FIELD
[0001]
The present invention relates to a hot stamped body.
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 Zn-based plated steel
material for hot pressing having at a surface-most layer an oxide layer mainly comprising Zn and
containing Mn in 1% or more in mass%, having underneath that a plating layer comprising a Zn35 based alloy, and containing in the Zn-based plated layer one or more of Ni: 0.01 to 20%, Cr: 0.01
to 10%, Mn: 0.01 to 10%, Mo: 0.01 to 5%, Co: 0.01 to 5%, Al: 0.01 to 60%, Si: 0.01 to 5%, Mg:
2
0.01 to 10%, Ca: 0.01 to 5%, and Sn: 0.01 to 10%.
[0006]
Further, PTL 4 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,
5 wherein the Zn-Al-Mg alloy layer has a Zn phase, the Zn 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 than 12.5%, Ca: 0% to 3.00%, Si: 0% to less
than 2.5%, etc.
[0007]
10 Similarly, PTL 5 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, 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-AlMgZn2 ternary eutectic structure is 0 to 5%, and the plating layer contains, by mass%, Zn: more
15 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.
[CITATIONS LIST]
[PATENT LITERATURE]
20 [0008]
[PTL 1] Japanese Unexamined Patent Publication No. 2012-112010
[PTL 2] Japanese Unexamined Patent Publication No. 2009-120948
[PTL 3] WO 2005-113233
[PTL 4] WO 2018/139619
25 [PTL 5] WO 2018/139620
SUMMARY
[TECHNICAL PROBLEM]
[0009]
30 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
embrittlement (LME)”. It is known that the fatigue characteristics of a steel material fall due to
the LME.
35 [0010]
On the other hand, if using a plated steel material containing Al as a constituent of the
3
plating layer in hot stamping, it is known that, for example, the hydrogen generated at the time of
heating in the hot stamping will sometimes penetrate the steel material and cause hydrogen
embrittlement cracking.
[0011]
5 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
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
penetration resistance.
10 [0012]
Therefore, an object of the present invention is to provide a hot stamped body improved in
the LME resistance and hydrogen penetration resistance and, further, excellent in the corrosion
resistance.
15 [SOLUTION TO PROBLEM]
[0013]
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
20 comprising, by mass%,
Al: 15.00 to 45.00%,
Mg: 4.50 to 12.00%,
Si: 0.05 to 3.00%,
Ca: 0.05 to 3.00%,
25 Fe: 20.00 to 50.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
30 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
35 balance: Zn and impurities,
the plating layer comprises an interfacial layer positioned at an interface with the steel base
4
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 85.0% of an Mg-Zn containing phase and
15.0 to 90.0% of an Fe-Al containing phase,
the Mg-Zn containing phase comprises at least one selected from the group consisting of an
5 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 and
an area ratio of the Fe-Al-Zn phase in the main layer is 10.0% or less.
(2) The hot stamped body according to the above (1), wherein the chemical composition
of the plating layer comprises, by mass%,
10 Al: 20.00 to 30.00% and
Mg: 5.50 to 10.00%.
(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.
15 (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
the MgZn phase and Mg2 Zn3 phase in the main layer is 25.0 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
20 is 5.0 to 55.0%.
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0014]
According to the present invention, it is possible to provide a hot stamped body improved in
25 the LME resistance and hydrogen penetration resistance and, further, excellent in the corrosion
resistance.
BRIEF DESCRIPTION OF DRAWINGS
[0015]
30 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
microscope (SEM) of a plating layer cross-section in a hot stamped body according to the
35 present invention.
FIG. 3 shows a backscattered electron image (BSE image) of a scanning electron
5
microscope (SEM) of a plating layer surface before hot stamping in a hot stamped body
according to the present invention.
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 according to an embodiment of the present invention comprises a
10 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 45.00%,
Mg: 4.50 to 12.00%,
Si: 0.05 to 3.00%,
15 Ca: 0.05 to 3.00%,
Fe: 20.00 to 50.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
20 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%,
25 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,
the main layer comprises, by area ratio, 10.0 to 85.0% of an Mg-Zn containing phase and
30 15.0 to 90.0% of an Fe-Al containing 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 and
an area ratio of the Fe-Al-Zn phase in the main layer is 10.0% or less.
35 [0017]
For example, if using a conventional Zn-based plated steel material or an Al-Zn-based
6
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
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
5 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 time of heating in the hot stamping, the water vapor
10 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
stamping at a 900C or higher temperature, a part of the Zn will evaporate and sometimes will
15 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 Znbased plated steel material or Al-Zn-based plated steel material, sometime parts thereof will
20 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]
Further, if the elements Zn and/or Mg having the effect of improving the corrosion
resistance at the time of hot stamping at a high temperature evaporate and parts of those elements
25 are lost, naturally a problem will arise in that it is not possible to maintain sufficient corrosion
resistance in the body after hot stamping. Furthermore, if the Zn and/or Mg in the plating layer
evaporate and are lost, in the plating layer after the hot stamping, relatively large amounts of AlFe-based intermetallic compounds and/or Zn-Fe-based intermetallic compounds will be formed
between the Fe which had been diffused from the base iron and the Al and/or Zn in the plating
30 layer. These intermetallic compounds become causes of red rust in corrosive environments.
[0020]
Therefore, the inventors studied the corrosion resistance, LME resistance, and hydrogen
penetration resistance in 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
35 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
7
remarkably reduce or suppress LME and penetration of hydrogen into the steel material due to
the heating in the hot stamping and to achieve sufficient corrosion resistance. In addition, the
inventors discovered that by limiting the amount of the Fe-Al-Zn phase contained in the plating
layer to within a predetermined range, the hydrogen penetration resistance of the hot stamped
5 body is further improved. Below, this will be explained more specifically while referring to the
drawings.
[0021]
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
10 containing an Al-Zn-Mg-based plating layer. Referring to FIG. 1, it will be understood that the
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
15 layer 1. The diffusion layer 3 forms 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.
[0022]
In a conventional hot stamped body containing an Al-Zn-Mg-based plating layer such as
20 shown in FIG. 1, the Zn and Mg evaporate during the heating in the hot stamping, therefore
LME and hydrogen penetration into the steel material occur. Furthermore, the corrosion
resistance of the hot stamped body greatly falls due to the loss of at least part of the Zn and Mg
due to evaporation of these elements and the decrease in the Zn and Mg in the metal phase
accompanying formation of the oxides. In addition, for example, LME cracking is liable to be
25 caused even when the concentration of Zn in the plating layer 1 relatively rises due to
evaporation of Mg.
[0023]
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
30 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
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
35 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
8
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”.
5 [0024]
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
10 sometimes includes, in addition to the FeAl phase 9a, a relatively small amount of an Fe-Al-Zn
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
the Fe-Al containing phase 9 (islands of FeAl 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
15 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 hydrogen into the steel material and to achieve
sufficient corrosion resistance. In addition, by controlling the amount of the Fe-Al-Zn phase
contained in the main layer 6 to within a predetermined range or by not including an Fe-Al-Zn
20 phase in the main layer 6, it is possible to further improve the hydrogen penetration resistance of
the hot stamped body.
[0025]
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
25 start of heating in the hot stamping, it is believed that the Ca leached out from the acicular AlZn-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
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
30 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.
[0026]
Due to the function of such a barrier layer, it is believed that evaporation of Zn and Mg in
35 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
9
body finally obtained after hot stamping, unlike the case of FIG. 1, Zn and Mg can be kept from
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 85.0% by area ratio in the main
layer 6, and therefore the drop in corrosion resistance due to the evaporation of Zn and Mg to the
5 outside can be remarkably suppressed.
[0027]
Further, the FeAl phase 9a contained in the Fe-Al containing phase 9, as shown in FIG. 2, is
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
10 of the main layer 6. Therefore, if the content of the Fe-Al-Zn phase in the main layer 6 becomes
greater, only naturally, the amount of the Fe-Al-Zn phase present near the surface of the main
layer 6 will also become greater. 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-Zn phase and hydrogen
will be generated. As a result, sometimes the generated hydrogen will penetrate the steel material
15 and cause hydrogen embrittlement cracking. In the hot stamped body according to the present
invention, it is believed that by limiting the Fe-Al-Zn phase in the main layer 6 to within a
predetermined range, i.e, to within an area ratio of 10.0% or less, the amount of hydrogen
generated due to the Fe-Al-Zn phase 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
20 becomes possible to further improve the hydrogen penetration resistance of the hot stamped
body.
[0028]
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
25 constituents means “mass%” unless otherwise indicated.
[0029]
[Steel Base Material]
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
30 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 cold rolled steel sheet), etc. Below, the
35 constituents contained in the steel base material preferably applied in the present invention will
be explained in detail.
10
[0030]
[C: 0.05 to 0.40%]
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,
5 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.
[0031]
[Si: 0 to 0.50%]
Silicon (Si) is an element effective for deoxidizing steel. However, if the Si content is too
10 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
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.
15 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
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
20 level, is generally 0.05%.
[0032]
[Mn: 0.50 to 2.50%]
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
25 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.
[0033]
[P: 0.03% or Less]
Phosphorus (P) is an impurity contained in steel. P segregates at the crystal grain boundaries
30 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,
therefore the lower limit of the P content is 0%.
35 [0034]
[S: 0.010% or Less]
11
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
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
5 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%.
[0035]
[sol. Al: 0 to 0.10%]
Aluminum (Al) is effective for deoxidation of steel. However, excessive inclusion of Al
10 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
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).
15 [0036]
[N: 0.010% or Less]
Nitrogen (N) is an impurity unavoidably contained in steel. N forms nitrides to cause a drop
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.
20 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,
therefore the lower limit of the N content is 0%.
[0037]
25 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
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
30 the contents of the elements are 0%.
[0038]
[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 steel base material. However, even if including B
35 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.
12
[0039]
[Ti: 0 to 0.10%]
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
5 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
content is 0 to 0.10%. The Ti content may be 0.01% or more.
[0040]
10 [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
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
15 content is 0 to 0.50%. The Cr content may also be 0.10% or more.
[0041]
[Mo: 0 to 0.50%]
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
20 content may also be 0.05% or more.
[0042]
[Nb: 0 to 0.10%]
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
25 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.
[0043]
[Ni: 0 to 1.00%]
Nickel (Ni) is an element able to suppress embrittlement caused by molten Zn at the time of
30 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.
[0044]
In the steel base material according to the embodiment of the present invention, the balance
other than the above constituents is comprised of Fe and impurities. The “impurities” in the steel
35 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
13
stamped body according to the embodiment of the present invention, and not intentionally added
to the hot stamped body.
[0045]
[Plating Layer]
5 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
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
10 the interfacial layer. The plating layer has the following average composition.
[0046]
[Al: 15.00 to 45.00%]
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
15 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
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
20 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 45.00%,
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
25 becomes difficult. Therefore, the Al content is 45.00% or less, preferably 40.00% or less or
35.00% or less.
[0047]
[Mg: 4.50 to 12.00%]
Mg is an element effective for improving the corrosion resistance of the plating layer and
30 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
5.00% or more. On the other hand, if the Mg content is too high, an excessive sacrificial
35 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.
14
[0048]
[Si: 0.05 to 3.00%]
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
5 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
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
10 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
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
15 less.
[0049]
[Ca: 0.05 to 3.00%]
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
20 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
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
25 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.
[0050]
[Fe: 20.00 to 50.00%]
30 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 positioned on the interfacial layer. If the Fe content is
35 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
15
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%
or more, preferably 25.00% or more. On the other hand, if the Fe content is too high, the amount
of the Fe-Al containing phase in the main layer becomes greater and the amount of the Mg-Zn
5 containing phase in the main layer relatively decreases, therefore the corrosion resistance falls.
Therefore, the Fe content is 50.00% or less, preferably 45.00% or less, more preferably 40.00%
or less.
[0051]
The chemical composition of the plating layer is as explained above. Furthermore, the
10 plating layer may optionally contain one or more of Sb: 0 to 0.50%, 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%, 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.
15 Below, these elements will be explained in detail.
[0052]
[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
20 stamped body. However, if the contents of the elements are excessive, at the time of the heating
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
25 0.50% or less, preferably 0.20% or less, while the contents of Cu, Sn, and Ti are 1.00% or less,
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%.
[0053]
30 [Sr: 0 to 0.50%]
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
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
35 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
16
deteriorate. Therefore, the Sr content is 0.50% or less, preferably 0.30% or less, more preferably
0.10% or less.
[0054]
[Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%]
5 Cr, Ni, and Mn concentrate near the interface of the plating layer and the steel base material
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
10 coating blistering and flow rust become greater and the corrosion resistance tends to deteriorate.
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.
[0055]
[Balance: Zn and Impurities]
15 The balance in the plating layer aside from the above constituents is comprised of Zn and
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
20 content is preferably 3.00% or more. The lower limit of the Zn content may be 10.00%, 15.00%,
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
25 Zn can form a solid solution with the Fe in the Fe-Al containing phase. Further, the “impurities”
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
30 present invention.

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 45.00%,
Mg: 4.50 to 12.00%,
Si: 0.05 to 3.00%,
Ca: 0.05 to 3.00%,
10 Fe: 20.00 to 50.00%,
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 85.0% of an Mg-Zn containing phase and
15.0 to 90.0% of an Fe-Al containing phase,
25 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 and
an area ratio of the Fe-Al-Zn phase in the main layer is 10.0% or less.
30 [Claim 2]
The hot stamped body according to claim 1, wherein the chemical composition of the
plating layer comprises, by mass%,
Al: 20.00 to 30.00% and
Mg: 5.50 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 25.0 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
55.0%