFECTS OF INVENTION]
[0014]
According to the present invention, it is possible to provide a hot stamped body improved in
the LME resistance and hydrogen penetration resistance and, further, excellent in the chemical
convertibilityBRIEF DESCRIPTION OF DRAWINGS
25 [0015]
FIG. 1 shows a backscattered electron image (BSE image) of a scanning electron
microscope (SEM) of a plating layer cross-section in a conventional hot stamped body including
an Al-Zn-Mg-based plating layer.
FIG. 2 shows a backscattered electron image (BSE image) of a scanning electron
30 microscope (SEM) of a plating layer cross-section in a hot stamped body according to the
present invention.
FIG. 3 shows a backscattered electron image (BSE image) of a scanning electron
microscope (SEM) of a plating layer cross-section before hot stamping in a hot stamped body
according to the present invention.
35 FIG. 4 is a graph showing a relationship between a point of change of a cooling speed when
cooling a plating layer and formation of an acicular Al-Zn-Si-Ca phase.
5
DESCRIPTION OF EMBODIMENTS
[0016]

The hot stamped body 5 according to an embodiment of the present invention comprises a
steel base material and a plating layer formed on a surface of the steel base material, wherein the
plating layer has a chemical composition comprising, by mass%,
Al: 15.00 to 55.00%,
Mg: 4.50 to 12.00%,
10 Si: 0.05 to 3.00%,
Ca: 0.05 to 3.00%,
Fe: 20.00 to 65.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
15 Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
Ti: 0 to 1.00%,
Sr: 0 to 0.50%,
Cr: 0 to 1.00%,
20 Ni: 0 to 1.00%,
Mn: 0 to 1.00%, and
balance: Zn and impurities,
the plating layer comprises an interfacial layer positioned at an interface with the steel base
material and containing Fe and Al and a main layer positioned on the interfacial layer,
25 the main layer comprises, by area ratio, 10.0 to 90.0% of an Mg-Zn containing phase, 5.0 to
less than 30.0% of an Fe-Al containing phase, and 2.0 to 25.0% of an Al-Si containing oxide
phase,
the Mg-Zn containing phase comprises at least one selected from the group consisting of an
MgZn phase, Mg2 Zn3 phase, and MgZn2 phase, and
30 the Fe-Al containing phase comprises at least one of an FeAl phase and Fe-Al-Zn phase.
[0017]
For example, if using a conventional Zn-based plated steel material or an Al-Zn-based
plated steel material for hot stamping, in general the plated steel material will be heated in the
hot stamping to about 900C or a higher temperature than that. Zn has a boiling point of about
35 907C, which is relatively low, therefore at such a high temperature, the Zn in the plating layer
will evaporate or melt, resulting in the partial formation of a high concentration Zn liquid phase
6
in the plating layer and the penetration of the liquid Zn into the crystal grain boundaries in the
steel in some cases causing liquid metal embrittlement (LME) cracking.
[0018]
On the other hand, in a conventional Al plated steel material not containing Zn, LME
cracking due to Zn will not occur, but at the tim 5 e of heating in the hot stamping, the water vapor
in the atmosphere will sometimes be reduced by the Al in the plating layer, resulting in the
generation of hydrogen. As a result, the generated hydrogen will sometimes penetrate the steel
material and cause hydrogen embrittlement cracking. Further, in an Al-Zn-based plated steel
material as well, since Zn has a relatively low boiling point as explained above, at the time of hot
10 stamping at a 900C or higher temperature, a part of the Zn will evaporate and sometimes will
react with the water vapor in the atmosphere and cause the generation of hydrogen. In such a
case, hydrogen embrittlement cracking is liable to occur due to hydrogen penetrating the steel
material due to not only the Al, but also the Zn. In addition, from the viewpoint of improvement
of the corrosion resistance, regarding the Mg and other elements which are added to the Zn15
based plated steel material or Al-Zn-based plated steel material, sometime parts thereof will
evaporate at the time of heating in hot stamping at a high temperature and, in the same way as
the case of Zn, cause production of hydrogen triggering hydrogen embrittlement cracking.
[0019]
Therefore, the inventors studied the LME resistance and hydrogen penetration resistance in
20 hot stamped bodies which include Al-Zn-Mg-based plating layers. As a result, the inventors
discovered that in a hot stamped body comprising an Al-Zn-Mg-based plating layer having a
predetermined chemical composition and containing a predetermined amount of an Mg-Zn
containing phase in the plating layer after hot stamping, it is possible to remarkably reduce or
suppress LME and penetration of hydrogen into the steel material due to the heating in the hot
25 stamping. In addition, the inventors discovered that by limiting the amount of the Fe-Al
containing phase contained in the plating layer to within a predetermined range, the hydrogen
penetration resistance of the hot stamped body is further improved. Furthermore, the inventors
discovered that the chemical convertibility of a hot stamped body is remarkably improved by the
Al-Si containing oxide phase formed in the plating layer, in particular the clumps of the Al-Si
30 containing oxide phase, when preparing a plating layer having the Mg-Zn containing phase and
Fe-Al containing phase. Below, this will be explained more specifically while referring to the
drawings.
[0020]
FIG. 1 shows a backscattered electron image (BSE image) of a scanning electron
35 microscope (SEM) of a plating layer cross-section in a conventional hot stamped body
containing an Al-Zn-Mg-based plating layer. Referring to FIG. 1, it will be understood that the
7
plating layer 1 contains a thick oxide layer 2 containing Zn and Mg. The oxide layer 2 is
believed to be the result of at least part of the Zn and Mg evaporating due to heating at about
900C in the hot stamping or a higher temperature than that depositing on the surface of the
plating layer as oxides. On the other hand, a diffusion layer 3 is positioned below the plating
layer 1. The diffusion layer 3 f 5 orms part of the steel base material 4. The diffusion layer 3 results
from the Al constituent in the plating layer diffusing into the steel base material 4 and forming a
solid solution due to the heating in the hot stamping.
[0021]
In a conventional hot stamped body containing an Al-Zn-Mg-based plating layer such as
10 shown in FIG. 1, the Zn and Mg evaporate during the heating in the hot stamping and these react
with the water vapor in the atmosphere and cause the generation of hydrogen, therefore LME
and hydrogen penetration into the steel material occur. In addition, for example, LME cracking is
liable to be caused even when the concentration of Zn in the plating layer 1 relatively rises due to
evaporation of Mg.
15 [0022]
FIG. 2 shows a backscattered electron image (BSE image) of a scanning electron
microscope (SEM) of a plating layer cross-section in a hot stamped body according to the
present invention. Referring to FIG. 2, the plating layer 1 comprises an interfacial layer 5
positioned at the interface with the steel base material 4, more specifically at the interface with
20 the diffusion layer 3 forming part of the steel base material 4, and containing Fe and Al and a
main layer 6 positioned on the interfacial layer 5. The interfacial layer 5, in normal hot stamping,
is formed at the interface with the steel base material and is mainly comprised of intermetallic
compounds containing Fe and Al. The interfacial layer 5 and the diffusion layer 3 positioned
beneath it are almost no different in chemical composition since the metal elements of the layers
25 diffuse into each other due to the relatively long heat treatment in the hot stamping for example.
Therefore, in the present invention, the interfacial layer 5 and the diffusion layer 3 will
sometimes not particularly be differentiated and the two together will sometimes be expressed as
the “Fe-Al layer 7”.
[0023]
30 Further, it will be understood that the main layer 6, in contrast to the case of FIG. 1,
contains an Mg-Zn containing phase 8 containing at least one selected from the group consisting
of an MgZn phase, Mg2 Zn3 phase, and MgZn2 phase, and an Fe-Al containing phase 9
comprising an FeAl phase 9a. While not shown in FIG. 2, the Fe-Al containing phase 9
sometimes includes, in addition to the FeAl phase 9a, a relatively small amount of an Fe-Al-Zn
35 phase. In particular, it will be understood that the main layer 6 shown in FIG. 2 has a structure
(island-in-sea structure) of a matrix phase of an Mg-Zn containing phase 8 in which islands of
8
the Fe-Al containing phase 9 (islands of Fe-Al phase 9a and islands of Fe-Al-Zn phase) are
present, in particular are present dispersed. In the hot stamped body according to the present
invention, by including an Mg-Zn containing phase 8 such as shown in FIG. 2 in the main layer
6 of the plating layer 1 in a relatively large amount, it is possible to remarkably reduce or
suppress occurrence of LME and penetration of 5 hydrogen into the steel material. In addition, by
controlling the amount of the Fe-Al containing phase contained in the main layer 6 to within a
predetermined range, it is possible to further improve the hydrogen penetration resistance of the
hot stamped body.
[0024]
10 While not intending to be bound by any specific theory, in the hot stamped body according
to the present invention, as explained in detail later in relation to the method of production, at the
start of heating in the hot stamping, it is believed that the Ca leached out from the acicular Al-
Zn-Si-Ca phase present in the surface structure of the plating layer is preferentially oxidized by
the oxygen in the atmosphere and forms a dense Ca-based oxide film at the surface-most part of
15 the plating layer. In other words, it is believed that the acicular Al-Zn-Si-Ca phase present in the
surface structure of the plating layer before hot stamping functions as a supply source of Ca for
forming a Ca-based oxide film at the start of heating in hot stamping, then the Ca-based oxide
film obtained by the oxidation of Ca supplied, more specifically a Ca- and Mg-containing oxide
film, functions as a barrier layer.
20 [0025]
Due to the function of such a barrier layer, it is believed that evaporation of Zn and Mg in
the plating layer to the outside and the related occurrence of LME and the penetration of
hydrogen from the outside can be decreased or suppressed. As a result, it is believed that in the
body finally obtained after hot stamping, unlike the case of FIG. 1, Zn and Mg can be kept from
25 forming a thick oxide layer 2 in the plating layer 1, can be made present as an Mg-Zn containing
phase 8 in a relatively large amount, i.e., in an amount of 10.0 to 90.0% by area ratio in the main
layer 6.
[0026]
Further, the FeAl phase 9a contained in the Fe-Al containing phase 9, as shown in FIG. 2, is
30 present in a relatively large amount near the interface of the main layer 6 and the interfacial layer
5, while the Fe-Al-Zn phase (not shown) is present in a relatively large amount near the surface
of the main layer 6. However, if the content of the Fe-Al containing phase 9 in the main layer 6
becomes greater, only naturally, the amount present near the surface of the main layer 6 will
become greater not only at the Fe-Al-Zn phase, but also the Fe-Al containing phase 9 including
35 the FeAl phase 9a as a whole. In such a case, at the time of the heating in the hot stamping, the
water vapor in the atmosphere will be reduced by the Al in the Fe-Al containing phase 9 present
9
near the surface of the main layer 6 and hydrogen will be generated. As a result, sometimes the
generated hydrogen will penetrate the steel material and cause hydrogen embrittlement cracking.
In the hot stamped body according to the present invention, it is believed that by limiting the Fe-
Al containing phase 9 in the main layer 6 to within a predetermined range, i.e, to within an area
ratio of less than 30.0%, the amount of hydrogen generated due 5 to the Fe-Al containing phase 9
can be reduced. As a result, it is believed that, compared with simply controlling the amount of
the Mg-Zn containing phase in the plating layer, it becomes possible to further improve the
hydrogen penetration resistance of the hot stamped body.
[0027]
10 Furthermore, in the hot stamped body of the present invention, the main layer 6 includes, in
addition to the above Mg-Zn containing phase 8 and Fe-Al containing phase 9, as shown in FIG.
2, an Al-Si containing oxide phase 10, in particular clumps of the Al-Si containing oxide phase
10. In general, in zinc phosphate treatment or other chemical conversion, if Al is present near the
surface of the plated steel material, sometimes the aluminum ions leached out into the treatment
15 solution cause the zinc phosphate precipitation reaction at the plating layer surface to be
inhibited. While not intending to be bound by any specific theory, it is believed that in the hot
stamped body of the present invention, by forming an Al-Si containing oxide phase in the main
layer, i.e., by including metal Al in the main layer in the form of an oxide, it is possible to reduce
the amount of Al present near the surface of the main layer of the plating layer and, as a result,
20 possible to improve the chemical convertibility of the hot stamped body.
[0028]
In addition, by forming the Al-Si containing oxide phase in the main layer, the amount of
the Al present near the surface of the main layer is reduced, therefore it is believed possible to
also reduce the amount of hydrogen generated due to the Al near the surface. As a result,
25 according to the present invention, it is believed that by limiting the amount of the Fe-Al
containing phase contained in the main layer to within a predetermined range and further by
including an Al-Si containing oxide phase in the main layer, it becomes possible to remarkably
improve the hydrogen penetration resistance of the hot stamped body.
[0029]
30 Below, the hot stamped body according to an embodiment of the present invention will be
explained in detail. In the following explanation, the “%” relating to the contents of the
constituents means “mass%” unless otherwise indicated.
[0030]
[Steel Base Material]
35 The steel base material according to the embodiment of the present invention may be a
material having any thickness and composition. It is not particularly limited, but, for example, is
10
preferably a material having a thickness and composition suitable for application to hot
stamping. Such a steel base material is known, and may include, for example, a steel sheet
having a 0.3 to 2.3 mm thickness and comprising, by mass%, C: 0.05 to 0.40%, Si: 0.50% or
less, Mn: 0.50 to 2.50%, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.010%
or less, and a balance: Fe and impurities (for example, a 5 cold rolled steel sheet), etc. Below, the
constituents contained in the steel base material preferably applied in the present invention will
be explained in detail.
[0031]
[C: 0.05 to 0.40%]
10 Carbon (C) is an element effective for raising the strength of a hot stamped body. However,
if the C content is too great, the hot stamped body will sometimes fall in toughness. Therefore,
the C content is 0.05 to 0.40%. The C content is preferably 0.10% or more, more preferably
0.13% or more. The C content is preferably 0.35% or less.
[0032]
15 [Si: 0 to 0.50%]
Silicon (Si) is an element effective for deoxidizing steel. However, if the Si content is too
great, the Si in the steel diffuses at the time of heating in the hot stamping and forms oxides at
the steel material surface. As a result, the efficiency of phosphate treatment sometimes falls.
Further, Si is an element making the Ac3 point of the steel rise. For this reason, since the heating
20 temperature of the hot stamping has to be the Ac3 point or more, if the amount of Si becomes
excessive, the heating temperature of the hot stamping of the steel will inevitably become higher.
In other words, steel with a large amount of Si is heated to a higher temperature at the time of hot
stamping and, as a result, Zn, etc., in the plating layer will unavoidably evaporate. To avoid such
a situation, the Si content is 0.50% or less. The Si content is preferably 0.30% or less, more
25 preferably 0.20% or less. The Si content may also be 0%, but to obtain the effect of deoxidation,
etc., the lower limit value of the Si content, while changing depending on the desired deoxidation
level, is generally 0.05%.
[0033]
[Mn: 0.50 to 2.50%]
30 Manganese (Mn) raises the hardenability and raises the strength of the hot stamped body.
On the other hand, even if including Mn in excess, the effect become saturated. Therefore, the
Mn content is 0.50 to 2.50%. The Mn content is preferably 0.60% or more, more preferably
0.70% or more. The Mn content is preferably 2.40% or less, more preferably 2.30% or less.
[0034]
35 [P: 0.03% or Less]
Phosphorus (P) is an impurity contained in steel. P segregates at the crystal grain boundaries
11
to cause a drop in the toughness of the steel and causes a drop in the delayed fracture resistance.
Therefore, the P content is 0.03% or less. The P content is preferably as small as possible and is
preferably 0.02% or less. However, excessive reduction of the P content invites a rise in costs,
therefore the P content is preferably 0.0001% or more. The inclusion of P is not essential,
5 therefore the lower limit of the P content is 0%.
[0035]
[S: 0.010% or Less]
Sulfur (S) is an impurity contained in steel. S forms sulfides to cause a drop in the
toughness of the steel and cause a drop in the delayed fracture resistance. Therefore, the S
10 content is 0.010% or less. The S content is preferably as small as possible and is preferably
0.005% or less. However, excessive reduction of the S content invites a rise in costs, therefore
the S content is preferably 0.0001% or more. The inclusion of S is not essential, therefore the
lower limit of the S content is 0%.
[0036]
15 [sol. Al: 0 to 0.10%]
Aluminum (Al) is effective for deoxidation of steel. However, excessive inclusion of Al
causes the Ac3 point of the steel material to rise and accordingly the heating temperature of the
hot stamping becomes higher and Zn, etc., in the plating layer unavoidably evaporate. Therefore,
the Al content is 0.10% or less, preferably 0.05% or less. The Al content may also be 0%, but to
20 obtain the effect of deoxidation, etc., the Al content may be 0.01% or more. In this Description,
the Al content means the content of so-called acid-soluble Al (sol. Al).
[0037]
[N: 0.010% or Less]
Nitrogen (N) is an impurity unavoidably contained in steel. N forms nitrides to cause a drop
25 in the toughness of the steel. If boron (B) is further contained in the steel, N bonds with B to
cause a reduction in the amount of B in solid solution and cause a drop in the hardenability.
Therefore, the N content is 0.010% or less. The N content is preferably as small as possible and
is preferably 0.005% or less. However, excessive reduction of the N content invites a rise in
costs, therefore the N content is preferably 0.0001% or more. The inclusion of N is not essential,
30 therefore the lower limit of the N content is 0%.
[0038]
The basic chemical composition of the steel base material suitable for use in the
embodiment according to the present invention is as explained above. Further, the above steel
base material may optionally contain one or more of B: 0 to 0.005%, Ti: 0 to 0.10%, Cr: 0 to
35 0.50%, Mo: 0 to 0.50%, Nb: 0 to 0.10%, and Ni: 0 to 1.00%. Below, these elements will be
explained in detail. The inclusion of these element is not essential, therefore the lower limits of
12
the contents of the elements are 0%.
[0039]
[B: 0 to 0.005%]
Boron (B) raises the hardenability of steel and raises the strength of the steel material after
hot stamping, therefore may be included in the 5 steel base material. However, even if including B
in excess, the effect becomes saturated. Therefore, the B content is 0 to 0.005%. The B content
may also be 0.0001% or more.
[0040]
[Ti: 0 to 0.10%]
10 Titanium (Ti) can bond with nitrogen (N) to form nitrides and keep the hardenability from
dropping due to formation of BN. Further, due to the pinning effect, Ti can refine the austenite
grain size and raise the toughness, etc., of the steel material at the time of heating in hot
stamping. However, even if including Ti in excess, the effect becomes saturated. Further, if Ti
nitrides precipitate in excess, sometimes the toughness of the steel will fall. Therefore, the Ti
15 content is 0 to 0.10%. The Ti content may be 0.01% or more.
[0041]
[Cr: 0 to 0.50%]
Chromium (Cr) is effective for raising the hardenability of steel and raising the strength of
the hot stamped body. However, if the Cr content is excessive and a large amount of Cr carbides
20 which are difficult to melt at the time of heating in hot stamping are formed, it becomes difficult
for the steel to transform to austenite, and conversely the hardenability falls. Therefore, the Cr
content is 0 to 0.50%. The Cr content may also be 0.10% or more.
[0042]
[Mo: 0 to 0.50%]
25 Molybdenum (Mo) raises the hardenability of steel. However, even if including Mo in
excess, the above effect becomes saturated. Therefore, the Mo content is 0 to 0.50%. The Mo
content may also be 0.05% or more.
[0043]
[Nb: 0 to 0.10%]
30 Niobium (Nb) is an element which forms carbides to refine the crystal grains at the time of
hot stamping and raise the toughness of the steel. However, if including Nb in excess, the above
effect becomes saturated and further the hardenability falls. Therefore, the Nb content is 0 to
0.10%. The Nb content may also be 0.02% or more.
[0044]
35 [Ni: 0 to 1.00%]
Nickel (Ni) is an element able to suppress embrittlement caused by molten Zn at the time of
13
the heating in the hot stamping. However, even if including Ni in excess, the effect becomes
saturated. Therefore, the Ni content is 0 to 1.00%. The Ni content may also 0.10% or more.
[0045]
In the steel base material according to the embodiment of the present invention, the balance
other than the above constituents i 5 s comprised of Fe and impurities. The “impurities” in the steel
base material mean constituents entering due to various factors in the production process, first
and foremost the raw materials such as the ore and scrap, when industrially producing the hot
stamped body according to the embodiment of the present invention, and not intentionally added
to the hot stamped body.
10 [0046]
[Plating Layer]
According to the embodiment of the present invention, a plating layer is formed on the
surface of the above steel base material. For example, if the steel base material is a steel sheet,
the plating layer is formed on at least one surface of the steel sheet, i.e., one surface or both
15 surfaces of the steel sheet. The plating layer comprises an interfacial layer positioned at the
interface with the steel base material and containing Fe and Al and a main layer positioned on
the interfacial layer. The plating layer has the following average composition.
[0047]
[Al: 15.00 to 55.00%]
20 Al is an element essential for suppressing the evaporation of the Zn and Mg at the time of
the heating in the hot stamping. As explained above, it is believed that due to the presence of the
acicular Al-Zn-Si-Ca phase in the surface structure of the plating layer before the hot stamping,
the Ca leaching out from the acicular Al-Zn-Si-Ca phase at the start of the heating in the hot
stamping is preferentially oxidized by the oxygen in the atmosphere and a dense Ca-based oxide
25 film, more specifically a Ca- and Mg-containing oxide film, is formed on the outermost surface
of the plating layer. Such a Ca-based oxide film is believed to function as a barrier layer for
suppressing evaporation of the Zn and Mg. To express the function of the barrier layer, the
content of Al in the plating layer after hot stamping has to be 15.00% or more, preferably is
20.00% or more or 25.00% or more. On the other hand, if the Al content is more than 55.00%,
30 Al4 Ca and other intermetallic compounds are preferentially formed at the plating layer before
the hot stamping and formation of the acicular Al-Zn-Si-Ca phase in a sufficient amount
becomes difficult. Therefore, the Al content is 55.00% or less, preferably 45.00% or less or
35.00% or less.
[0048]
35 [Mg: 4.50 to 12.00%]
Mg is an element effective for improving the corrosion resistance of the plating layer and
14
improving the coating blistering, etc. Further, Mg has the effect of forming liquid phase Zn-Mg
and suppressing LME cracking at the time of heating in the hot stamping. If the Mg content is
low, the possibility of LME occurring increases. From the viewpoint of improvement of the
corrosion resistance and suppression of the LME, the Mg content is 4.50% or more, preferably is
5.00% or more, 5.50% 5 or more, or 6.00% or more. On the other hand, if the Mg content is too
high, an excessive sacrificial corrosion action tends to cause coating blistering and flow rust to
rapidly become larger. Therefore, the Mg content is 12.00% or less, preferably 10.00% or less.
[0049]
[Si: 0.05 to 3.00%]
10 Si is an element essential for suppressing evaporation of Zn and Mg at the time of the
heating in the hot stamping. As explained above, due to the presence of the acicular Al-Zn-Si-Ca
phase in the surface structure of the plating layer before the hot stamping, it is possible to form a
barrier layer comprised of a Ca-based oxide film for suppressing evaporation of Zn and Mg at
the time of heating in the hot stamping. To express the function of the barrier layer, the Si
15 content in the plating layer after hot stamping has to be 0.05% or more, preferably is 0.10% or
more or 0.40% or more. On the other hand, if the Si content is excessive, an Mg2 Si phase is
formed at the interface of the steel base material and the plating layer at the plating layer before
hot stamping and the corrosion resistance greatly deteriorates. Further, if the Si content is
excessive, the Mg2 Si phase is preferentially formed at the plating layer before the hot stamping
20 and it becomes difficult to make the acicular Al-Zn-Si-Ca phase form in a sufficient amount.
Therefore, the Si content is 3.00% or less, preferably 1.60% or less, more preferably 1.00% or
less.
[0050]
[Ca: 0.05 to 3.00%]
25 Ca is an element essential for suppressing evaporation of Zn and Mg at the time of heating
in the hot stamping. As explained above, due to the presence of the acicular Al-Zn-Si-Ca phase
in the surface structure of the plating layer before the hot stamping, it is possible to form a
barrier layer comprised of a Ca-based oxide film for suppressing evaporation of Zn and Mg at
the time of the heating in the hot stamping. To express the function of the barrier layer, the Ca
30 content in the plating layer after hot stamping has to be 0.05% or more, preferably is 0.40% or
more. On the other hand, if the Ca content is excessive, Al4 Ca and other intermetallic
compounds are preferentially formed at the plating layer before the hot stamping and it becomes
difficult to make the acicular Al-Zn-Si-Ca phase form in a sufficient amount. Therefore, the Ca
content is 3.00% or less, preferably 2.00% or less, more preferably 1.50% or less.
35 [0051]
[Fe: 20.00 to 65.00%]
15
If heating the plated steel material at the time of hot stamping, the Fe from the steel base
material diffuses in the plating layer, therefore the plating layer inevitably contains Fe. Fe bonds
with the Al in the plating layer to form at the interface with the steel base material an interfacial
layer mainly comprised of an intermetallic compound containing Fe and Al and further form an
Fe-Al containing phase in the main layer positi 5 oned on the interfacial layer. If the Fe content is
low, the amount of the Fe-Al containing phase decreases, therefore the structure of the main
layer easily collapses. More specifically, if the Fe content is low, the Zn and Mg contents
relatively increase, therefore at the time of the heating in the hot stamping, these elements easily
evaporate and as a result hydrogen penetration easily occurs. Therefore, the Fe content is 20.00%
10 or more, preferably 25.00% or more. On the other hand, the Fe content may be 65.00% or less,
55.00% or less, or 50.00% or less.
[0052]
The chemical composition of the plating layer is as explained above. Furthermore, the
plating layer may optionally contain one or more of Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to
15 1.00%, Sn: 0 to 1.00%, Ti: 0 to 1.00%, Sr: 0 to 0.50%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0
to 1.00%. While not particularly limited to this, from the viewpoint of causing the actions and
functions of the above basic constituents forming the plating layer to be sufficiently manifested,
the total content of these elements is preferably 5.00% or less, more preferably 2.00% or less.
Below, these elements will be explained in detail.
20 [0053]
[Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, and Ti: 0 to 1.00%]
Sb, Pb, Cu, Sn, and Ti can be contained in the MgZn2 phase present in the main layer, but
if within predetermined ranges of contents, do not detrimentally affect the performance of the hot
stamped body. However, if the contents of the elements are excessive, at the time of the heating
25 in the hot stamping, sometimes oxides of these elements will precipitate and cause deterioration
of the surface properties of the hot stamped body and the phosphate treatment will become poor
and the corrosion resistance after coating will deteriorate. Furthermore, if the Pb and Sn contents
become excessive, the LME resistance will tend to fall. Therefore, the contents of Sb and Pb are
0.50% or less, preferably 0.20% or less, while the contents of Cu, Sn, and Ti are 1.00% or less,
30 preferably 0.80% or less, more preferably 0.50% or less. On the other hand, the contents of
elements may also be 0.01% or more. These elements are not essential. The lower limits of the
contents of these elements are 0%.
[0054]
[Sr: 0 to 0.50%]
35 Sr can be included in the plating bath at the time of production of the plating layer so as to
suppress the formation of the top dross formed on the plating bath. Further, Sr tends to suppress
16
oxidation by air at the time of heating in hot stamping, therefore can suppress color changes in
the body after hot stamping. These effects are exhibited even in small amounts, therefore the Sr
content may be 0.01% or more. On the other hand, if the Sr content is excessive, the occurrence
of coating blistering and flow rust becomes larger and the corrosion resistance tends to
deteriorate. Therefore, the Sr content is 0.50% or 5 less, preferably 0.30% or less, more preferably
0.10% or less.
[0055]
[Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%]
Cr, Ni, and Mn concentrate near the interface of the plating layer and the steel base material
10 and have the effect of eliminating spangles of the plating layer surface, etc. To obtain such an
effect, the contents of Cr, Ni, and Mn are preferably respectively 0.01% or more. On the other
hand, these elements may be included in the interfacial layer or included in the Fe-Al containing
phase present in the main layer. However, if the contents of these elements are excessive, the
coating blistering and flow rust become greater and the corrosion resistance tends to deteriorate.
15 Therefore, the contents of Cr, Ni, and Mn are respectively 1.00% or less, preferably 0.50% or
less, more preferably 0.10% or less.
[0056]
[Balance: Zn and Impurities]
The balance in the plating layer aside from the above constituents is comprised of Zn and
20 impurities. Zn is an essential constituent in the plating layer from the viewpoint of preventing
corrosion. Zn is present mainly as the Mg-Zn containing phase in the main layer of the plating
layer and greatly contributes to improvement of the corrosion resistance. If the Zn content is less
than 3.00%, sometimes a sufficient corrosion resistance cannot be maintained. Therefore, the Zn
content is preferably 3.00% or more. The lower limit of the Zn content may be 10.00%, 15.00%,
25 or 20.00%. On the other hand, if the Zn content is too high, at the time of the heating in the hot
stamping, Zn easily evaporates and as a result LME and hydrogen penetration easily occur.
Therefore, the Zn content is preferably 50.00% or less. The upper limit of the Zn content may be
45.00%, 40.00%, or 35.00%. Further, Zn can be substituted by Al, therefore a small amount of
Zn can form a solid solution with the Fe in the Fe-Al containing phase. Further, the “impurities”
30 in the plating layer mean constituents entering due to various factors in the production process,
first and foremost the raw materials, when producing the plating layer, and not intentionally
added to the plating layer. In the plating layer, the impurities may contain elements other than
the elements explained above in trace amounts to an extent not detracting from the effect of the
present invention.
35 [0057]
The chemical composition of the plating layer is determined by dissolving the plating layer
17
in an acid solution to which an inhibitor is added for inhibiting corrosion of the steel base
material and measuring the obtained solution by the ICP (high frequency inductively coupled
plasma) emission spectrometry method. In this case, the measured chemical composition is the
average composition of the total of the main layer and the interfacial layer.
5 [0058]
The thickness of the plating layer may be, for example, 3 to 50 m. Further, if the steel base
material is a steel sheet, the plating layer may be provided at both surfaces of the steel sheet or
may be provided at only one surface. The amount of deposition of the plating layer is not
particularly limited, but for example may be 10 to 170 g/m2 per surface. The lower limit may be
20 or 30 g/m2 and the upper limit may be 150 or 130 g/m2 10 . In the present invention, the amount
of deposition of the plating layer is determined from the change in weight before and after acid
washing by dissolving the plating layer in an acidic solution to which an inhibitor for inhibiting
corrosion of the base iron has been added.
[0059]
15 [Interfacial Layer]
The interfacial layer is a layer containing Fe and Al, more specifically a layer at which, at
the time of the heating in the hot stamping, the Fe from the steel base material diffuses in the
plating layer and bonds with the Al in the plating layer and is mainly comprised of an
intermetallic compound containing Fe and Al.
20 [0060]
[Main Layer]
The main layer includes an area ratio of 10.0 to 90.0% of an Mg-Zn containing phase and
5.0 to 30.0% of an Fe-Al containing phase, and 2.0 to 25.0% of an Al-Si containing oxide phase.
The main layer has the effect of inhibiting the formation of scale at the time of hot stamping and
25 contributes to corrosion resistance of the hot stamped body as well. The main layer has a mixed
structure of an Mg-Zn containing phase and Fe-Al containing phase and generally, as shown in
FIG. 2, has the structure (island-in-sea structure) of a matrix phase of an Mg-Zn containing phase
8 in which a relatively amount of islands of Fe-Al containing phase 9 are present. If referring to
FIG. 2, in addition to the islands of the Fe-Al containing phase 9, a relatively small amount of
30 Al-Si containing oxide phase 10, in particular clumps of the Al-Si containing phase 10, are
present in the Mg-Zn containing phase 8.
[0061]
[Mg-Zn Containing Phase]
In an embodiment according to the present invention, by configuring the plating layer after
35 hot stamping so that Zn and Mg are present as an Mg-Zn containing phase in the main layer in an
area ratio of an amount of 10.0 to 90.0%, occurrence of LME and hydrogen penetration to the
18
steel material due to the heating at the time of hot stamping can be remarkably reduced or
suppressed. If the area ratio of the Mg-Zn containing phase is less than 10.0%, it is not possible
to sufficiently obtain such an effect. Therefore, the area ratio of the Mg-Zn containing phase is
10.0% or more, preferably 15.0% or more, more preferably 25.0% or more. On the other hand,
the area ratio of the Mg-Zn containing phase may 5 be 90.0% or less, for example, may be 85.0%
or less, 80.0% or less, or 75.0% or less.
[0062]
The Mg-Zn containing phase includes at least one phase selected from the group consisting
of an MgZn phase, Mg2 Zn3 phase, and MgZn2 phase. Here, the MgZn phase, Mg2 Zn3 phase,
10 and MgZn2 phase are intermetallic compounds, therefore while the atomic ratios of Mg and Zn
of the phases may be considered to be substantially constant, in actuality they fluctuate
somewhat since sometimes Al, Fe, etc., dissolve partially. Therefore, in the present invention, in
phases having a chemical composition in which the total of the Mg and Zn contents is 90.0% or
more, a phase where the atomic ratio of Mg/Zn is 0.90 to 1.10 is defined as an MgZn phase, a
15 phase where an atomic ratio of Mg/Zn is 0.58 to 0.74 is defined as an Mg2 Zn3 phase, and a
phase where an atomic ratio of Mg/Zn is 0.43 to 0.57 is defined as an MgZn2 phase. In
particular, when the Mg-Zn containing phase includes an MgZn phase and/or Mg2 Zn3 phase, it
is possible to suppress LME at the time of hot stamping. To reliably obtain such an effect, the
Mg-Zn containing phase preferably includes an MgZn phase with a large Mg content. The area
20 ratio of the MgZn phase in the main layer is preferably 5.0% or more and 10.0% or more is more
preferable. 30.0% or more or 40.0% or more is also possible. Further, the Mg-Zn containing
phase preferably includes an MgZn phase and Mg2 Zn3 phase. The area ratio of the total of the
MgZn phase and Mg2 Zn3 phase in the main layer is preferably 10.0% or more or 25.0% or
more. 40.0% or more or 50.0% or more is also possible. On the other hand, it may be 85.0% or
25 less, 80.0% or less, 75.0% or less, or 70.0% or less.
[0063]
[Fe-Al Containing Phase]
As explained above, the main layer includes an area ratio of 5.0 to less than 30.0% of an Fe-
Al containing phase. The Fe-Al containing phase becomes a barrier at the time corrosion
30 progresses in the Mg-Zn containing phase, therefore by establishing the presence of the Fe-Al
containing phase, the corrosion resistance can be improved. Explaining this in more detail, the
Fe-Al containing phase (Fe-Al-Zn phase and FeAl phase) is present in the main layer not as a
laminar structure, but as island structures, therefore if corrosion progresses in the Mg-Zn
containing phase having the corrosion resistance improving effect, the corrosion will proceed in
35 a spotted state avoiding these islands of the Fe-Al containing phase. As a result, it is believed
possible to delay progress of corrosion of the Mg-Zn containing phase.
19
[0064]
The Fe-Al containing phase includes at least one of the Fe-Al-Zn phase and FeAl phase. In
the present invention, the Fe-Al containing phase means a phase having a chemical composition
where the total of Fe, Al, and Zn is 90.0% or more. In the Fe-Al containing phase having such a
chemical composition, a phase where the Zn 5 content is 1.0% or more is defined as an Fe-Al-Zn
phase and a phase where the Zn content is less than 1.0% is defined as an FeAl phase. While not
intending to be bound by any specific theory, it is believed that the Fe-Al-Zn phase and FeAl
phase do not grow at the interface of the plating layer and the steel base material from the steel
base material to inside the plating layer in a layer shape, but form nuclei of spherical shapes in
10 the plating layer in the molten state at the time of the heating in the hot stamping and then grow
into island shapes.
[0065]
As explained in detail later, by suitably controlling the production conditions of the plated
steel material before hot stamping, it is possible to establish the presence of the acicular Al-Zn-
15 Si-Ca phase dispersed in the surface structure of the plating layer. As a result, it is possible to
suppress the evaporation of Zn and Mg at the time of the heating in the hot stamping. By
suppressing the evaporation of Zn and Mg, it is believed that nuclei are formed inside the main
layer in the molten state and the Fe-Al containing phase grows to island shapes. As explained
above, the Fe-Al containing phase, in particular the Fe-Al-Zn phase and FeAl phase, has island
20 shapes. While not particularly limited, the aspect ratio almost never is more than 5.0. In general,
the Fe-Al containing phase has island shapes of an aspect ratio of 5.0 or less, for example, 4.0 or
less or 3.0 or less. The lower limit of the aspect ratio is not particularly prescribed, but, for
example, may be 1.0 or more, 1.2 or more, or 1.5 or more. In the present invention, the “aspect
ratio” means the ratio of the longest axis of the Fe-Al containing phase (Fe-Al-Zn phase and
25 FeAl phase) (long axis) and the longest axis in the axes of the Fe-Al containing phase
perpendicular to the same (short axis).
[0066]
As explained above, the FeAl phase contained in the Fe-Al containing phase as shown in
FIG. 2 is present in a relatively large amount near the interface of the plating layer and Fe-Al
30 layer, while the Fe-Al-Zn phase is present in a relatively large amount near the surface of the
plating layer. However, if the content of the Fe-Al containing phase in the plating layer
increases, only naturally the amount of not only the Fe-Al-Zn phase, but also Fe-Al containing
phase as a whole including the FeAl phase, present near the surface of the plating layer will
become greater. In such a case, at the time of the heating in the hot stamping, water vapor in the
35 atmosphere is reduced by the Al in the Fe-Al-Zn phase and hydrogen is generated. Therefore, in
an embodiment according to the present invention, by limiting the Fe-Al containing phase in the
20
main layer to an area ratio of less than 30.0% in range, it is possible to reduce the amount of
hydrogen generated due to the Fe-Al containing phase. As a result, compared with when just
controlling the amount of the Mg-Zn containing phase in the main layer, it becomes possible to
further improve the hydrogen penetration resistance of the hot stamped body.
5 [0067]
Further, by suitably adjusting the heat treatment in the hot stamping, it is possible to control
the contents of the Fe-Al-Zn phase and FeAl phase in the main layer. In an embodiment
according to the present invention, the area ratio of the Fe-Al-Zn phase in the main layer is
preferably 5.0% or less, more preferably 3.0% or less, most preferably 2.0% or less and may also
10 be 0%. Further, in an embodiment according to the present invention, the area ratio of the FeAl
phase in the main layer may for example, be 5.0% or more, 6.0% or more, or 8.0% or more and
may be less than 30.0%, 20.0% or less, or 15.0% or less.
[0068]
[Al-Si Containing Oxide Phase]
15 As explained above, the main layer contains an area ratio of 2.0 to 25.0% of an Al-Si
containing oxide phase. By forming the Al-Si containing oxide phase in the main layer, i.e.,
including metal Al in the main layer in the form of oxides, it is possible to reduce the amount of
Al present near the surface of the main layer of the plating layer. As a result, it is possible to
improve the chemical convertibility of the hot stamped body. Furthermore, by reducing the
20 amount of Al present near the surface of the main layer, it is possible to also reduce the amount
of hydrogen generated due to the Al near the surface. As a result, it becomes possible to further
improve the hydrogen penetration resistance of the hot stamped body. To obtain these effects,
the area ratio of the Al-Si containing oxide phase has to be 2.0% or more and is preferably 3.0%
or more, more preferably 4.0% or more.
25 [0069]
On the other hand, if the area ratio of the Al-Si containing phase is more than 25.0%, along
with the decrease in the Al constituent in the main layer, sometimes the corrosion resistance of
the hot stamped body falls. Therefore, the area ratio of the Al-Si containing oxide is 25.0% or
less. For example, it may also be 20.0% or less or 15.0% or less. Further, the Al-Si containing
30 oxide phase is for example 1.0 m or more, 2.0 m or more, or 3.0 m or more and 15.0 m or
less, 12.0 m or less, or 10.0 m or less in particle size. In the present invention, the “particle
size of the Al-Si containing oxide phase” means the circle equivalent diameter found by image
analysis using a scanning electron microscope (SEM) observed image and electron backscatter
diffraction analysis (EBSP or EBSD).
35 [0070]
[Other Intermetallic Compounds]
21
The main layer may contain other intermetallic compounds besides those contained in the
Mg-Zn containing phase, Fe-Al containing phase, and Al-Si containing phase. The other
intermetallic compounds are not particularly limited, but, for example, intermetallic compounds
containing Si and Ca or other elements contained in the plating layer, specifically Mg2 Si,
Al4 Ca, etc., may be mentioned. How 5 ever, if the area ratio of the other intermetallic compounds
in the main layer becomes too large, sometimes it is not possible to sufficiently secure the Mg-
Zn containing phase, Fe-Al containing phase, and/or Al-Si containing oxide phase. Therefore,
the area ratio of the other intermetallic compounds, for example, the area ratio of the Mg2 Si and
Al4 Ca, is preferably a total of 10.0% or less. 5.0% or less is more preferable.
10 [0071]
[Oxide Layer]
The surface of the plating layer is sometimes formed with an oxide layer due to oxidation of
the plating constituents. Such an oxide layer is liable to cause a drop in the chemical
convertibility and electrodeposition coatability after hot stamping. Therefore, the thickness of the
15 oxide layer is preferably small. For example, it is preferably 1.0 m or less. If the Zn and Mg
evaporate at the time of hot stamping, a thick Mg-Zn containing oxide layer of more than 1.0 m
is formed.
[0072]
[Fe-Al Layer]
20 In an embodiment according to the present invention, as shown in FIG. 2, sometimes an Fe-
Al layer 7 is formed beneath the main layer 6. The Fe-Al layer contains mainly Fe and Al. More
specifically, it is believed that the Fe-Al layer is formed by further diffusion of the metal
elements of the above-explained interfacial layer and the diffusion layer positioned beneath the
interfacial layer due for example to the relatively high temperature in the hot stamping. If the Fe-
25 Al layer becomes too thick, the Al constituent in the plating layer, in particular the main layer,
becomes too small, therefore this is not preferable. Therefore, the thickness of the Fe-Al layer is
generally 25.0 m or less, preferably 20.0 m or less, more preferably 15.0 m or less, most
preferably 10.0 m or less.
[0073]
30 The thicknesses of the plating layer, the Fe-Al layer, and the oxide layer are determined by
cutting out a test piece from the hot stamped body, burying it in a resin, etc., then polishing the
cross-section and measuring the image observed by an SEM. Further, if examining these in a
backscattered electron image of the SEM, the contrast at the time of observation will differ
depending on the metal constituents, therefore it is possible to identify the layers and confirm the
35 thicknesses of the layers. The thicknesses of the plating layer, the Fe-Al layer, and the oxide
layer are determined by performing similar observation in three or more different fields and
22
finding the averages of these.
[0074]
In the present invention, the area ratios of the phases of the main layer are determined in the
following way. First, a prepared sample is cut into a 25 mm15 mm size, and any cross-section
of the plating layer is photographed by a power o 5 f 1500X by a scanning electron microscope
(SEM). From the BSE image of the same and an SEM-EDS mapping image, the area ratios of
the phases at the main layer were measured by computer image processing. The averages of the
measurement values at any five fields (however, the measured areas in the fields are 400 m2 or
more) were determined as the area ratios of the MgZn phase, Mg2 Zn3 phase, MgZn2 phase,
10 FeAl phase, Fe-Al-Zn phase, Al-Si containing oxide phase, and other intermetallic compounds.
Further, the area ratio of the Mg-Zn containing phase was determined as the area ratio of the
total of the MgZn phase, Mg2 Zn3 phase, and MgZn2 phase. Similarly, the area ratio of the Fe-Al
containing phase was determined as the area ratio of the total of the FeAl phase and Fe-Al-Zn
phase.
15 [0075]

Next, a preferred method for producing the hot stamped body according to the embodiment
of the present invention will be explained. The following explanation is intended to illustrate a
characteristic method for producing a hot stamped body according to the embodiment of the
20 present invention and is not intended to limit the hot stamped body to one produced by a
production method as explained below.
[0076]
The above production method comprises forming the steel base material, forming a plating
layer on the steel base material, and hot stamping (hot pressing) the steel base material on which
25 the plating layer is formed. Below, each step will be explained in detail.
[0077]
[Step of Forming Steel Base Material]
In the step of forming the steel base material, for example, first, molten steel having the
same chemical composition as that explained for the steel base material is produced. The
30 produced molten steel is used to produce a slab by a casting method. Alternatively, the produced
molten steel may be used to produce an ingot by the ingot making method. Next, the slab or
ingot is hot rolled to produce the steel base material (hot rolled steel sheet). In accordance with
need, the hot rolled steel sheet may be pickled, then the hot rolled steel sheet may be cold rolled.
The obtained cold rolled steel sheet may be used as the steel base material.
35 [0078]
[Step of Forming Plating Layer]
23
Next, in the step of forming the plating layer, a plating layer having the chemical
composition explained above is formed on at least one surface of the steel base material,
preferably on both surfaces.
[0079]
More specifically, first, the above steel base material is 5 reduced by heating in an N2 -H2
mixed gas atmosphere at a predetermined temperature and time, for example, a temperature of
750 to 850C, then is cooled in a nitrogen atmosphere or other inert atmosphere until near the
plating bath temperature. Next, the steel base material is dipped in a plating bath having the a
predetermined chemical composition for 0.1 to 60 seconds, then is pulled up and adjusted in
10 amount of deposition of the plating layer to within a predetermined range by immediately
blowing N2 gas or air by the gas wiping method.
[0080]
Further, the amount of deposition of the plating layer is preferably 10 to 170 g/m2 per
surface. In the present step, as an aid to plating deposition, it is also possible to apply Ni
15 preplating, Sn preplating, or other preplating. However, these preplatings cause changes in the
alloying reactions, therefore the amount of deposition of the preplating is preferably 2.0 g/m2
per surface or less.
[0081]
Finally, the steel base material on which the plating layer is deposited is cooled, whereby
20 the plating layer is formed on one surface or both surfaces of the steel base material. In the
present method, at the time of this cooling, it is important to form the acicular Al-Zn-Si-Ca phase
of the intermetallic compound comprised of mainly Al, Zn, Si, and Ca in the surface structure of
the plating layer. FIG. 3 shows a backscattered electron image (BSE image) of a scanning
electron microscope (SEM) of the plating layer surface before hot stamping of the hot stamped
25 body according to the present invention. Referring to FIG. 3, it will be understood that in the
surface structure of the plating layer, in addition to an  phase 11 (dendrite structure in FIG. 3)
and / eutectic phase 12, the acicular Al-Zn-Si-Ca phase 13 is present in a relatively large
amount. The  phase is a structure mainly comprised of Al and Zn, while the  phase is a
structure mainly comprised of Mg, Zn, and Al.
30 [0082]
While not intending to be bound by any specific theory, it is believed that the acicular Al-
Zn-Si-Ca phase 13 shown in FIG. 3 functions as the supply source of Ca for forming a Ca-based
oxide film at the start of heating in the hot stamping. More specifically, it is believed that due to
the presence of the acicular Al-Zn-Si-Ca phase 13 in the surface structure of the plating layer
35 before the hot stamping, the Ca leaching out from the acicular Al-Zn-Si-Ca phase 13 at the start
of the heating in the hot stamping is preferentially oxidized by the oxygen in the atmosphere and
24
forms a dense Ca-based oxide film, more specifically a Ca- and Mg-containing oxide film, at the
surface-most part of the plating layer. It is believed that such a Ca-based oxide film functions as
a barrier layer for suppressing evaporation of Zn and Mg. In particular, by the acicular Al-Zn-Si-
Ca phase 13 being present in a predetermined amount, more specifically an area ratio of 2.0% or
more, in the surface structure of the plating layer, such a 5 function as a barrier layer is effectively
exhibited. Therefore, it is possible to reduce or suppress the evaporation of Zn and Mg in the
plating layer to the outside and penetration of hydrogen from the outside at the time of hot
stamping.
[0083]
10 In the present method, suitably controlling the cooling conditions at the time of
solidification of the plating layer in the liquid phase state, more specifically cooling the steel
base material on which the plating layer is deposited in two stages, is extremely important for the
acicular Al-Zn-Si-Ca phase to be formed in a predetermined amount in the surface structure of
the plating layer. Explained in more detail, the specific value of the cooling speed can change in
15 accordance with the chemical composition, etc., of the plating layer, but to make the acicular Al-
Zn-Si-Ca phase be reliably formed in a predetermined amount, it is effective to first cool the
steel base material on which the plating layer is deposited by a 14C/s or more, preferably
15C/s or more, average cooling speed from the bath temperature (in general, 500 to 700C)
down to 450C, then cool it by a 5.5C/s or less, preferably 5C/s or less, average cooling speed
20 from 450C to 350C. By such cooling conditions, i.e., by two-stage cooling of fast cooling and
slow cooling, at the time of the first fast cooling, a supersaturated state is created to produce a
state in which nuclei of the acicular Al-Zn-Si-Ca phase can easily form and a large amount of
nuclei is formed and, at the time of the next slow cooling, the nuclei is made to slowly grow,
whereby an area ratio of 2.0% or more of the acicular Al-Zn-Si-Ca phase is formed in the surface
25 structure of the plating layer, in particular is formed dispersed. As a result, even in the case of a
heating temperature of 900C or more in hot stamping, it becomes possible to suppress
evaporation of Zn and Mg and is possible to remarkably reduce or suppress LME and hydrogen
penetration into the steel material. On the other hand, if not performing the above two-stage
cooling, it is not possible to form the acicular Al-Zn-Si-Ca phase in the surface structure of the
30 plating layer or not possible to form it in a sufficient amount, therefore at the time of the heating
in the hot stamping, much of the Zn and Mg in the plating layer evaporates. Part of the
evaporated Zn and Mg is deposited as oxides on the steel base material. In general, a thick Mg-
Zn containing oxide layer of more than 1.0 m, for example, 2.0 m or more or 3.0 m or more,
is formed. As a result, the LME resistance and hydrogen penetration resistance of the obtained
35 hot stamped body greatly fall. In this case, it is not possible to form an Al-Si containing oxide
phase at the obtained hot stamped body and as a result not possible to achieve the desired
25
chemical convertibility. Further, if an acicular Al-Zn-Si-Ca phase is formed, but the amount
formed is not necessarily sufficient, sometimes the desired area ratio of the Mg-Zn containing
phase or Fe-Al containing phase cannot be achieved.
[0084]
If the point of change of the cooling 5 speed of the fast cooling and slow cooling becomes
higher than about 450C, sometimes nuclei of the acicular Al-Zn-Si-Ca phase are not sufficiently
formed. On the other hand, if the point of change of the cooling speed becomes lower than about
450C, sometimes the nuclei formed cannot be made to sufficiently grow. Whatever the case, it
becomes difficult to render the acicular Al-Zn-Si-Ca phase present in a predetermined amount,
10 more specifically an area ratio of 2.0% or more in amount in the surface structure of the plating
layer. Therefore, the point of change of the cooling speed, as explained later, has to be selected
from 425 to 475C in range. To reliably form 2.0% or more of the acicular Al-Zn-Si-Ca phase,
as explained above, 450C is preferable.
[0085]
15 [Step of Hot Stamping (Hot Pressing)]
Finally, in the step of hot stamping (hot pressing), the steel base material provided with the
plating layer is hot pressed. The present step is performed by loading the steel base material
provided with the plating layer in a heating furnace, holding it for a predetermined holding time
after reaching 1000C, then hot pressing it. The above “holding time” means the holding time
20 from 1000C or more to 1300C or less after reaching 1000C. The specific value of the holding
time can change according to the holding temperature and the chemical composition of the
plating layer, etc., but in general is more than 1 minute or more. To reliably obtain the hot
stamped body according to the embodiment of the present invention having the plating layer
provided with the main layer including the above explained Mg-Zn containing phase, Fe-Al
25 containing phase, and Al-Si containing oxide phase, the time is 1.5 minutes or more and 2
minutes or less or 3 minutes or less.
EXAMPLES
[0086]
30 Below, examples will be used to explain the present invention in more detail, but the
present invention is not limited to these examples in any way.
[0087]
[Example A]
In the present example, hot stamped bodies according to embodiments of the present
35 invention were produced under various conditions and were investigated for characteristics.
[0088]
26
First, molten steel comprising, by mass%, a C content of 0.20%, Si content of 0.20%, Mn
content of 1.30%, P content of 0.01%, S content of 0.005%, sol. Al content of 0.02%, N content
of 0.002%, B content of 0.002%, Ti content of 0.02%, Cr content of 0.20%, and balance of Fe
and impurities was used to produce a slab by continuous casting. Next, the slab was hot rolled to
produce hot rolled steel sheet, the 5 hot rolled steel sheet was pickled, then the sheet was cold
rolled to produce a cold-rolled steel sheet (steel base material) having a 1.4 mm sheet thickness.
[0089]
Next, the produced steel base material was cut to 100 mm200 mm, then the steel base
material was plated using a batch type hot dip coating apparatus made by Rhesca. More
10 specifically, first, the produced steel base material was reduced by heating in a furnace with an
oxygen concentration of 20 ppm or less in an N2 -5%H2 mixed gas atmosphere at 800C, then
was cooled in N2 down to the plating bath temperature+20C. Next, the steel base material was
dipped in a plating bath having a predetermined chemical composition for about 3 seconds, then
was pulled up by a pull-up speed of 20 to 200 mm/s and adjusted by N2 gas wiping to an amount
15 of deposition of the plating layer of the value shown in Table 1. Next, the steel base material on
which the plating layer was deposited was cooled in two stages under the conditions shown in
Table 1, whereby a plated steel material on the two surfaces of which plating layer was formed
was obtained. The sheet temperature was measured using a thermocouple spot welded to the
center part of the steel base material.
20 [0090]
Next, the obtained plated steel material was hot stamped. Specifically, the hot stamping was
performed by loading the plated steel material into a heating furnace, then heating it to 900C or
heating to1000C or more in temperature and holding it there for a predetermined time, then hot
pressing it by a die equipped with a water cooling jacket. As the heat treatment conditions at the
25 time of hot stamping (HS), either of the following conditions X to Z was selected. The
quenching by the die was controlled to give a cooling speed of 50C/s or more up to about the
martensite transformation start point (410C).
X: Holding at 1000C for 2 minutes
Y: Holding at 1250C for 1 minute
30 Z: Holding at 900C for 1 minute
[0091]
27
[Table 1]
Table 1
No. Class
Chemical composition of plating layer (mass%)
Amount of
deposition of
plating layer per
surface
Method of production
Bath
temperature
Bath temperature
to 450C average
cooling speed
450 to 350C
average cooling
speed
HS heat
treatment
Zn Al Mg Si Ca Fe
Others
Type Total value (g/m
2
) (C) (C/s) (C/s)
1 Comp. ex. 7.87 15.10 0.00 0.03 0.00 77.00 - 0.00 0.0 520 15.0 5.0 X
2 Comp. ex. 13.38 12.10 0.01 0.00 0.01 74.50 - 0.00 0.2 520 15.0 5.0 X
3 Example 39.70 15.00 4.50 0.60 0.80 39.40 - 0.00 30.4 530 15.0 5.0 X
4 Comp. ex. 14.64 20.60 16.27 0.60 0.79 47.10 - 0.00 30.1 630 15.0 5.0 X
5 Comp. ex. 12.00 11.50 0.00 0.00 0.00 76.50 - 0.00 0.1 530 15.0 5.0 X
6 Example 30.72 18.40 4.60 0.40 0.76 45.10 Pb:0.02 0.02 25.1 580 15.0 5.0 X
7 Example 31.28 18.50 4.70 0.46 0.76 44.30 - 0.00 35.5 580 15.0 5.0 Y
8 Example 30.55 20.40 4.50 0.45 0.60 43.50 - 0.00 30.1 580 15.0 5.0 X
9 Example 26.88 22.60 4.60 0.41 0.60 44.90 Sr:0.01 0.01 29.8 580 15.0 5.0 X
10 Example 27.49 22.10 5.00 0.43 0.10 44.80 Sn:0.08 0.08 33.5 580 15.0 5.0 X
11 Example 27.50 21.50 4.50 0.40 1.10 45.00 - 0.00 33.1 580 15.0 5.0 X
12 Example 25.97 22.50 5.90 0.46 0.07 45.10 - 0.00 0.0 580 15.0 5.0 X
13 Example 24.92 22.60 5.90 0.50 0.69 45.30 Sb:0.09 0.09 0.0 580 15.0 5.0 X
14 Example 18.17 22.70 12.00 0.42 0.76 45.10 Ni:0.85 0.85 45.8 620 15.0 5.0 Y
15 Comp. ex. 14.70 10.90 0.00 0.20 0.10 74.10 - 0.00 0.2 580 10.0 5.0 X
16 Comp. ex. 9.05 12.80 0.00 0.00 0.05 78.10 - 0.00 0.1 580 15.0 15.0 X
17 Comp. ex. 13.30 11.50 0.10 0.20 0.10 74.80 - 0.00 0.3 580 5.0 5.0 X
18 Comp. ex. 13.20 10.80 0.10 0.20 0.10 75.60 - 0.00 0.3 580 15.0 7.0 X
19 Comp. ex. 24.60 22.00 4.80 3.40 0.10 45.10 - 0.00 41.1 580 15.0 5.0 X
20 Comp. ex. 25.50 20.60 4.10 0.40 3.10 46.30 - 0.00 42.3 580 15.0 5.0 Z
21 Example 16.77 25.50 9.10 0.53 2.00 46.10 - 0.00 0.0 580 15.0 5.0 X
22 Example 19.18 27.80 6.25 0.06 0.50 46.20 Cr:0.01 0.01 0.0 580 15.0 5.0 Y
23 Example 16.29 29.80 6.71 0.10 0.50 46.40 Ni:0.20 0.20 20.2 640 15.0 5.0 X
24 Example 17.40 30.50 6.50 0.50 0.70 44.10 Mn:0.30 0.30 20.1 630 15.0 5.0 X
25 Example 16.18 33.40 6.90 0.50 0.72 42.30 - 0.00 19.5 630 15.0 5.0 X
26 Example 6.09 36.90 12.00 1.60 0.81 42.10 Cu:0.50 0.50 17.5 630 15.0 5.0 X
27 Example 12.21 38.00 4.50 2.50 0.75 42.00 Ti:0.04 0.04 19.5 650 15.0 5.0 X
28 Example 9.40 39.40 6.10 3.00 0.73 41.37 - 0.00 18.4 650 15.0 5.0 X
29 Example 3.78 42.90 5.70 1.71 0.86 45.05 - 0.00 20.0 680 15.0 5.0 X
30 Comp. ex. 3.68 55.60 4.50 0.56 0.56 35.10 - 0.00 22.0 700 15.0 5.0 X
31 Comp. ex. Commercially available hot dip galvannealed steel sheet X
32 Comp. ex. Commercially available hot dip Al coated steel sheet X
Bold underlines indicate outside scope of present invention or outside preferable range.
[0092]
28
[Table 2]
Table 2
No. Class
Fe-Al layer
(interfacial
layer+diffusion
layer)
Main layer
Results of evaluation
Mg-Zn containing phase (%) Fe-Al containing phase (%)
Al-Si containing oxide phase
Other
intermetallic
compounds
Mg-Zn
containing
oxide layer
Thickness (m)
Total MgZn Mg2Zn3 MgZn2 Total Fe-Al-Zn phase
FeAl
phase
LME
Chemical
convertibility
Hydrogen
Area penetration
ratio
Area
ratio
Area ratio Area ratio
Area
ratio
Area ratio Area ratio
Particle size
(m)
Area ratio
(%)
Area ratio
Thickness
(m)
1 Comp. ex. 14.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.0 1.9 D C D
2 Comp. ex. 16.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.1 1.7 D C D
3 Example 10.1 90.0 0.0 40.0 50.0 6.5 1.5 5.0 1.6 3.5 0.0 <0.2 B B A
4 Comp. ex. 12.2 78.0 28.9 49.1 0.0 22.0 1.5 20.5 - 0.0 0.0 <0.2 A C C
5 Comp. ex. 14.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.0 3.2 D C D
6 Example 12.1 89.8 39.2 33.3 17.3 7.8 2.9 4.9 5.1 2.3 0.1 <0.2 A B A
7 Example 10.1 88.0 47.8 27.2 13.0 5.5 2.0 3.5 6.1 6.5 0.0 <0.2 A B A
8 Example 10.5 87.4 41.4 31.5 14.5 6.5 2.0 4.5 5.0 6.1 0.0 <0.2 A A A
9 Example 10.6 88.1 44.1 29.9 14.1 5.8 1.2 4.6 5.1 6.1 0.0 <0.2 A A A
10 Example 11.5 87.4 76.4 11.0 0.0 5.7 1.6 4.1 5.3 6.8 0.1 <0.2 A A A
11 Example 11.3 87.9 71.5 3.3 13.1 5.0 1.4 3.6 4.9 7.1 0.0 <0.2 A A A
12 Example 11.4 86.9 86.9 0.0 0.0 5.0 2.0 3.0 5.5 8.1 0.0 <0.2 A A A
13 Example 11.2 85.0 85.0 0.0 0.0 5.0 0.0 5.0 6.1 9.9 0.1 <0.2 A A A
14 Example 11.5 84.1 84.1 0.0 0.0 5.1 0.0 5.1 5.4 10.5 0.3 <0.2 A A A
15 Comp. ex. 18.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.0 4.1 D C D
16 Comp. ex. 19.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.0 6.4 D C D
17 Comp. ex. 18.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.0 4.5 D C D
18 Comp. ex. 18.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 - 0.0 0.0 5.4 D C D
19 Comp. ex. 11.5 19.5 0.0 0.0 19.5 69.0 0.0 69.0 - 0.0 11.5 <0.2 C C D
20 Comp. ex. 12.5 22.5 0.0 0.0 22.5 66.3 0.0 66.3 - 0.0 11.2 <0.2 C C D
21 Example 10.5 79.8 73.1 6.7 0.0 10.1 0.0 10.1 5.0 10.1 0.0 <0.2 A A A
22 Example 19.5 82.6 80.1 2.5 0.0 5.1 0.0 5.1 4.8 12.2 0.1 <0.2 A A A
23 Example 12.6 81.5 81.5 0.0 0.0 5.3 0.0 5.3 3.5 13.0 0.2 <0.2 A A A
24 Example 12.3 80.2 80.2 0.0 0.0 5.2 0.0 5.2 4.1 14.5 0.1 <0.2 A A A
25 Example 11.7 79.1 79.1 0.0 0.0 5.4 0.0 5.4 3.7 15.5 0.0 <0.2 A A A
26 Example 11.8 72.6 72.6 0.0 0.0 10.1 0.0 10.1 8.4 17.2 0.1 <0.2 A A A
27 Example 14.5 75.5 75.5 0.0 0.0 11.1 0.0 11.1 8.2 13.1 0.3 <0.2 A A A
28 Example 15.1 68.1 68.1 0.0 0.0 16.4 0.0 16.4 8.1 15.5 0.0 <0.2 A A A
29 Example 20.0 60.0 60.0 0.0 0.0 20.0 0.0 20.0 7.8 20.0 0.0 <0.2 A A A
30 Comp. ex. 11.5 16.0 0.0 0.0 16.0 73.9 0.0 73.9 - 0.0 10.1 <0.2 D C D
31 Comp. ex. Commercially available hot dip galvannealed steel sheet D D A
32 Comp. ex. Commercially available hot dip Al coated steel sheet A D D
Bold underlines indicate outside scope of present invention.
29
[0093]
The chemical compositions and structures of the plating layers in the hot stamped bodies
obtained in the examples and comparative examples and the various characteristics when hot
stamping the plated steel materials were investigated by the following methods: The results are
shown in Tables 1 and 2. In Tables 1 and 2, Comparative Examples 3 5 1 and 32 respectively relate
to hot dip galvannealed (Zn-11%Fe) steel sheet and hot dip aluminum coated (Al-10%Si) steel
sheet conventionally used as plated steel materials for hot stamping and show the results when
hot stamping these steel sheets. The chemical compositions and structures of the plating layers of
Comparative Examples 31 and 32 clearly differ from the chemical compositions and surface
10 structures of the plating layers according to the present invention, therefore analysis of the
chemical compositions and structures of these plating layers were omitted. Further, Comparative
Examples 31 and 32 are just commercially available products which were evaluated. Therefore,
details of the production methods of these steel sheets are not known. Further, while not shown
in Table 2, the Fe-Al containing phase (Fe-Al-Zn phase and FeAl phase) has island shapes. In
15 the Fe-Al containing phase, the aspect ratio was 5.0 or less.
[0094]
[Chemical Composition of Plating Layer]
The chemical composition of the plating layer was determined by dissolving the plating
layer in an acid solution to which an inhibitor inhibiting corrosion of the steel base material was
20 added and measuring the obtained solution by ICP emission spectrometry.
[0095]
[Thicknesses of Fe-Al Layer and Oxide Layer]
The thicknesses of the Fe-Al layer and the oxide layer were determined by cutting out a test
piece from the hot stamped body, burying it in a resin, etc., then polishing the cross-section,
25 measuring the image observed by an SEM, and averaging the measured values of these in three
different fields as the thicknesses of the Fe-Al layer and the oxide layer.
[0096]
[Area Ratio and Composition of Each Phase in Main Layer]
The area ratio of each phase in the main layer was determined as follows: First, a prepared
30 sample was cut into 25 mm15 mm size. The plating layer surface was photographed by a
1500X power by an SEM. From the BSE image obtained and SEM-EDS mapping, the area ratio
of each phase in the main layer was measured by computer image processing. The averages of
these measured values at any five fields were determined as the area ratios of the MgZn phase,
Mg2 Zn3 phase, MgZn2 phase, FeAl phase, Fe-Al-Zn phase, and Al-Si containing oxide phase.
35 Further, the area ratio of the Mg-Zn containing phase was determined as the area ratio of the
total of the MgZn phase, Mg2 Zn3 phase, and MgZn2 phase. Similarly, the area ratio of the Fe30
Al containing phase was determined as the area ratio of the total of the FeAl phase and Fe-Al-Zn
phase.
[0097]
[Particle Size of Al-Si Containing Oxide Phase]
A sample was taken using any cross-5 section of the plating layer as the observed surface.
The observed surface was polished, the observed surface was examined under an SEM, then
image processing was performed to calculate the circle equivalent diameter of the particles of the
Al-Si containing oxide phase. The average value of these was found and determined as the
particle size of the Al-Si containing oxide phase.
10 [0098]
[LME Resistance]
The LME resistance was evaluated by subjecting a sample of the plated steel material
before hot stamping to a hot V-bending test. Specifically, a sample 170 mm30 mm of the plated
steel material before hot stamping was heated in a heating furnace and taken out from the
15 furnace when the temperature of the sample reached 900C. A precision press was used to
conduct a V-bending test. The V-bending die had a shape of a V-bending angle of 90 and R1,
2, 3, 4, 5, and 10 mm. The LME resistance was ranked as follows: Rankings of AAA, AA, A,
and B were deemed passing.
AAA: No LME cracking occurred even with R of 1 mm.
20 AA: LME cracking occurred with R of 1 mm, but LME cracking did not occur with R of 2
mm
A: LME cracking occurred with R of 2 mm, but LME cracking did not occur with R of 3
mm
B: LME cracking occurred with R of 3 mm, but LME cracking did not occur with R of 4
25 mm
C: LME cracking occurred with R of 4 mm, but LME cracking did not occur with R of 5
mm
D: LME cracking occurred with R of 5 mm, but LME cracking did not occur with R of 10
mm
30 [0099]
[Chemical Convertibility]
The chemical convertibility of the hot stamped body was evaluated as follows: First, a 50
mm100 mm sample of the hot stamped body was treated by zinc phosphate (SD5350 system:
standard set by Japan Paint and Industrial Coating), then the coverage rate of the chemically
35 converted crystals was measured by SEM examination and evaluated as follows: Evaluations of
A and B were deemed passing.
31
A: Coverage rate of 95% or more
B: Coverage rate of 90% or more and less than 95%
C: Coverage rate of 85% or more and less than 90%
D: Coverage rate of less than 85%
5 [0100]
[Hydrogen Penetration Resistance]
The hydrogen penetration resistance of the hot stamped body was found as follows: First, a
sample of the hot stamped body was stored in liquid nitrogen. Thermal desorption spectroscopy
was used to find the concentration of hydrogen penetrating the hot stamped body. Specifically,
10 the sample was heated in a heating furnace equipped with a gas chromatograph and the amount
of hydrogen released from the sample up to 250C was measured. The measured amount of
hydrogen was divided by the mass of the sample to find the amount of hydrogen penetration.
This was ranked as follows: Rankings of AAA, AA, A, and B were deemed passing.
AAA: Amount of hydrogen penetration of 0.1 ppm or less
15 AA: Amount of hydrogen penetration of more than 0.1 to 0.2 ppm
A: Amount of hydrogen penetration of more than 0.2 to 0.3 ppm
B: Amount of hydrogen penetration of more than 0.3 to 0.5 ppm
C: Amount of hydrogen penetration of more than 0.5 to 0.7 ppm
D: Amount of hydrogen penetration of 0.7 ppm or more
20 [0101]
Referring to Tables 1 and 2, in Comparative Example 1, the Si and Ca contents in the
plating layer were small, therefore the acicular Al-Zn-Si-Ca phase was not formed in the surface
structure of the plating layer before hot stamping. It is believed that a barrier layer comprised of
a Ca-based oxide film was not formed at the time of heating in the hot stamping. As a result, at
25 the time of the above heating, Zn and Mg in the plating layer evaporated, a thick Mg-Zn
containing oxide layer of more than 1.0 m was formed, an Mg-Zn containing phase was not
formed in the main layer, and the LME resistance and hydrogen penetration resistance were
evaluated as being poor. In Comparative Examples 2 and 5, similarly the Al, Si, and/or Ca
content in the plating layer was small or nonexistent, therefore at the time of heating in the hot
30 stamping, no barrier layer was formed and the LME resistance and hydrogen penetration
resistance were evaluated as poor. In Comparative Example 4, the Mg content in the plating
layer was large. Hydrogen penetration occurred due to the evaporation of Mg at the time of hot
stamping. In Comparative Examples 15 to 18, the cooling of the plating layer did not satisfy the
predetermined two-stage cooling conditions, therefore an acicular Al-Zn-Si-Ca phase was not
35 sufficiently formed at the surface structure of the plating layer before the hot stamping, and, at
the time of the heating in the hot stamping, Zn and Mg in the plating layer evaporated and, as a
32
result, the LME resistance and hydrogen penetration resistance were evaluated as poor. In
Comparative Example 19, the Si content in the plating layer was too high, therefore in the
plating layer before the hot stamping, an Mg2 Si phase (other intermetallic compound in Table 2)
was preferentially formed, the acicular Al-Zn-Si-Ca phase was not sufficiently formed, and, as a
result, the LME resistance and hydrogen penetration 5 resistance were evaluated as poor. In
Comparative Examples 20 and 30, the Ca content or the Al content in the plating layer was too
high, therefore in the plating layer before hot stamping, Al4 Ca and other intermetallic
compounds (other intermetallic compounds in Table 2) were preferentially formed, an acicular
Al-Zn-Si-Ca phase was not sufficiently formed, and, as a result, the LME resistance and
10 hydrogen penetration resistance were evaluated as poor. Further, in all of the above comparative
examples, an Al-Si containing oxide phase was not formed, therefore the chemical convertibility
was evaluated as poor. In Comparative Example 31 using conventional hot dip galvannealed
steel sheet, the hydrogen penetration resistance was excellent, but the LME resistance and
chemical convertibility were evaluated as poor. In Comparative Example 32 using conventional
15 hot dip aluminum coated steel sheet, the LME resistance was excellent, but the chemical
convertibility and hydrogen penetration resistance were evaluated as poor.
[0102]
In contrast to this, in all of the examples according to the present invention, by suitably
controlling the chemical composition of the plating layer and the phases contained in the plating
20 layer and the area ratios of the same, a hot stamped body in which the LME resistance and
hydrogen penetration resistance were improved and, furthermore, the chemical convertibility
was excellent could be obtained. From the BSE image of the SEM of the plating layer surface
before hot stamping (and in accordance with need the SEM-EDS mapping image), in all
examples, an acicular Al-Zn-Si-Ca phase was present in an area ratio of 2.0% or more at the
25 surface structure of the plating layer before hot stamping.
[0103]
[Example B]
In this example, the inventors studied the point of change of the cooling speed between fast
cooling and slow cooling in two-stage cooling of a plating layer. First, except for using a plating
30 bath for forming a plating layer similar to Example 13, etc. (bath temperature 600C), and
further changing the point of change of the cooling speed to 375C, 400C, 425C, 450C,
475C, and 500C and making the average cooling speed of the first stage 15C/s and the
average cooling speed of the second stage 5C/s, they followed the same procedure as in the case
of Example A to obtain plated steel materials with plating layers formed on both surfaces of the
35 steel base materials. They examined the area ratios of the acicular Al-Zn-Si-Ca phases at the
surface structures of the plating layers at the obtained plated steel materials. The results are
33
shown in FIG. 4.
[0104]
Referring to FIG. 4, if the point of change of the cooling speed is 400C, the area ratio of
the acicular Al-Zn-Si-Ca phase is 1.9%, and therefore 2.0% or more could not be secured, while
if the point of change of the cooling speed 5 is 425C, 450C, and 475C, 2.0% or more of the
acicular Al-Zn-Si-Ca phase could be formed. In particular, if the point of change of the cooling
speed is 450C, the highest area ratio of the acicular Al-Zn-Si-Ca phase could be achieved.
REFERENCE SIGNS LIST
10 [0105]
1 plating layer
2 oxide layer
3 diffusion layer
4 steel base material
15 5 interfacial layer
6 main layer
7 Fe-Al layer
8 Mg-Zn containing phase
9 Fe-Al containing phase
20 9a FeAl phase
10 Al-Si containing oxide phase
11  phase
12 / eutectic phase
13 acicular Al-Zn-Si-Ca phase.

CLAIMS
[Claim 1]
A hot stamped body comprising a steel base material and a plating layer formed on a
surface of the steel base material, wherein the plating layer has a chemical composition
5 comprising, by mass%,
Al: 15.00 to 55.00%,
Mg: 4.50 to 12.00%,
Si: 0.05 to 3.00%,
Ca: 0.05 to 3.00%,
10 Fe: 20.00 to 65.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
15 Ti: 0 to 1.00%,
Sr: 0 to 0.50%,
Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Mn: 0 to 1.00%, and
20 balance: Zn and impurities,
the plating layer comprises an interfacial layer positioned at an interface with the steel base
material and containing Fe and Al and a main layer positioned on the interfacial layer,
the main layer comprises, by area ratio, 10.0 to 90.0% of an Mg-Zn containing phase, 5.0 to
less than 30.0% of an Fe-Al containing phase, and 2.0 to 25.0% of an Al-Si containing oxide
25 phase,
the Mg-Zn containing phase comprises at least one selected from the group consisting of an
MgZn phase, Mg2 Zn3 phase, and MgZn2 phase, and
the Fe-Al containing phase comprises at least one of an FeAl phase and Fe-Al-Zn phase.
30 [Claim 2]
The hot stamped body according to claim 1, wherein the chemical composition of the
plating layer comprises, by mass%,
Al: 25.00 to 35.00% and
Mg: 6.00 to 10.00%.
35
[Claim 3]
35
The hot stamped body according to claim 1 or 2, wherein the Mg-Zn containing phase
comprises an MgZn phase, and an area ratio of the MgZn phase in the main layer is 30.0% or
more.
5 [Claim 4]
The hot stamped body according to any one of claims 1 to 3, wherein the Mg-Zn containing
phase comprises an MgZn phase and Mg2 Zn3 phase, and an area ratio of a total of the MgZn
phase and Mg2 Zn3 phase in the main layer is 40.0 to 85.0%.
10 [Claim 5]
The hot stamped body according to any one of claims 1 to 4, wherein the Fe-Al containing
phase comprises an FeAl phase and an area ratio of the FeAl phase in the main layer is 5.0 to
20.0%.