Description
Title of Invention
AUTOMOBILE PART AND METHOD FOR MANUFACTURING AUTOMOBILE
5 PART
Technical Field
[OOOl]
10 The present invention relates to an automobile part and a method for
manufacturing the automohile parts.
Background Art
[0002]
15 Recently, it has been increasingly demanded to restrain the consumption of
fossil fuels in order to control global warming and protect the environment, which
has affected various manufacturing industries. For example, automobiles, which
are an indispensable part of transportation means in daily life and activities, are not
exception. There is a demand to improve fuel economy by, for example, reducing
20 vehicle body weight. It is not allowed, however, to simply reduce the vehicle body
weight by neglecting product qualities. It is necessary to secure appropriate safety.
[0003]
Many of the structural parts of an automohile are made of ferrous material,
in particular a steel sheet. For reducing the vehicle body weight, it is important to
25 reduce the weight of the steel sheet. lnstead of simply reducing the weight of the
steel sheet, which is not allowed as mentioned above, the weight reduction must he
accompanied with obtaining the mechanical strength of the steel sheet. Such
demand becomes higher not only in thc car manufacturing industry hut also in
various other manufacturing industries. Rescarch and development efforts have
30 been directed to a stcel sheet that can have the same or a larger mechanical strength
as compared to conventional one even -when the sheet is made thinner.
Our Ref.: NNSC14041-IN
2140
In general, a material having a high mechanical strength tends to become
lower in formability and shape lixability in shape formation work such as bending.
It is difficult to carry out the process for forming such material into a complicated
5 shape. One of the solutions to the formability problem is what is called "a hot
pressing method (also referred to as hot stamping, hot pressing, die quenching, or
press hardening)'?. In the hot pressing method, a material to be formed is heated
temporarily to a high temperature (in an austenite region) and the steel sheet soften
by the heating is formed by pressing. The steel sheet is then cooled. By using the
10 hot pressing method, the material is once soften by heating to a high temperature so
that the material is easy to be pressed. The mechanical strength of the material
becomes larger due to a quenching effect during cooling after the shaping.
Accordingly, the hot pressing can provide a product having both a good shape
fixability and a high mechanical strength.
15 [0005]
When the hot pressing method is applied to a steel sheet, however, iron and
other substances on the surface are oxidized to generate scales (oxides) due to
heating to a high temperature of, for example, 800°C or more. Accordingly, a
descaling process is necessary after hot pressing to remove the scales, which
20 deteriorates productivity. For the members and the like that require corrosion
resistance, it is necessary to carry out anti-corrosion treatment and metal cover
installation on the surfaces of the members after the shaping process. A surface
cleaning process and a surface treatment process are also necessary, which further
deteriorates productivity.
25 [0006]
As an example of restraining such deterioration in productivity, a covering
layer can be installed on a steel sheet. In general, various materials including
organic and inorganic materials are used for the covering layer on a steel sheet.
Among them, galvanized steel sheets that have a sacrificial protection effect on steel
30 sheets are widely used for steel sheets for automobiles and other products because
the galvanized steel sheets provide a good anti-corrosion effect and suitability to
Our Ref.: NNSC14041-IN
3/40
steel sheet production technology. Ilowever, this may cause to considerable
deterioration in thc surface properties because heating temperatures used in the hot
pressing (700 to 1000°C) arc higher than the temperatures at which the organic
materials decompose or the zinc boils so that the plating layer evaporates at a time of
5 heating by hot press.
[0007]
For this reason, it is desirable to use, for example, what is called an Alplated
steel sheet for the hot pressing that heats the steel sheet to high temperatures.
The AI-plated steel sheet is a steel sheet having an Al-based metal cover that has the
10 boiling point higher than that of an organic material cover or Zn-based metal cover.
[OOOS]
The Al-based metal cover can prevent scales from depositing on the surface
of the steel sheet, which leads to omitting a process such as the descaling process and
improving productivity. The Al-based melal cover also has an anti-corrosion eKect
15 so that the corrosion resistance of the steel sheet after coated with paint is improved.
Patent Literature 1 listed below discloses a method for using an Al-plated steel sheet
in hot pressing, the Al-plated steel sheet being obtained by covering a steel sheet
having predetermined steel components with Al-based metal, as explained above.
[0009]
20 However, in the case that the Al-based metal cover is applied like Patent
Literature 1, the A1 cover is melted and transformed into an A1-Fe compound due to
the dispersion of Fe from the steel sheet, depending on preheating conditions before
a pressing step in the hot pressing method. The A1-Fe compound grows until the
Al-Fe compound reaches to the surface of the steel sheet. The compound layer is
25 hereinafter referred to as the A1-Fe alloy layer. The Al-Fe alloy layer is so hard.
That is because the A1-Fe alloy layer is intrinsically not smooth on the surface and is
inferior in lubricity, comparatively. In addition, since the A1-Fe alloy layer tends to
break, develop cracks in a plating layer, and come off in a powder form. Moreover,
flaked materials from the Al-Fe alloy layer and coming-off materials by strong
30 abrasion on the AI-Fe surface attach on the dies The Al-Fe compound then adheres
to and deposits on the dies, which leads to deterioration in ihe quality of pressed
Our Ref: NNSC14041-IN
4140
products. To prevent this, it is necessary to remove Al-Fe alloy powder adhered to
the dies during maintenance, which is one of the causes for lowering productivity
and increasing the cost.
[OOI 01
5 Further~noret,h e A1-Fe alloy layer is less reactive in phosphate treatment so
that a chemical conversion coating (a phosphate coating), which is a treatment before
electrodeposition painting, is difficult to generated. Although the chemical
conversion coating is not formed, the Al-Fe alloy layer itself has a good coating
adhesion ability with paint so that corrosion resistance after coated with paint
10 becomes better if A1 plating amount is large enough. An increase in the amount,
however, tends to worsen the aforementioned adhesion to the dies.
[OOIl]
On the other hand, Patent Literature 2 listed below discloses a technique in
which a wurtzite-type compound is applied to the surface of an Al-plated steel sheet.
15 According to the Patent Literature 2 listed below, such a process improves in
lubricity in hot state and in chemical conversion treatability. This technique is
effective for improving lubricity and also corrosion resistance after coated with paint.
[OO 121
In addition, Patent Literature 3 listed below discloses a technique for
20 controlling the average section length of the crystal grains that are in an intermetallic
compound phase and contain A1 at an amount of 40% or more and 65% or less
among the crystal grains of A1-Fe that is a main ingredient of the intermetallic
compound phase formed on the surface of the steel sheet, and also for controlling the
thickness of thc intermetallic compound phase. The technique also includes
25 forming of a lubricating coating containing ZnO on the surface of the A1 plating layer.
In Patent Literature 3 listed below, the corrosion resistance after coated with paint
and the formability during hot stamping can be improved by using such techniques
30
Patent Literature
Citation List
Our Ref.: NNSC14041-IN
5140
[0013]
Patent Literature 1 JP 2000-38640A
Patent Literature 2 WO 20091131 233A1
Patent Literature 3 WO 20121137687Al
5
Summary of Invention
Technical Problem
[0014]
As described in the foregoing, the Al-plated steel sheet plated with A1
10 having the relatively high melting point is regarded as a promising member, for use
as an automobile steel sheet, etc., that requires corrosion resistance. Modified
techniques have been proposed in applying the Al-plated steel sheet to the process of
hot pressing.
[0015]
15 However, the above-described techniques known in the art have
presupposed that the film thickness of the electrodeposition painting has been
approximately 20 pm, which is relatively thick. However, in the electrodeposition
painting that uses a method of immersing an automobile body, the film thickness
affects cost largely. As a coated film of the electrodeposition painting has become
20 thinner recently, it is necessary to maintain the properties in the thinner
electrodeposition paint.
[0016]
Patent Literature 1 listed above does not mention electrodeposition painting
as is described above. Patent Literature 2 listed above indicates the thickness of the
25 electrodeposition painting to be 20 pm. In addition, Patent Literature 3 listed above
mentions a value of 1 to 30 pm as a thickness of the electrodeposition painting in
general. These known techniques have been fine as far as relatively thick
electrodeposition paints are presupposed, as described above. The situation
changes drastically, however, when it comes to the thickness of an electrodeposition
30 film being less than 15 pm.
[0017]
Our Ref.: NNSC14041-1N
6/40
More specifically, it is known that the surface roughness of a A1 plated steel
sheet is large after it is alloyed, which is substantially 2 pm as Ra in JIS B0601
(2001) (Ra is the arithmetic mean of roughness, which is the arithmetic mean of
height Sa as specified in IS0 25178). When the surface having a large surface
5 roughness is covered with a thin paint film, the actual paint film thickness on top of
peaks of the alloy layer becomes small. As a result, corrosion under the paint film
will start from the portions having a locally thin paint film. When a material has an
average mean of roughness Ra of 2 km, Rt (maximum profile height) according to
JIS B0601 (2001) becomes about 20 pm for the material. The maximum profile
10 height Rt of about 20 pm indicates that the peaks of about 10 pm may appear on the
surface of the material. The Present Inventors found that, in such a case, when the
film thickness of the electrodeposition painting is 14 pm, about 4pm thick portions
exist locally, and such portions may be corroded preferentially.
[00 1 81
15 Note that Patent Literature 3 listed above only discloses an example of
about 20 pm thick film alone of the electrodeposition painting in the embodiment,
and it is not known whether to stably obtain the effect disclosed in Patent Literature 3
listed above also in a region where the thickness of the electrodeposition painting is
less than 15 pm. In addition, Patent Literature 3 listed above does not disclose any
20 knowledge about the relationship between corrosion and the maximum profile height
Rt as described above.
[0019]
The present invention is achieved in view of the above-described problems,
and is directed to provide automobile pai-ts that have an excellent corrosion
25 resistance after coated with an electrodeposition paint film being less thick than ever
before, that improve formability and productivity in hot pressing work, and that
improve chemical conversion treatability after hot press-forming, and is also directed
to provide a method for manufacturing the automobile parts.
30 Solution to Problem
[0020]
Our Ref.: NNSC14041-IN
7140
As the rcsults of studies to solve the above-described problems, the Present
Inventors have found that a steel sheet comes to have a sufficient corrosion resistance
aRer coated with paint, even if the thickness of the electrodeposition paint film is less
than 15 pm, when the steel sheet is treated to have an intermetallic compound layer
5 formed of an A1-Fe intermctallic compound on the surface of the steel sheet, and has
a surface coating layer including a coating containing ZnO and a coating mainly
containing zinc phosphate on the surface of the intermetallic compound layer, and
when the surface roughness of the surface coating layer is controlled to have a
predetermined threshold value or less. The Present hventors have further found the
10 conditions of A1 plating and heating to achieve such surface roughness, and
subsequently achieved the present invention. The gist of the present invention
conceived on the basis of the above findings is as follows.
[002 11
(1)
15 An automobile part, including:
a formed steel sheet having an intermetallic compound layer formed on a
surface of the steel sheet, the intermetallic compound layer being formed of A1-Fe
intermetallic compound having a thickness of 10 pm or more and 50 pm or less, the
intermetallic compound layer including a diffusion layer positioned in closest
20 proximity to the steel sheet, the diffusion layer having a thickness of 10 pm or less;
a surface coating layer provided on a surface of the intermetallic coinpound
layer, the surface coating layer including a coating containing ZnO and a zinc
phosphate coating and having a surface roughness of 3 pm or more and 20 pm or less
as a maximum profile height Rt in accordance with JIS B060l (2001); and
25 an electrodeposition paint film provided on a surface of the surface coating
layer and having a thickness of 6 pin or inore and less than 15 pm.
(2)
The automobile part according to (I), wherein the maxiinum profile height
Rt is 7 pm or more and 14 pm or less.
30 (3)
The automobile part according to (1) or (2), wherein the ZnO has an average
Our Ref NNSC14041-IN
8/40
grain sire of 50 nm or more and 1000 nm or less in diameter.
(4)
The automobile part according to any one of (1) to (3), wherein a content of
ZnO is 0.3 g/m2 or more and 3 g/m2 or less in nletallic Zn equivalent for one surface.
5 (5)
The automobile part according to any one of (1) to (4), wherein the content
of ZnO is 0.5 g/m2 or more and 1.5 g/m2 or less in metallic Zn equivalent for one
surface.
(6)
10 The automobile part according to any one of (1) to (5), wherein the steel
sheet is an A1 plated steel sheet having an A1 plating layer formed on a surface of the
steel sheet serving as a base metal.
(7)
The automobile part according to (6), wherein the A1 plating layer has an
15 average primary crystal diameter of 4 pm or more and 40 pm or less.
(8)
The automobile part according to (6) or (7), wherein the A1 plating layer has
an average primary crystal diameter of 4 pm or more and 30 pm or less.
(9)
20 The automobile part according to any one of (6) to (S), wherein an amount
of the A1 plating layer is 30 g/m2 or more and 110 g/m2 or less for one surface.
(10)
The automobile part according to any one of (6) to (8), wherein an amount
of the A1 plating layer is 30 g/m2 or more and less than 60 g/m2 for one surface.
25 (11)
The automobile part according to any one of (6) to (8), wherein an amount
of the A1 plating layer is 60 g/m2 or more and 110 g/m2 or less for one surface.
(12)
A method for manufacturing an automobile part, the method including:
30 using an A1 plated steel sheet including a coating containing ZnO on a
surface of the A1 plzted steel sheet;
Our Ref.: NNSC14041-IN
9/40
usiug a hot pressing method;
causing an Al plating layer having an avcrage primary crystal diameter of 4
pm or more and 40 bm or less to have an amount of plating of 30 g/m2 or more and
11 0 g/m2 or less for one surface;
5 causing a ZnO amount of the A1 plating layer to be 0.3 g/m2 or more and 3
g/m2 or less in metallic Zn equivalent for one surface;
causing a rate of temperature increase during a heating process in hot
pressing to be 12"C/second or more;
causing a reaching steel sheet temperature to be 870°C or more and 1100°C
10 or less; and
causing a electrodeposition paint film to have thickness of 6 pm or more and
less than 15 km.
(13)
The method for manufacturing an automobile part according to (12),
15 wherein an amount of the Al plating layer is 50 g/m2 or more and 80 g/m2 or less for
one surface.
(14)
A method for manufacturing a high-strength automobile part, the method
including:
20 using an A1 plated steel sheet including a coating containing ZnO on a
surface of the A1 plated steel sheet;
using a hot pressing method;
causing an Al plating layer having an average primary crystal diameter of 4
pm or more and 40 pni or less to have an amount of plating of 30 g/m2 or more and
25 less than 60 g/m2 for one surface;
causing a ZnO amount of the Al plating layer to be 0.3 g/m2 or more and 3
g/m2 or less as metallic Zn for one surface;
causing a rate of temperature increase during a heating process in hot
pressing to be less than 12"C/second;
30 causing a reaching steel sheet temperahre to be 850°C or more and 950°C
or less; and
Our Ref.: NNSC14041-1N
10140
causing a electrodeposition paint film to have thickncss of 6 pm or more and
less than 15 pm.
(1 5)
The method for manufacturing an automobile part according to (14),
5 wherein an amount of the A1 plating layer is 35 g/m2 or more and 55 g1m2 or less for
one surface.
(16)
A method for manufacturing a high-strength automobile part, the method
including:
10 using an A1 plated steel sheet including a coating containing ZnO on a
surface of the A1 plated steel sheet;
using a hot pressing method;
causing an A1 plating layer having an average primary crystal diameter of 4
pm or more and 40 pm or less to have an amount of plating of 60 g/m2 or more and
15 110 g/m2 or less for one surface;
causing a ZnO amount of the A1 plating layer to be 0.3 dm2 or more and 3
g/m2 or less as metallic Zn for one surface;
causing a rate of temperature increase during a heating process in hot
pressing to be less than 12"C/seeond;
20 causing a reaching steel sheet temperature to be 920°C or more and 970°C
or less; and
causing a electrodeposition paint film to have thickness of 6 pin or more and
less than 15 pm.
(17)
25 The method for manufacturing an automobile part according to (16),
wherein an amount of the A1 plating layer is 60 g/m2 or more and 90 gild or less for
one surface.
(18)
The method for manufacturing an automobile part according to any one of
30 (12) to (17), wherein the content of ZnO is 0.5 g/m2 or more and 1.5 g1m2 or less in
metallic Zn equivalent for one surface.
Our Ref.: NNSC14041-IN
11140
(19)
The method for manufacturing an automobile part according to any one of
(12) to (18), wherein the A1 plating layer has an average primary crystal diameter of
4 pm or more and 30 pm or less.
5 (20)
The method for manufacturing an automobile part according to any one of
(12) to (19), further including:
treating the A1 plated steel sheet with chemical conversion by using a
chemical conversion liquid containing phosphates, before hot pressing.
10
Advantageous Effects of Invention
[0022]
As described above, the present invention can provide the automobile parts
that have an excellent corrosion resistance after coated with an electrodeposition
15 paint film being less thick than ever before, that improve formability and
productivity in hot pressing work, and that improve chemical conversion treatability
after hot press-forming, and also can provide the method of manufacturing such
automobile parts.
20 Brief Description of Drawings
[0023]
[FIG. 11 FIG. 1 is a cross-sectional photograph showing the cross-sectional
structure of a typical A1 plating layer.
[FIG. 21 FIG. 2 is a cross-sectional photograph showing a typical Al-Fe layer
25 and a diffusion layer.
[FIG. 31 FIG. 3 is a perspective view illustrating a shape of a hat-shaped
product manufactured in Example 1
Description of Embodiments
30 [0024]
Hereinafter, referring to the appended drawings, preferred embodiments of
Our Ref.: NNSC14041-IN
12140
the present invention will be described in detail. It should be noted that, in this
specification and the appended drawings, structural elements that have substantially
the same function and structure are denoted with the same reference signs, and
repeated explanation thereof is omitted.
5 [0025]
(Plated Steel Sheet)
A plated steel sheet according to an embodiment of the present invention
will be described.
A plated steel sheet according to the embodiment has a layered structure
10 including at least two layers on one surface or each of both surfaces of the steel sheet.
In other words, an A1 plating layer containing at least A1 is formed on one surface or
each of both surfaces of the steel sheet, and a surface coating layer containing at least
ZnO is further stacked on the A1 plating layer.
[0026]
15
For the steel sheet, it is desirable to use a steel sheet formed to have, for
example, a high mechanical strength (which refers to properties related to
mechanical deformation and failure, including, for example, tensile strength, yield
point, elongation, contraction of area, hardness, impact value, fatigue strength, creep
20 strength, etc.). A composition example of the steel sheet that achieves a high
mechanical strength and can be employed in an embodiment of the present invention
is described as follows
100271
For example, the steel sheet includes, in mass%, C: 0.1% or more and 0.4%
25 or less, Si: 0.01% or more and 0.6% or lcss, Mn: 0.5% or more and 3% or less, Ti:
0.01% or more and 0.1% or less, B: 0.0001% or more and 0.1% or less, and the
balance: Fe and impurities.
[0028]
Each component added to steel will now be explained. Note that the
30 term % represents "mass% unless otherwise stated
100291
I Oul. Ref.: NNSC14041 -IN
[C: 0.1% or more and 0.4% or less]
C is added to secure a target mechanical strength. A content of C of less
than 0.1% does not provide enough mechanical strength improvement, and makes C
addition less effective. In contrast, the content of C exceeding 0.4% makes the steel
5 sheet harden more, but is more likely to cause melting cracks. Accordingly, it is
preferable to add C at a content of, in mass%, 0.1% or more and 0.4% or less. The
content of C is more preferably 0.15% or more and 0.35% or less.
[0030]
[Si: 0.01% or more and 0.6% or less]
10 Si is one of the elements for improving mechanical strength and is added to
secure a target mechanical strength in a way similar to C. If the content of Si is less
than 0.01%, it is difficult to exhibit a strength-improving effect, and enough
mechanical strength is not obtained. In contrast, Si is an element that is easily
oxidized. Thus, the content of Si exceeding 0.6% lowers wettability during hot-&p
15 A1 plating, which is likely to cause the generation of non-plated portions.
Accordingly, it is preferable to add Si at a content of, in mass%, 0.01% or more and
0.6% or less. The content of Si is more preferably 0.01% or more and 0.45% or less.
[0031]
[Mn: 0.5% or more and 3% or less]
20 Mn is one of the elements for strengthening steel and also one of the
elements for increasing hardenability. Mn is also effective in preventing hotbrittleness
caused by S that is one of the impurities. A content of Mn of less than
0.5% does not provide such an effect, which is exhibited when the content of Mn is
0.5% or more. In contrast, the content of Mn exceeding 3% may lower strength due
25 to residual y-phase becoming excessive. Accordingly, it is preferable to add Mn at
a content of, in mass%, 0.5% or more and 3% or less. The content of Mn is more
preferably 0.8% or more and 3% or less.
[0032]
[Ti: 0.01% or more and 0.1% or less]
30 Ti is one of the elements for improving strength and also an clement for
inlproving the heat resistance of the Al plating layer. A content of Ti of less than
Our Ref: NNSC14041-IN
0.01% cannot provide a strength-improving effect or an oxidation-resistanceimproving
effect, while these effects arc achieved at a content of Ti of 0.01% or more.
In contrast, Ti is also an element that may soften steel by forming, for example,
carbides and nitrides if added excessively. In particular, if the content of Ti exceeds
5 0.1%, it is not likely to obtain a target mechanical strength. Accordingly, it is
preferable to add Ti at a content of, in mass%, 0.01% or more and 0.1% or less.
The content ofTi is more preferably 0.01% or more and 0.07% or less.
[0033]
[B: 0.0001% or more and 0.1% or less]
10 B is an element for improving strength by contributing to quenching. A
content of B of less than 0.0001% does not provide such a strength-improving effect
sufliciently. In contrast, the content of B exceeding 0.1% may lower fatigue
strength by forming inclusions and making a brittle steel sheet. Accordingly, it is
preferable to add B at a content of, in mass%, 0.0001% or more and 0.1% or less.
15 The content of B is more preferably 0.0001% or more and 0.01% or less.
[0034]
[Optional Element]
As optional elements other than the above-described elements, the steel
sheet contains, in many cases, Cr: 0.01% or more and 0.5% or less, Al: 0.01% or
20 more and 0.1% or less, N: 0.001% or more and 0.02% or less, P: 0.001% or more and
0.05% or less, S: approximately, 0.001% or more and 0.05% or less. Cr exhibits a
hardenability effect as is Mn, and A1 is applied as a deoxidizer. It is needless to say
that not all the optional elements must be added in the steel sheet.
[0035]
25 [Impurity]
Incidentally, the steel sheet may have impurities that comes to be inevitably
included in other manufacturing processes. Such impurities may include, for
exanlple, Ni, Cu, Mo, 0 and others.
[0036]
30 A steel sheet formed of such components is quenched after heated by, for
example, a hot pressing method so that the steel sheet may have a mechanical
Our Ref: NNSCl4041-IN
15/40
strength of about 1500 MPa or more. Although the steel sheet has such a high
mechanical strength, it can be shaped easily when the hot pressing method is used
because the steel sheet is soften by heating and is hot-pressed in a soft state.
Moreover, a high mechanical strength can be achieved for the steel sheet, and the
5 steel sheet can maintain or improve the mechanical strength even if the thickness of
the steel sheet is reduced for the purpose of weight reduction.
[003 71
< A1 plating layer >
The A1 plating layer is formed on one surface or both surfaces of the steel
10 sheet as described above. The Al plating layer may be formed on the surface of the
steel sheet by using, for example, a hot-dip plating method. The forming method of
the A1 plating layer according to the present invention, however, is not limited to
such an example.
[003 S]
15 The A1 plating layer contains A1 as a plating component, and also contains
Si in many cases. The content of Si in the plating composition can control an AI-Fe
alloy layer that is generated when a metal cover is formed by hot-dip plating. If the
content of Si is less than 3%, an A1-Fe alloy layer grows thick during Al plating,
which may aggravate crack development during working, and may negatively impact
20 on corrosion resistance. Ln contrast, the content of Si exceeding 15% may hamper
the workability and corrosion resistance of the plating layer. Accordingly, it is
preferable to add Si at a content of, in mass%, 3% or more and 15% or less.
[0039]
Elements present in the A1 plating bath, other than Si, include Fe at an
25 anlount of 2 to 4%, which is eluted from the equipment or steel strips in the plating
bath. In addition to Si and Fe, elements such as Mg, Ca, Sr, Li, etc., may be
included in the Al plating bath at an amount of approximately 0.01 to 1%.
[0040]
The A1 plating layer foimed of such components can prevent the steel sheet
30 from corroding. The A1 plating layer can also prevent the steel sheet from
generating the scales (i:-on oxides) that are generated by the oxidization of the stecl
Our Ref.: NNSCl4041-IN
sheet surfaces that are heated to a high temperature when shaping the steel sheet by
the hot pressing method. Accordingly, forming of such Al plating layer can omit
such processes as scale removing, surface cleaning, and surface treatment, and thus
can improve productivity. The A1 plating layer has the boiling point higher than
5 that of a plating cover formed by organic-based materials or by metal-based
materials (for example, Zn-based material). This allows the steel sheet to be shaped
at high temperature in the shaping work using the hot pressing method, which leads
to further improvement in formability during the hot pressing and also leading to
easiness in shaping.
10 [0041]
Note that an average primary crystal diameter in the A1 plating layer is 4 pm
or more and 40 pm or less. Incidentally, the average primary crystal diameter in the
A1 plating layer can be measured by observing a polished cross section using an
optical microscope. In the A1 plating, primary crystals are often Al, and eutectic
15 crystals of A1-Si (Al-Si eutectic crystals) solidify at an end stage of solidification.
Consequently, eutectic crystal portions made of Al-Si eutectic crystals are first
identified, and then a structure present between adjacent eutectic crystal portions can
be determined as the primary crystal portion made of the A1 primary crystal. With
the average primary crystal diameter in the A1 plating layer being in such a range, a
20 desired surface roughness is achieved in the surface coating layer, which will be
described later.
100421
FIG. 1 shows a cross-sectional structure of a typical A1 plating layer. By
observing the cross-sectional structure, the location of the primary crystal portions
25 can be determined. In FIG. 1, regions surrounded by dotted lines are the primary
crystal portions made of the A1 primary crystal, and a region present between
adjacent primary crystal portions is the eutectic crystal portion. Here, by converting
an ellipse representing the primary crystal portion into a circle having the area
equivalent to the ellipse, the diameter of the primary crystal portion (diameter of
30 circle) is to be obtained. In calculating an average of the diameters of primary
crystal portions obtained as described above, 10 diameters of the primary crystal
Our Ref: NNSC14041-IN
17/40
pollions in arbitral two field of views, in which 5 diameters are measured per one
field of view, are to be averaged.
[0043]
The average primary crystal diameter depends on the situation in which the
5 alloy (in other words, eutectic crystal portion) is generated, and also depends on the
cooling rate after plating. In reality, it is difficult to obtain a diameter of less than 4
pm. Consequently, the lower limit of the average primary crystal diameter is set at
4 km or more. On the other hand, when the average primary crystal diameter is too
large, which means the plating structure is partially not uniform, the partially
10 nonuniform plating structure tends to cause the surface irregularities to be larger after
heating. Consequently, the upper limit of the average primary crystal diameter is
set at 40 pm. The average primary crystal diameter is more preferably 4 pm or
more and 30 pm or less.
[0044]
15 An amount of the A1 plating may be (1) 30 g/m2 or more and 110 g/m2 or
less per surface, (2) 30 g/m2 or more and less than 60 g/m2 per surface, or (3) 60 g/m2
or more and 110 g/m2 or less per surface. In the hot pressing method according to
the embodiment of the present invention, a rate of temperature increase, a maximum
steel sheet temperature to be reached, and the like, in the heating process of the hot
20 pressing method are controlled according to the amount of the A1 plating, which will
be described later.
[0045]
Here, the amount indicated in (1) above is more preferably 50 g/m2 or more
and 80 g/m2 or less. The amount indicated in (2) above is more preferably 35 g/m2
25 or more and 55 g/m2 or less, and the amount indicated in (3) above is more
preferably 60 g/m2 or more and 90 g/m2 or less.
[0046]
Incidentally, the amount of the Al plating can be measured by using a
known method such as, for example, the fluorescent X-ray analysis. For example, a
30 calibration curve showing the relation between the intensity of fluorescent X-ray and
the amount is determined in advance by using specinlens of which the Al amount is
Our Ref.: NNSC14041-IN
18/40
known, and then the amount of the Al plating can be determined from the
measurement results of the intensity of fluorescent X-ray by using the calibration
curve.
[0047]
5 In the embodiment of the present invention, the above-described A1 plated
steel sheet is shaped into a part by hot forming. Thereby, the components of the A1
plating and the steel sheet are reacted during the hot forming, and change to an Al-Fe
based intermetallie compound. As the A1-Fe type or a type in which the AI-Fe type
contains Si, many compounds are known, and thus the alloyed plating layer has a
10 complicated structure. As a typical example, the alloyed plating layer has a
structure that is similar to 5 layers being stacked. Such a plating layer including a
plurality of alloyed layers is hereinafter referred to as an "intermetallie compound
layer".
[0048]
15 In the embodiment of the present invention, the thickness of a digusion
layer, which is located closest to the steel sheet in the Al-Fe layer (intermetallic
compound layer), is specified as 10 pm or less. FIG. 2 shows a typical Al-Fe layer
and a typical diffusion layer. A polished cross section is subjected to nital etching
to obtain such a cross-sectional structure. Here, an intermetallie compound layer
20 according to the embodiment of the present invention has a structure that is similar to
5 layers a to e being stacked as shown in FIG. 2 by way of example, and the layers d
and e together are defined as a "diffusion layer". Note that the number of layers in
the intermetallic compound layer in the embodiment of the present invention is not
limited to five as shown in FIG. 2 by way of example. Even if the intermetallic
25 compound layer has layers other than five, the first and the second layer in the
intennetallie compound layer, which are located closest to the steel sheet, can be
regarded as the diffusion layer.
[0049]
The thickness of the diffusion layer is specified as 10 pm or less. This is
30 because spot weldability is dependent on this thickness. The thickness of the
diffusion layer exceeding 10 pm tends to generate welding dust and causes the
Our Ref.: NNSC14041-IN
19/40
proper range of welding current to be narrower. Although the lower limit of the
thickness of the diffusion layer is not specified here, the diffusion layer of I pm or
more in thickness is normally present, and thus 1 pm practically becomes the lower
limit.
5 [0050]

The surface coating layer is layered on the surface of an A1 plating layer as
described above. The surface coating layer contains at least ZnO. The surface
coating layer may be formed by using a liquid in which ZnO particles are suspended
10 in an aqueous solution and applying the suspension onto the A1 plating with a roll
coater, etc. The surface coating layer provides an effect of improving lubricity in
hot pressing and reactivity in the reaction with a chemical conversion liquid.
[0051]
Besides ZnO, the surface coating layer may contain, for example, an organic
15 binder component. A water-soluble resin such as, for example, polyurethane resin,
polyester resin, acrylic resin, and a silane coupling agent may be used as the organic
binder component. As oxides besides ZnO, the surface coating layer may contain,
for example, SiO2, TiO2, and A1203, etc.
[0052]
20 The methods for applying the suspension may include, for example, a
method in which the above-described suspension containing ZnO is mixed with a
predetermined organic binder and is applied on the surface of the A1 plating layer,
and a method for applying by using powder coating.
[0053]
25 Although a grain size (average grain size) of ZnO is not specifically limited
here. it is preferable to have a grain size of, for example, approximately 50 nm or
more and 1000 nm or less in diameter, and more preferably, 50 nm or more and 400
nm or less. Note that the grain size of ZnO is defined as a grain size after hot
pressing. Typically, the grain size is to be determined by observation with a
30 scanning electron microscope (SEM) or an equivalent device after undergoing the
process in which a sample is retained in a furnace at 900°C of a sheet temperature for
Our Rcf.: NNSC14041-IN
20140
5 to 6 minutes and rapidly cooled with dies. The organic contents in the binder is
decon~posed during hot pressing, and only oxides remain to exist in the surface
coating.
[0054]
5 Although the amount of the surface coating including ZnO is not
specifically limited, it is preferable to be 0.3 g/m2 or more and 3 g/m2 or less in
metallic Zn equivalent for one surface of the steel sheet. The ZnO amount of 0.3
g/m2 or more in metallic Zn equivalent can efficiently provide effects such as
lubricity improvement, etc. In contrast, if the amount of ZnO exceeds 3g/m2 in
10 metallic Zn equivalent, the thickness of the above-described A1 plating layer and the
surface coating layer becomes excessive, thereby deteriorating weldability. Thus, it
is preferable that the surface coating layer on one surface contains ZnO of 0.3 g/m2
or more and 3 g/m2 or less in metallic Zn equivalent. A ZnO amount of 0.5 g/m2 or
more and 1.5 g/m2 or less is especially preferable. By keeping the ZnO amount in a
15 range of 0.5 g/m2 or more and 1.5 g/m2 or less, the lubricity in hot pressing is secured,
and weldability and paint adhesion become better as well. The surface coating
layer may contain, besides ZnO and the binder, compounds such as, for example, Mg,
Ca, Ba, Zr, P, B, V, and Si.
[0055]
20 Methods for baking and drying after coating application, which use, for
example, an air-heating furnace, an induction heating furnace, a near infrared ray
furnace, and the like, may be utilized separately or in combination. Depending on
the type of binder used in coating application, hardening treatment may be carried
out by using, for example, ultraviolet ray, electron beam, or the like, instead of the
25 baking and drying afler coatiug application. The baking temperature after coating
application is approximately in a range of 60 to 200°C in many cases. The methods
of forming the surface coating layer is not limited to such examples, but can include
various other methods.
[0056]
30 When the binder is not used, the adhesion of coating after apphcd onto the
A1 plating layer and before heating is slightly low and the coating may be coming off
Our Ref NNSClIO41 IN
21/40
when rubbed strongly.
[0057]
Now, a zinc phosphate coating will be described.
In a typical painting process for automobiles, an immersion-type chemical
5 conversion is carried out before electrodeposition painting. The chemical
conversion is carried out by using a known chemical conversion liquid containing
phosphates. The chemical conversion causes zinc in the coating, including ZnO, to
react with phosphates contained in the chemical conversion liquid to form a zinc
phosphate coating on the surface of the steel sheet on which the A1 plating layer and
10 the surface coating layer have been formed. The zinc phosphate coating improves
adhesion to a paint film and also contributes to the corrosion resistance after coated
with paint. For example, in the case of a known A1 plated steel sheet as described in
Patent Literature 1 listed above, the alloyed Al-Fe surface, which is covered with a
stiff Al-oxide coating, has exhibited a low reactivity with the chemical conversion
15 liquid. Patent Literature 2 listed above describes a technique to improve the
reactivity with the chemical conversion liquid. The zinc phosphate coating
(chemical conversion coating) similar to that described in Patent Literature 2 listed
above is also used in the embodiment of the present invention. Depositing the
coating containing ZnO improves the reactivity between the A1 plated steel sheet and
20 the chemical conversion liquid, enabling the zinc phosphate coating to be formed.
[OOSS]
The amount of zinc phosphate coating is governed almost by the content of
ZnO. When the coating containing ZnO has ZnO of 0.3 g/m2 or more and 3 g/m2 or
less for one surface in metallic Zn equivalent, the coating amount of zinc phosphate
25 becomes approximately 0.6 g/m2 or more and 3 g/m2 or less for one surface.
Although the zinc phosphate coating is formed on the surface of the surface coating
layer, it is difficult to distinguish the zinc phosphate coating from the surface coating
layer in a part product. Consequently, the thickness is regarded as a total thickness
of the surface coating layer and the zinc phosphate coating in the part product. The
30 total thickness of the surface coating layer and the zinc phosphate coating is
app:oximately 0.5 pm or more and 3 pm or less when the ZnO amolint for one
Our Ref.: NNSC14041 -IN
22140
surface is 0.3 g/m2 or more and 3 g1m2 or less in metallic Zn equivalent
[0059]
Incidentally, the ZnO amount of the surface coating layer and the coating
amount of zinc phosphate can be measured by using a known analysis method such
5 as the fluorescent X-ray analysis. For example, calibration curves showing the
relation between the intensity of fluorescent X-ray and the amounts are determined in
advance by using specimens of which the amount of Zn and the amount of
phosphorus are known, and the ZnO amount and the coating amount of zinc
phosphate can be determined from the measurement results of the intensity of
10 fluorescent X-ray by using the calibration curves.
[0060]
(Processing Using Hot Pressing Method)
The plated steel sheet according to the embodiment, which can be
preferably utilized as a raw material of an automobile part according to the
15 embodiment of the present invention, has so far been described. The plated steel
sheet that is formed in a manner as described above is especially useful when the
plated steel sheet is subjected to the processing in which the hot pressing method is
used. Thus, the case in which the plated steel sheet having the above-described
configuration is processed by using the hot pressing method will be described below.
20 [0061]
In the hot pressing method according to the embodiment, the plated steel
sheet is heated first to a high temperature to soften the plated steel sheet. The
softened plated steel sheet is pressed and shaped, and then the shaped plated steel
sheet is cooled. The temporarily-softened plated steel sheet can make the following
25 pressing work easier. The plated steel sheet having the aforementioned components
is, by undergoing heating and cooling, quenched to obtain a high mechanical strength
of about 1500 MPa or more.
[0062]
The plated steel sheet according to the embodiment is heated in the hot
30 pressing method. As the heating method in the hot pressing method, a heating
method using as a typical electric furnace, a radiant tube furnace, or infrared heating
Our Ref NNSC14041-IN
23140
can be utilized
100631
In the heating, the A1 plated steel sheet melts at the melting point or a
temperature higher than the melting point and, at the same time, changes into an Al-
5 Fe-based A1-Fe alloy layer (in other words, intermetallic compound layer) due to
counter diffusion with Fe. The A1-Fe alloy layer has the high melting points, i.e.,
around 1150°C. A plurality of species of such Al-Fe compounds and Al-Fe-Si
compounds that includes Si additionally exist and are transformed into compounds
having a higher Fe concentration by heating to a high temperature or heating for a
10 long period of time. The surface state desirable for a final product is that alloying
proceeds to the surface and, at the same time, the Fe concentration in the alloy layer
is not high. If unalloyed A1 remains to exist, the portion in which unalloyed A1
remains corrodes rapidly, resulting in being quite vulnerable to cause blistering of the
paint coating in terms of the corrosion resistance after coated with paint, which is not
15 desirable. On the other hand, if the Fe concentration in the Al-Fe alloy layer
becomes too high, the corrosion resistance of the A1-Fe alloy layer itself becomes
lower, which also results in being vulnerable to cause blistering of the paint coating
in terms of the corrosion resistance after coated with paint. This is because the
corrosion resistance of the A1-Fe alloy layer depends on the A1 concentration in the
20 alloy layer. Consequently, there exist a desirable alloying state in terms of the
corrosion resistance after coated with paint, and the alloying state is determined
based on the A1 amount of plating and the heating conditions.
roo641
Moreover, in the embodiment of the present invention, the A1 plated steel
25 sheet, which has a coating containing ZnO (in other words, surface coating layer), is
formed using hot pressing, in which surface roughness after forming becomes
important. In terms of controlling the surface roughness after the A1-Fe alloy layer
is formed, it is important to control three factors such as the amount of A1 plating, a
rate of temperature increase, and a reaching steel sheet temperature.
30 [0065]
An especially influcncing factor is the rate of temperature increase. The
Our Ref.: NNSC14041 -IN
24140
surface roughness can be reduced by increasing temperature at a temperaturc
increase rate of 12"C/second or more, irrespective of the amount of A1 plating and
the steel sheet temperature to be reached. Here, the rate of temperature increase is
the average rate of temperature increase from 50°C to "a reaching steel sheet
5 temperature-30°C". With this temperature increase pattern, the amount of Al
plating is set at 30 g1m2 or more and 110 g1m2 or less. The reason is that the amount
of plating of less than 30 g/m2 causes the corrosion resistance provided by the A1
plating to be not enough, while the amount of plating of more than 110~/mca~u ses
excessively thick plating, which tends to come off and adhere to dies during forming.
10 The amount of Al plating is more preferably 50 g'rn2 or more and 80 g1m2 or less.
The upper limit of the rate of temperature increase is not specified here, but it is
difficult to obtain a rate of temperature increase of 30O0C/second or more even by
using a method such as electric heating, etc. With this temperature increase pattern,
the rate of temperature increase is preferably 12"C/second or more and 150"CIsecond
15 or less. In addition, with this temperature increase pattern, the reaching steel sheet
temperature is set at 870°C or more and 1100°C or less although it does not affect the
surface roughness. When the reaching steel sheet temperature is less than 870°C, it
may not complete alloying. On the other hand, when the reaching steel sheet
temperature exceeds 1 10O0C, the alloying proceeds excessively, which may cause a
20 defect in the corrosion resistance.
[0066]
In contrast, if the rate of temperature increase is less than 12"C/second, the
surface roughness varies, depending on the amount of A1 plating and the reaching
steel sheet temperature. There is a tendency in which the surface roughness
25 becomes smaller when the amount of A1 plating is smaller. Consequently, with this
temperature increase pattern, the amount of A1 plating is set at 30 g1m2 or more and
less than 60 g/m2 for one surface. In addition, when the plated steel sheet with this
amount of A1 plating is heated at a rate of temperature increase of less than 12"C, the
reaching steel sheet temperature is set at 850°C or more and 950°C or less. In this
30 case, it is difficult to obtain the corrosion resistance if the amount of A1 plating is less
than 30 g1m2. In addition, the reaching steel sheet temperature of less thzn 850°C
Our Ref.: NNSC14041-IN
25140
may cause insufficient hardness alter quenching, whereas the reaching steel sheet
temperature of more than 950°C causes the diffusion of A1-Fe to progress too far,
which deteriorates the corrosion resistance. In this temperature increase pattern, the
lower limit of the rate of temperature increase is not specified, but the rate of
5 temperature increase of less than l0C/second lacks economic rationality dramatically,
regardless of the amount of plating. Moreover, in this temperature increase pattern,
the amount of A1 plating is preferably 35 g/m2 or more and 55 g/m2 or less, the
reaching steel sheet temperature is preferably 850°C or more and 900°C or less, and
the rate of temperature increase is preferably 4"C/second or more and 12"C/second
10 or less.
[0067]
In contrast, if the rate of temperature increase is less than 12"C/second, and
the amount of A1 plating is large, the surface roughness tends to be larger, and thus it
is important to strictly control the reaching steel sheet temperature. When the
15 reaching steel sheet temperature is high, the surface roughness tends to he small.
Thus, when the amount of A1 plating is 60 g/m2 or more and 110 g/m2 or less for one
surface, it is important to control the reaching steel sheet temperature to be 920°C or
more and 970°C or less with this temperature increase pattern. When the amount of
A1 plating exceeds 1l0g/m2 for one surface, excessively thick A1 plating tends to
20 come off and may adhere to the dies during forming. On the other hand, when the
reaching steel sheet temperature is less than 920°C, the surface roughness tends to
become large, and it is difficult to maintain the corrosion resistance when the
clectrodeposition paint film is thin. The amount of A1 plating is more preferably 60
g/m2 or more and 90 g/m2 or less. The lower limit of the rate of temperature
25 increase is not specified here, but the rate of temperature increase of less than
l0C/second lacks economic rationality dramatically, regardless of the amount of
plating. In addition, with this tenlperature increase pattern, the reaching steel sheet
temperature is preferably 940°C or more and 970°C or less, and ihe rate of
temperature increase is preferably 4"CIsecond or more and 12"C/second or less.
30 [0068]
When the amount of A1 plating is set at 30 g/m2 or more and 110 g/m2 or
Our Ref: NNSC14041-IN
less, the thickness of the A1-Fe alloy layer (in other words, the thickness of the
intermetallic compound layer) in a hot-p~essed part product becomes approximately
10 pm or more and 50 pm or lcss. Accordingly, it is preferable that the thickness of
the A1-Fc alloy layer falls in this range.
5 [0069]
Next, the reason to limit the surface roughness after hot pressing will be
described. The embodiment of the present invention provides parts having a better
corrosion resistance after coated with paint by controlling the surface roughness to
have a specified value or less as described above when the thickness of the
10 electrodeposition paint film is less than 15 pm. As an index of the surface
roughness, a maximum profile height (Rt) according to JIS B0601 (2001) (JIS B0601
(2001) is a standard corresponding to IS0 4287), is used. The maximum profile
height(Rt) is defined as the sum of the maximum peak height and the maximum
valley depth in a length to be evaluated in a roughness curve. This value roughly
15 corresponds to the difference between the maximum value and the minimum value in
the roughness curve. In the high-strength automobile parts according to the
embodiment of the present invention, the maximum profile height Rt of the surface
coating layer is set at 3 pm or more and 20 pm or less. It is not practically possible
to make the maximum profile height Rt less than 3 pm, and thus the lower limit is set
20 at this value. If the maximum profile height Rt exceeds 20pm, corrosion starts to
occur from a thin portion of the electrodeposition paint film, which is generated due
to surface irregularities, and thus the upper limit is set at 20 pm. The maximum
profile height Rt of the surface coating layer is more preferably 7 pm or more and 14
pm or less.
25 [0070]
(Example of Effect by Plated Steel Sheet and Hot Pressing Method)
The plated steel sheet to be used for the automobile parts according to the
embodiment of the present invention, and the hot pressing method for the plated steel
sheet, have so far been described. The automobile part formed using the plated
30 steel sheet according to the embodiment has the surface coating layer containing
ZnO, zinc phosphate, etc., so that, for example, a high degree of lubricity is achieved
Our Ref.: NNSC14041-IN
and chemical conversion treatability is improved, as described above.
[0071]
The reason why ZnO contributes to the adhesion of the chemical conversion
coating is that the chemical conversion reaction is triggered and made to proceed by
5 the etching reaction in which acid reacts with a material. On the other hand, ZnO
itself is an amphoteric compound and is solved in acid so that ZnO reacts with the
chemical conversion liquid.
[0072]
(Automobile Parts)
10 The above-described A1 plated steel sheet is subjected to the abovedescribed
hot pressing work so that the automobile parts according to the
embodiment of the present invention are manufactured. The automobile part has
the intermetallic compound layer formed of the A1-Fe intermetallic co~npound of 10
pm or more and 50 pm or less in thickness on the surhce of the formed steel sheet
15 (steel sheet as the base metal), and the thickness of the diffusion layer located closest
to the steel sheet in the intermetallic compound layer is 10 pm or less. In addition,
the surface coating layer including the coating containing ZnO and the zinc
phosphate coating is provided on the surface of the intermetallic compound layer,
and the surface roughness of the surface coating layer is 3 pm or more and 20 pm or
20 less as a maximum profile height Rt in accordance with JIS B0601 (2001).
Moreover, the electrodeposition paint film having a thickness of 6 pm or more and
less than 15 pm is provided on the above-described surface coating layer. This
automobile part exhibits a high mechanical strength such as, for example, 1500 MPa
or more.
25 [0073]
Incidentally, the electrodeposition paint film to be formed on the surface of
the surface coating layer is not specifically limited, but a known electrodeposition
paint fill11 can be formed by using a known method. The thickness of the
elecirodeposition paint film is desirably 8 pm or more and 14 pnl or less. The
30 surface coating layer of the automobile part according to the embodiment of the
present invention has a very flat surface whose surface roughness is 3 pm cr more
Our Ref.: NNSCl4041-IN
28/40
and 20 pm or less as a maximum profile height Rt. Thereby, the automobile part
can stably provide excellent effects such as excellent corrosion resistance after
coated with paint, excellent formability and productivity in the hot pressing work,
and excellent chemical conversion treatability after hot press-forming, even if the
5 electrodeposition paint film is made very thin as described above.
[Examples]
[0074]
The automobile part according to the embodiment of the present invention
10 will now be described more specifically with reference to examples. Note that
Examples as described below are merely examples of the automobile part according
to the embodiment of present invention, and the automobile part according to the
embodiment of present invention is not limited to those examples described below.
[0075]
15
In Example 1, a cold-rolled steel sheet (sheet thickness of 1.2 mm) having
steel composition as shown in Table 1 was used, and the cold-rolled steel sheet was
plated with Al. The annealing temperature used was about 800°C. The A1 plating
bath contained Si: 9% and an about 2% amount of Fe that had been eluted from steel
20 strips. The amount after plating was adjusted, by using a gas wiping method, to 20
g/m2 or more and 120 g/m2 or less for one surface. After the plated steel sheet was
cooled, the suspension, which contained ZnO of which a particle diameter was about
50 nm, and an acrylic binder of which the amount was 20% as a ratio to the ZnO
amount, was applied with a roll coater, and the plated steel sheet was baked at about
25 80°C. The amount was set in the range of 0.1 g/m2 or more and 4 g/m2 or less as an
amount of metallic Zn. The average primary crystal diameter was adjusted by
changing the amount of plating and the cooling rate. The average primary crystal
diameter was calculated by the method described above by observing a cross-section
of the structure using an optical microscope
30 [0076]
[Table 11
Our Ref.: NNSC14041-IN
29140
[0077]
5 The plated steel sheet was hot-stamped on the conditions as described below.
Table 1 Steel Conlponents of Specimens (unit: mass%)
There were employed two heating methods: a method in which the plated steel sheet
was inserted into an air atmosphere furnace being set at a constant temperature, and a
C
method in which a far-infrared ray furnace having two zones. In the latter method,
one zone was kept at 1150°C and the other zone was kept at 900°C. The plated
10 steel sheets were heated to 800°C in the 1150°C furnace, and then transferred to the
900°C furnace. Thermocouples were welded to each of the plated steel sheets to
actually measure the sheet temperature, and the average rate of temperature increase
from 50°C to "a reaching steel sheet temperature - 3 0 "C was measured.
[0078]
15 After the reaching steel sheet temperature and the sample holding time at
0.22 1 0.13 1 1.20 0.005 1 0.002 0.02 0.004 0.03
Si
the reaching steel sheet temperature were adjusted, the plated steel sheet was pressed
into a hat shape, and was quenched by cooling it for 10 seconds at the bottom dead
center. Subsequently, a sample was cut out from the hat-shaped product to evaluate
Mn I P
the corrosion resistance. FIG. 3 illustrates the shape of the product used at that time
20 and a cut-out portion. The cut-out sample was subjected to chemical conversion
treatment using a chemical conversion liquid (PB-SX35) containing phosphates
available from Nihon Parkerizing Co., Ltd. The sample was then coated with
electro-deposition paint (Powemics 110) available from Nippon Paint Co., Ltd. so as
S
to target the film thickness for 5 pm or more and 20 pm or less, and the sample was
25 bakedat17O0C.
[0079]
The corrosion resistance after coated with paint was evaluated in accordance
with JASO M609 established by the Society of Automotive Engineers of Japan,
The samplc was subjected to a corrosion test of 180 cycles (60 days) with the edges
Ti B AI
Our Ref.: NNSC14041-IN
30140
of the sample being sealed and with no scratch being provided on the paint film.
Corrosion condition after the test was observed and evaluated according to a criteria
listed below. As a comparative sample, an alloyed hot-dip galvanized steel sheet of
45 g/m2 on one side was cold-formed into the hat shape and was evaluated in a
5 similar way. The result was "B".
[OOSO]
A: with red rnst, no blistering
B: with red rust, a blistered area of 3% or less
C: with red rust, a blistered area of 5% or less
10 D: with red rust, a blistered area exceeding 5%
[0081]
In addition, the surface roughness (Rt) was measured for the samples that
had undergone chemical conversion in accordance with JIS B0601 (2001). The
thickness of the diffusion layer was then determined by observing, with an optical
15 microscope, the cross-section of the sample that had been treated by 3% nital etching
after observing the pretreated cross-section with the microscope.
[0082]
After the hat forming, the detachment of AI-Fe from the internal surface of
an R portion (compressive stress portion) was observed. The degree of the
20 detachment was then evaluated by visual observation. Such detachment is not
desirable because the Al-Fe detached from the compressive stress portion adheres to
the die and causes press products to be scratched.
[OOS3]
A: almost no detachment
25 B: small detachment
C: large detachment
[0084]
For spot weldability, a 1.4 millimeter-thick flat sheet was heated and
subjected to die quenching under thc heating conditions same as the hat forming test.
30 Proper range of welding current was evaluated for this sample at 12 cycle with a
single-phase AC current source (60 Hz) and a pressure of 400 kgf (I kgf
Our Ref.: NNSC14041-lN
3 1/40
approximately equals to 9.8 N). The evaluation was conducted using a criteria
listed below with the lower limit set at 4 x (t)' (t is thickness) and the upper limit set
at dust generation.
[0085]
A: proper range is 1.5 kA or more
B: proper range is less than 1.5 kA
[0086]
Obtained results were summarized in Table 2. In this Table, the amount of
plating and the amount of ZnO are both for one surface, and the amount of ZnO is
expressed as an amount of metallic Zn. As the surface coating layer, the coating
containing ZnO and the coating containing zinc phosphate have been confirmed to be
formed in any of the samples corresponding to the present invention.
LO0871
[Table 21
Our Ref.: NNSC14041-IN
[OOSS]
In Table 2, it is shown that excellent corrosion resistance after coated with
Our Ref.: NNSC14041-IN
paint is exhibited when the amount of A1 plating, the ZnO amount, the average
primary crystal diameter, the rate of temperature increase, the reaching steel sheet
temperature, and the thickness of the electrodeposition paint film are appropriate.
However, a sufficient corrosion resistance is not obtained in the cases in which, for
5 example, the amount of Al plating is small (no. I), the ZnO amount is sinall (no. 30),
the electrodeposition paint film is excessively thin (no. 31), the average primary
crystal diameter is excessively large (no. 32). In addition, the corrosion resistance
is reduced in the case in which the reaching steel sheet temperature is excessively
low (no. 10) or excessively high (no. 11). In no. 11, the reaching steel sheet
10 temperature is too high, which causes A1-Fe itself to melt so that the surface
roughness becomes large. When the rate of temperature increase is small, an
appropriate range of the reaching steel sheet temperature varies depending on the
amount of A1 plating. Especially when the amount of plating is thick and the
reaching steel sheet temperature is set at around 900°C (no. 29), the surface
15 roughness increases, and thus sufficient corrosion resistance cannot be obtained. It
has become apparent that, in such a case, it is thus necessary to set the reaching steel
sheet temperature higher (no. 21, no. 22).
[0089]
Heretofore, preferred embodiments of the present invention have been
20 described in detail with reference to the appended drawings, but the present invention
is not limited thereto. It should be understood by those skilled in the art that various
changes and alterations may he made without departing from the spirit and scopc of
the appended claims.
[0090]
25 As described in the foregoing, owing to the present invention, the lubricity
has become better and the workability has improved in carrying out hot pressing of
the Al-plated steel sheet, which enables more complicated pressing. Also enabled
are labor saving in maintenance work of hot pressing equipment and an increase in
productivity. The paint coating and the corrosion resistance of finished products are
30 confirmed to improve because the chemical conversion treatahility of the processed
products after hot pressing becomes better. In view of the above, the present
Our Ref.: NNSC14041-IN
34/40
invention is sure to expand the application range of hot pressing of Al-plated steel
and to enhance applicability of Al-plated steel materials to final products such as
automobiles and industrial machines.
Our Ref.: NNSC14041-IN
35140
CLAIMS
Claim 1
An automobile part, comprising:
a formed steel sheet having an intermetallic compound layer formed on a
surface of the steel sheet, the intermetallic compound layer being formed of Al-Fe
intermetallic compound having a thickness of 10 pm or more and 50 pm or less, the
intermetallic compound layer including a diffusion layer positioned in closest
proximity to the steel sheet, the diffusion layer having a thickness of 10 pm or less;
a surface coating layer provided on a surface of the intermetallic compound
layer, the surface coating layer including a coating containing ZnO and a zinc
phosphate coating and having a surface roughness of 3 pm or more and 20 Fm or less
as a maximum profile height Rt in accordance with JIS B0601 (2001); and
an electrodeposition paint film provided on a surface of the surface coating
layer and having a thickness of 6 pm or more and less than 15 pm.
Claim 2
The automobile part according to claim 1, wherein the maximum profile
height Rt is 7 pm or more and 14 pm or less.
Claim 3'
The automobile part according to claim 1 or 2, wherein the ZnO has an
average grain size of 50 nm or more and 1000 nm or less in diameter.
Claim 4
The automobile part according to any one of claims 1 to 3, wherein a
content of ZnO is 0.3 g/m2 or more and 3 g/m2 or less in metallic Zn equivalent for
one surface.
Claim 5
The automobile part according to any one of claims 1 to 4, wherein the
content of ZnO is 0.5 glm2 or morc and 1.5 g/m2 or less in mctallic Zn equivalent for
! Our Ref.: NNSC14041-IN I
I 36/40
one surface.
Claim 6
The automobile part according to any one of claims 1 to 5, wherein the steel
5 sheet is an Al plated steel shect having an A1 plating layer formed on a surface of the
steel sheet serving as a base metal.
Claim 7
The automobile part according to claim 6, wherein the Al plating layer has
10 an average primary crystal diameter of 4 pm or more and 40 pm or less.
Claim 8
The automobile part according to claim 6 or 7, wherein the A1 plating layer
has an average primary crystal diameter of 4 pm or more and 30 pm or less.
15
Claim 9
The automobile part according to any one of claims 6 to 8, wherein an
amount of the A1 plating layer is 30 g/m2 or more and 110 g/m2 or less for one
surface.
20
Claim 10
The automobile part according to any one of claims 6 to 8, wherein an
amount of the Al plating layer is 30 g/m2 or more and less than 60 g/m2 for one
surface.
25
Claim 11
The automobile part according to any one of claims 6 to 8, wherein an
amount of the Al plating layer is 60 g/m2 or more and 110 g/m2 or less for one
surface.
30
Claim 12
Our Ref.: NNSC14041-IN
37140
A method for manufacturing an automobile part, the method comprising:
using an A1 plated steel sheet including a coating containing ZnO on a
surface of the A1 plated steel sheet;
using a hot pressing method;
5 causing an Al plating layer having an average primary crystal diameter of 4
pm or more and 40 pm or less to have an amount of plating of 30 g/m2 or more and
110 g/m2 or less for one surface;
causing a ZnO amount of the A1 plating layer to be 0.3 g/m2 or more and 3
g1m2 or less in metallic Zn equivalent for one surface;
10 causing a rate of temperature increase during a heating process in hot
pressing to be 12"C/second or more;
causing a reaching steel sheet temperature to be 870°C or more and 1100°C
or less; and
causing a electrodeposition paint film to have thickness of 6 pm or more and
15 less than 15 wm.
Claim 13
The method for manufacturing an autoinobile part according to claim 12,
wherein an amount of the A1 plating layer is 50 g/m2 or more and 80 g/m2 or less for
20 one surface.
Claim 14
A method for manufacturing a high-strength automobile part, the method
comprising:
25 using an A1 plated steel shcet including a coating containing ZnO on a
surface of the A1 plated steel shcet;
using a hot pressing method;
causing an A1 plating layer having an average primary crystal diameter of 4
pm or morc and 40 pm or less to have an amount of plating of 30 g/m2 or more and
30 less than 60 g/m2 for one surface;
causing a ZnO amount of the A1 plating layer to be 0.3 g/m2 or more and 3
Our Kef.: NNSC14041-IN
38140
g/m2 or less as ruetallic Zn for one surface;
causing a rate of temperature increasc during a heating process in hot
pressing to be less than 12"C/second;
causing a reaching steel sheet temperature to be 850°C or more and 950°C
5 or less; and
causing a electrodeposition paint film to have thickness of 6 pm or more and
less than 15 bm.
Claim 15
10 The method for manufacturing an automobile part according to claim 14,
wherein an amount of the A1 plating layer is 35 g/m2 or more and 55 g/m2 or less for
one surface.
Claim 16
15 A method for manufacturing a high-strength automobile part, the method
comprising:
using an A1 plated steel sheet including a coating containing ZnO on a
surface of the A1 plated steel sheet;
using a hot pressing method;
20 causing an A1 plating layer having an average prirnary crystal diameter of 4
pm or more and 40 pm or less to have an amount of plating of 60 g/m2 or more and
11 0 g/m2 or less for one surface;
causing a ZnO amount of the A1 plating layer to be 0.3 g/m2 or more and 3
g/m2 or less as metallic Zn for one surface;
25 causing a rate of temperature increase during a heating process in hot
pressing to be less than 12"C/second;
causing a reaching steel sheet temperature to be 920°C or inore and 970°C
or less; and
causing a electrodepositioil paint film to have thicklless of 6 pm or more and
30 less than 15 pm.
Our Ref.: NNSC14041.IN
Claim 17
The method for manufacturing an automobile part according to claim 16,
wherein an amount of the A1 plating layer is 60 glm2 or more and 90 g/m2 or less for
one surface.
5
Claim 18
The method for manufacturing an automobile part according to any one of
claims 12 to 17, wherein the content of ZnO is 0.5 dm2 or more and 1.5 g/m2 or less
in metallic Zn equivalent for one surface.
10
Claim 19
The method for manufacturing an automobile part according to any one of
claims 12 to 18, wherein the A1 plating layer has an average primary crystal diameter
of 4 pm or more and 30 pm or less. *
15
Claim 20
The method for manufacturing an automobile part according to any one of
claims 12 to 19, further comprising:
treating the A1 plated steel sheet with chemical conversion by using a
20 chemical conversion liquid containing phosphates, before hot pressing.