Description
Title of Invention
AI-PLATED STEEL SHEET USED FOR HOT PRESSING AND METHOD FOR
5 MANUFACTURING A1-PLATED STEEL SHEET USED FOR HOT PRESSING
Technical Field
[OOOl]
The present invention relates to a plated steel sheet used for hot pressing
10 that has an A1 plating cover mainly made of Al, and a method for manufacturing an
Al-plated steel sleet used for hot pressing.
Bgckground Art
[0002]
15 In recent years, it has been increasingly demanded to suppress the
consumption of fossil fuels in order to prevent global warnling 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 an exception. There is a demand to itnprove file1 economy by, for
20 example, reducing vehicle body weight. It is not allowed for automobiles, howevet;
to simply reduce the vehicle body weight in terms of product filnctions. It is
necessary to secure appropriate safety.
[0003]
Many of the structural parts of an a~rtomobile are 'made of an iron-based
25 ~natesial (in particular, a steel sheet). For reducing the vehicle body weight, it is
inlpostant to reduce the weight of the steel sheet. Instead of simply reducing the
weight of the steel sheet, which is not allowed as mentioned above, the weight
reduction must be accompanied with securing the mechanical strength of the steel
sheet. Such demand arises not only in the car manufacturing industry but also in
30 various other n~anufacturing industries. Research and developn~ent efforts have
been directed to a steel sheet that can have, due to improved mechanical strength, the
same or a higher mechanical strength as compared with a conventional one even
when the sheet is made thinner.
[0004]
In general, a material having a high mechanical strength tends to become
5 lower in shape fixability in shape fortnation work such as bending. It is difficult to
form such material into a con~plicated shape. One of the solntions to the
fornlability problem is what is called a "hot pressing method (also referred to as hot
stamping, hot pressing, or die quenching)". In the hot pressing method, a material
to be formed is heated temporarily to a high tet~peraturea nd the steel sheet softened
10 by the heating is formed by pressing, and then the steel sheet is cooled. By using
the hot pressing method, the material is once softened by heating to a high
temperature, so that the material is easy to press. Furthermore, the mechanical
strength of the material becomes higher due to a quenching effect during cooling
after the shaping. Accordingly, the hot pressing method can provide a product
15 having both a favorable shape fixability and a high mechanical strength.
[OOOS]
When the hot pressing method is applied to a steel sheet, however, the
surface of the steel sheet is oxidized to generate scales (conlpounds) due to heating
the steel sheet to a high temperature of 800°C or higher. Accordingly, a process to
20 remove the scales (descaling process) is necessary after hot pressing, which
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 treatment on the surfaces of the members after the work process. A
surface cleaning process and a surface treatment process are also necessary, which
25 further deteriorates productivity.
[0006]
As an exarnple of a method for suppressing 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
30 a steel sleet. Especially, galvanized steel sheets that have a sacrificial protection
cffect on steel sheets are wvidely used for steel sheets for automobiles and other
products because the galvanized steel sheets provide anti-corrosion perfor~~~anancde
suitability to steel sheet production tecl~nology. However, this may cause
considerable deterioration in the surface properties because heating temperatures
used in the hot pressing (700 to 1000°C) are higher than the tenlperatnres at which
5 the organic materials deco~nposeo r the metal (e.g., zinc) melts or boils, so that the
surface coating and the plating layer evaporate at the time of heating by hot pressing.
[0007]
For this reason, it is desirable to use a steel sheet having an Al-based metal
cover that has the boiling point higher than that of an organic material cover or a
10 zinc-based metal cover (i.e., an Al-plated steel sheet) for the hot pressing method that
involves high temperature heating.
[OOOS]
The Al-based metal cover can prevent scales fiom being generated on the
surface of the steel sheet, which leads to omitting a process such as descaling and
15 improving the productivity of products. The Al-based metal cover also has an anticorrosion
effect and thus improves corrosion resistance. Patent Literature 1 below
discloses a method in which hot pressing is performed on a steel sheet obtained by
covering a steel sheet having a predetermined component composition with Al-based
metal.
20 [0009]
However, in the case where the Al-based metal cover is applied as in Patent
Literature 1, the A1 cover is melted and then an AI-Fe alloy layer is generated due to
the dispersion of Fe fiom the steel sheet, depending 011 preheating conditions before
hot pressing. Fu~-thennore, the A1-Fe alloy layer grows until the AI-Fe alloy layer
25 reaches the surface of the steel sheet in some cases. This AI-Fe alloy layer is so
hard that there is a problem in that scratches are formed in the product by contacting
with dies during the pressing work.
[OOl 01
The AI-Fe alloy layer is not smooth on the surface and is inferior in lubricity.
30 In addition, since the AI-Fe alloy layer is hard and tends to break, cracks are
developed in a plating layer and powdering occurs, for example, resulting in lower
formability. Moreover, flaked materials from the AI-Fe alloy layer and coliling-off
materials by strong abrasion on the surface of the Al-Fe alloy layer of the stccl sheet
attach on the dies. The A1-Fe intermetallic cotnpoutid caused by the Al-Fe alloy
layer then adheres to tlie dies, \vl~ichle ads to deterioration in the quality of products.
5 To prevent this, it is necessary to periodically relilove the Al-Fe illterllletallic
cornpound adhered to the dies, ~vliich is one of the causes for lowering the
productivity of products and increasing the production cost.
[OOll]
Furtherriiore, the AI-Fe alloy layer is less reactive in phosphate treatment.
10 Therefore, a chemical cotiversion coating (a phosphate coating), which is a treatment
before electrodeposition coating, cannot be generated on the surface of the AI-Fe
alloy layer. Even when a chemical conversion coating is not generated, cossosion
resistance after paint coating would be favorable if adhesion with paint is favorable
and A1 deposition amount is large enough. However, an increase in Al deposition
15 amount leads to a larger amount of tlie Al-Fe intermetallic compound adhered to the
dies.
[OO 121
The adhesion of the A1-Fe intermetallic compound includes the case where
the flaked materials from the A1-Fe alloy layer attach on the dies and the case where
20 the coming-off materials by strong abrasion on the surface of the Al-Fe alloy layer
attach on the dies. In performing hot pressing on a steel sheet having a surface
coating, an increase in lubricity makes an improvenlent for the case where comingoff
materials by strong abrasion on the surface of the Al-Fe alloy layer attach on the
dies, but this increase in lubricity is not effective for an improvement for the case
25 \vliere flaked materials from the Al-Fe alloy layer attach on the dies. To alleviate
the attachment of the flaked materials from the AI-Fe alloy layer on tlie dies, it is
most effective to lower the Al deposition amount in the A1 plating. Ho\\lever,
loxtering the A1 deposition amount causes deterioration in the corrosion resistance.
[00 131
30 Hence, Patent Literature 2 below discloses a steel sheet with an objective to
prevent scratches from occurring in a product during work. In the steel sheet
disclosed in Patent Literature 2, an Al-based metal cover is fornled on the surface of
a steel sheet having a predetennined component conlposition, and, on the surface of
the Al-based metal cover, there is formed a coating made of an inorganic comnpound
containing at least one of Si, ZI; Ti, and P, an organic conlpound, or a complex
5 compound thereof. For the steel sheet with a surface coating fornled thereon as
disclosed in Patent Literature 2, the surface coating does not come off during
pressing after heating, so that the surface coating can prevent scratches from being
formed during pressing.
[0014]
10 Patent Literature 3 below discloses a method in which a wwrtzite comnpound,
particularly zinc oxide, is formed on the surface of an Al-plated steel sheet. The
method disclosed in Patent Literature 3 is a method that improves lubricity in hot
state and chemical conversion treatability, and adds a binder component to a surface
coating it1 order to secure the adhesion of the surface coating before hot pressing.
15
Citation List
Patent Literah~re
[00 151
Patent Literature 1:
Patent Literature 2:
Patent Literahue 3:
Summary of Invention
Technical Proble~n
25 [0016]
Unforh~nately, the surface coating disclosed in Patent Literaturc 2 cannot
provide enough lu~bricity during pressing, and there is a demand to improve a
lubricant, for examnple. In addition, for the method disclosed in Patent Literature 3,
zinc oxide itself does not have enough electrical conductivity, \vhich tends to
30 deteriorate spot weldability.
[00 171
The present invention has been made in view of such circumstances, and
aims to provide an Al-plated steel sheet used for hot pressing and a method for
manufacturing an Al-plated steel shect used for hot pressing that provide excellent
lubricity in hot state, corrosion resistance after paint coating, and spot weldability.
5
Solution to Proble~n
[OO 1 81
Based on the results of intensive studies to solve the aforementioned
problems, the present inventors have found that by uniformly depositing a zinc
10 compound, particularly zinc oxide, with a deposition amount within a certain range
on a surface of an Al-plated steel sheet, it is possible to significantly i~nprove spot
weldability while securing lubricity in hot state and corrosion resistance after paint
coating.
The gist of the present invention based on this finding is described as
15 follows.
[0019]
(1)
An Al-plated steel sheet used for hot pressing, including:
a steel sheet;
20 an A1 plating layer formed on one surface or both surfaces of the steel sheet;
and
a surface coating layer formed on the Al plating layer,
wherein the surface coating layer contains a zinc compound, and the zinc
compound is deposited on the Al-plated steel sheet in an amount of equal to or
25 greater than 0.3 g/~na2n d equal to or less than 1.5 g/m2 as an atnount of metallic zinc
per one smface, and a zinc deposition amount in arqr continuous 1 nnn 2 l.eg l.o ll is 1.5
g/n12 or less.
(2)
The Al-plated steel sheet used for hot pressing according to (I),
30 wherein the zinc colnpontld in the surface coating layer is zinc oxide.
(3)
The Ai-plated steel sheet used for hot pressing according to (1) or (2),
wwri~erein an average length RSni of a profile element in a roughness curve of
a surface of the Al-plated steel sheet used for hot pressing after the fannation of the
surface coating layer is SO0 pm or less.
5 (4)
A method for ~nanufacturinga n Al-plated steel sheet used for hot pressing,
including the steps of:
applying an application liquid containing a zinc con~ponnd and having a
contact angle of equal to or greater than 20" and equal to or less than SOo with
10 respect to the Al-plated steel sheet onto the Al-plated steel sheet including a steel
sheet and the A1 plating layer formed on one surface or both surfaces of the steel
sheet; and
drying the Al-plated steel sheet onto whicl~ the application liquid has been
applied, at a peak metal temperature of equal to or higher than 60°C and equal to or
15 lower than 200°C, to form a surface coating layer containing the zinc compound on
the A1 plating layer.
(5)
The method for manufacturing an Al-plated steel sheet used for hot pressing
according to (4),
20 wvherein a surface tension of the application liquid at 2S°C is equal to or
greater than 20 mNlm and equal to or less than 60 mN/rn.
(6)
The method for manufacturing an Al-plated steel sheet used for hot pressing
according to (4) or (S),
25 wherein a viscosity of the application liquid at 25°C is equal to or greater
than 2 mPa.s and equal to or less than 20 mPas.
(7)
The tnetllod for manufacturing an Al-plated steel sheet used for hot pressing
according to any one of (4) to (6),
30 wherein the application liquid is applied onto the Al plating layer with a roll
coater,
(8)
The method for t~~auufacturinagn Al-plated steel sheet used for hot pressing
according to ally one of (4) to (7),
wherein the zinc compound is zinc oxide.
5 (9)
The method for manufactnriag an Al-plated steel sheet used for hot pressing
according to any one of (4) to (8),
wherein in the manufactured surface coating layer, the zinc compound is
deposited on the Al-plated steel sheet in an amount of equal to or greater than 0.3
10 g/m2 and equal to or less than 1.5 g/m2 as an amount of metallic zinc per one surface,
and a zinc deposition amount in any continuous 1 mrn 2l.eg l.o n is 1.5 g/m2 or less. '
(10)
The method for manufacturing an Al-plated steel sheet used for hot pressing
according to any one of (4) to (9),
15 wherein an average length RSm of a profile element in a roughness curve of
a surface of the manufactured Al-plated steel sheet used for hot pressing after the
formation of the surface coating layer is 500 pm or less.
Advantageous Effects of Invention
20 [0020]
According to the present invention, a plated steel sheet used for hot pressing
that has excellent lubricity in hot state, corrosion resistance after paint coating, and
spot weldability can be provided, and prod~~ctiviitny a hot pressing process can be
improved.
Brief Description of Drawiugs
[0021]
[FIG. I] FIG. 1 is a schematic diagram illustrating a Bo\vden tester.
Description of Embodiments
Hereinafter, preferred cmbodilllcnts of the prcscnt invention will be
described in detail with reference to the appended drawings.
In the following dcscription, " % Iileans mass% unless otlierwise
specified.
5 [0023]
(Plated steel sheet used for hot pressing)
First, description is given on a plated steel sheet used for hot pressing
according to an embodiment of the present invention. The plated steel sheet used
for hot pressing according to the present embodiment has an A1 plating layer fanned'
10 on one surface or both surfaces of a steel sheet, and a surface coating layer
containing a zinc compound is further fonned on a surface of the Al plating layer.
[0024]

As a steel sheet before plating, it is desirable to use a steel sheet having a
15 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
strength, etc.). Described below is an example of a steel sheet before plating, wliich
is used for the steel sheet used for hot pressing according to the present embodiment.
20 [0025]
First, a component coniposition ~vilbl e described.
[0026]
The conlponent composition of the steel sheet before plating preferably
contains, in mass%, C: 0.1 to 0.4%, Si: 0.01 to 0.6%, and Mn: 0.5 to 3%.
25 Furtliern~ore, the component cor~lposition of tlie steel sheet before plating may
contain at least one of Cr: 0.05 to 3.0%, V: 0.01 to 1.0%, Mo: 0.01 to 0.3%, Ti: 0.01
to 0.1%, and B: 0.0001 to 0.1%. The balance of the compone~ltc on~positiono f the
, steel slleet before plating includes Fe and impurities.
[0027]
30 [C: 0.1 to 0.4%]
C is contained to secure a desired n~eclianicals trength of the steel sheet. A
co~ltento f C of less than 0.1% does not provide enough n~echa~iicastlr ength. In
contrast, the C content exceeding 0.4% makes the steel sheet harden but is more
likely to cause inelting cracks. Accordingly, the C content is preferably 0.1 to 0.4%.
[0028]
5 [Si: 0.01 to 0.6%]
Si is an element for improving the meclianical strength of the steel sheet and
is contained to secure a desired mechanical strength of the steel sheet in a way
similar to C. If the Si content is less than 0.01%, it is difficult to exhibit a strengthimproving
effect and obtain mechanical strength improved enough. Si is also an
10 element that is easily oxidized and thus Si content exceeding 0.6% lowers wettability
during hot-dip A1 plating, which is likely to cause the generation of non-plated
portions. Accordingly, the Si content is preferably 0.01 to 0.6%.
[0029]
[Mn: 0.5 to 3%]
15 Mn is an element for improving the mechanical strength of the steel sheet
and also an element for increasing hardenability. Mti is also effective in preventing
hot-brittleness caused by S, which is an impurity. A content of Mn of less than
0.5% does not provide such effects. In contrast, the Mn content exceeding 3% may
lower strength due to residual y-phase becoming excessive. Accordingly, the Mn
20 content is preferably 0.5 to 3%.
[0030]
[Cr: 0.05 to 3.0%]
[V: 0.01 to 1.0%]
[Mo: 0.01 to 0.3%]
25 Cr, V, and Mo are elements for improving mechanical properties of the steel
sheet and also an elenlent for suppressing generation of pearlite during cooling fiotn
an annealing teml~erature. A content of Cr of less than 0.05%, a content of V of less
than 0.01%, or a content of Mo of less than 0.01% does not provide such effects. In
contrast, the Cr content exceeding 3.0%, the V content exceeding 1.0%, or the Mo
30 content exceeding 0.3% makes an area ratio of hard phase excessive and causes
deterioration in formability. Accordingly, the Cr content is preferably 0.05 to 3.0%,
the V content is preferably 0.01 to 1.0%, and the Mo content is preferably 0.01 to
0.3%.
[0031]
[Ti: 0.01 to 0.1%]
5 Ti is an element for improving the mechanical strength of the steel sheet and
also an element for improving the heat resistance of the A1 plating layer. A Ti
content of less than 0.01% cannot provide a nnechanical-strength-inlproving effect or
an oxidation-resistance-improving effect. In contrast, Ti may soften steel by
forming carbides and nitrides if contained excessively. In particular, if the Ti
10 content exceeds 0.1%, a desired mechanical strength cannot be obtained.
Accordingly, tlle Ti content is preferably 0.01 to 0.1%.
[0032]
[B: 0.0001 to 0.1%]
B is an element for improving strength by contributing to quenching. A
15 content of B of less than 0.0001% does not provide such strength-improving effect.
In contrast, the B content exceeding 0.1% may lower fatigue strength by forming
inclusions in the steel sheet and causing embrittlement. Accordingly, the B content
is preferably 0.0001 to 0.1%.
[0033]
20 [Optionally added element]
The conlponent composition of the steel sheet before plating described
above is merely an example, and another component conlposition may be en~ployed.
For exatnple, the conlponent co~nposition of the steel sheet before plating may
contain, as a deoxidation element, Al: 0.001 to 0.08%. In addition, the component
25 composition of the steel sheet before plating may contain impurities that are
inevitably mixed in from manufacturing processes, etc.
[0034]
A steel sheet before plating with such a component composition is quenched
after heated by, for example, a hot pressing method, so that the steel sheet may have
30 a tensile strength of approximately 1500 MPa or more, even after plating. Altl~oough
the steel sheet has such a high terlsiie strength, when the hot pressing method is used,
it can be shaped easily in a softened state by heating. Moreover, the obtained
product can have a high n~echanical strength, and the steel sheet can maintain or
improve the mechanical strength even if its thickness is reduced for the purpose of
weight reduction.
5 [0035]

The A1 plating layer is formed on one surface or both surfaces of the steel
sheet before plating. The deposition amount of this A1 plating layer is preferably,'
for example, 20 g/~n2to 120 dm2 per one surface. The A1 plating layer is fonned
10 on one surface or both surfaces of the steel sheet by a hot-dip plating method, for
example; the method for forming the A1 plating layer is not limited thereto.
[0036]
The component compositioti of the A1 plating layer may be any component
composition containing at least 50 mass% of Al, and includes a component
15 composition to which an element other than A1 is added to improve characteristics of
the plating layer. Although the element other than Al is not particularly limited, Si
may be contained positively in the A1 plating layer for the following reason.
[0037]
When Si is contained in the A1 plating layel; an Al-Fe-Si alloy layer is
20 generated at the interface between the plating and base iron, suppressing generation
of a weak Al-Fe alloy layer during hot-dip plating. A Si content of less than 3%
makes an AI-Fe alloy layer grow tlick at the stage of applying Al plating, thus
promoting crack developn~enti n the plating layer during work, which may adversely
affect corrosion resistance. A Si content exceeding 15%, on the other hand,
25 increases the volutne ratio of a layer containing Si, and may cause a decrease in the
workability and corrosion resistance of the plating layer. Accordingly, the Si
content in the A1 plating layer is preferably 3% to 15%.
[0038]
The A1 plating layer prevents corrosion of the steel sheet used for hot
30 pressing according to the present embodiment. In addition, in the case ~vliere the
steel sheet for hot pressing according to the present embodiment is subjected to work
by the hot pressing method, the surface is not oxidized to generate scalcs (iron
compounds) even when the steel sheet is heated to a high temperature. The A1
plating layer preventing generation of scales allows omission of such processes as
scale removing, surface cleaning, and surface treatment, and thus can iniprove the
5 productivity of products. The A1 plating layer has the boiling point and melting
point higher than those of a plating cover fornied by organic-based materials or by
other metal-based materials (e.g., a zinc-based material). This allows the steel sheet
to be shaped at high temperature in the shaping work using the hot pressing method,
because tl~e cover does not evaporate, which leads to fi~~theimr provement in
10 formability during the hot pressing, leading to easiness in shaping.
[0039]
The A1 plating layer can be alloyed with Fe contained in the steel sheet, by
heating during hot-dip plating and hot pressing. Accordingly, the A1 plating layer is
not necessarily formed as one single layer having a constant componer~ct omposition,
15 and may contain a partially alloyed layer (alloy layer) therein.
[0040]

The surface coating layer is fornied on the surface of the A1 plating layer.
The surface coating layer contains a zinc compound. Examples of the zinc
20 con~pound include various compounds, such as zinc oxide, zinc hydroxide, zinc
sulfate, zinc nitrate, zinc phosphate, zinc acetate, zinc citrate, zinc oxalate, zinc
oleate, and zinc gluconate; it is pa~ticularlyp referable to use zinc oxide. These zinc
compounds provide an effect of iinproving lubricity during hot pressing and
reactivity in the reaction with a cliemical conversion liquid. The surface coating
25 layer may contain only one kind of these zinc colnpounds, or may use a plurality of
qinc conlpounds in mixture.
[0041]
Described below is an example in which the surface coating layer contains
zinc oxide. It should be noted that the same description applies to a case where the
30 surface coating layer contains any of the above zinc compou~ldso ther than zinc oxide.
[0042]
The surface coating layer contaitiing zinc oxide can be fornicd on the A1
plating layer through, for cxanlple, a process of applying a paint containing zinc
oxide particles and a hardening process by baking and drying after tlie application.
Examples of a method for applying zinc oxide include a method in which a sol
5 containing zinc oxide is mixed with a predetermined organic binder and applied onto
the surface of the aluminum plating layer, and a method of applying by powder
coating. The predeternlincd organic binder may be, for exaniple, polyurethane resin,
polyester resin, acrylic resin, or a silane coupling agent. These organic binders are
water-soluble so as to be dissolved with the sol containing zinc oxide. An
10 application liquid obtained in this nlanner is applied onto the surface of the
aluminum-plated steel sheet.
[0043]
The size of zinc oxide is not particularly limited, but the size is preferably,
for example, approximately 50 titn to 300 nm in diameter. Tlie grain size of zinc
15 oxide includes two types, the grain size of powder itself, and the grain size in the sol
when powder is made into a sol; in the present embodiment, the grain size in the sol
is described. 111 general, subsequent precipitation of fine powder occurs in the sol,
which makes the grain size in the sol larger than the grain size of powder itself.
When the grain size of powder itself is less than 50 ntn, kneading is difficult, and
20 also subsequent precipitation easily occurs, resulting in coarsening. Therefore, it is
practically difficult to make the grain size in the sol less than 50 tim. When the
grain size in the sol exceeds 300 nm, particles easily precipitate, which causes
unevenness. The grain size in the sol of zinc oxide is further preferably
approximately 50 nm to 150 ~m. Note that tlie grain size in the sol of zinc oxide
25 can be measured by a known method, such as a dynanlic light scattering method, an
induced grating method, or a laser diffraction and scattering method.
[0044]
A content of the binder conlponent, such as a resin cotnponent andlor a
silane coupling agent, in the surface coating is preferably approxitnately 5 to 30% as
30 a tnass ratio of the binder component to zinc oxide. A binder content of less than
5% does not provide the binder effect sufficiently and tends to cause tlie separation
of the paint coat, and further, as will be described latel; may greatly affect lubricity
because holes do not occur after organic solvent evaporation. To obtain the binder
effect stably, it is more preferable to contain the binder co~nponenat t a mass ratio of
10% or more. On the other hand, if the content of the binder conlponent exceeds
5 30%, odor generation during heating becotnes noticeable, which is not preferable.
[0045]
It has also been found that tlie content of the binder component in the above
range improves surface lubricity during hot pressing. This is presumably because
the evaporation of the organic solvent of the binder at the heating stage causes holes
10 to occur in the zinc oxide coating, bringing zinc oxide having a lubricating effect and
die metal into point contact.
[0046]
A deposition amount of zinc oxide needs to be equal to or greater than 0.3
g/m2 and equal to or less than 1.5 g/n12 as an amount of metallic zinc per one surface
15 of the steel sheet. If the deposition amount of zinc oxide is less than 0.3 g/m2 of
metallic zinc per one surface of the steel sheet, lubricity in hot state and corrosion
resistance after paint coating are not enough. If the deposition atnount of zinc oxide
exceeds 1.5 g/m2 of metallic zinc per one surface of the steel sheet, tlie thickness of
the surface coating layer becomes excessive, which deteriorates spot weldability.
20 Accordingly, for the surface coating layer on one side, zinc oxide needs to be formed
on the surface of the aluminum plating layer with a deposition atnount of equal to or
greater than 0.3 g/1ii2 and equal to or less than 1.5 g/m2 as an amount of metallic zinc.
In particular, the deposition amount of zinc oxide is preferably approxi~nately 0.6
g/rn2 to 1.0 g/m2 as an amount of metallic zinc; in such a range, lubricity during hot
25 pressing can be secured and weldability is favorable.
[0047]
Here, the deposition amount of the zinc co~npound can be measured as an
amount of metallic zinc by a kno\vn method, such as fluorescent X-ray spectroscopy.
For example, a sample whose deposition amount of metallic zinc is already kno\vn is
30 used, a calibration curve indicating the relation bet~vee~fliu orescent X-ray intensity
and deposition atnount is created in advance, and the deposition amount of ~netallic
zinc is determined from tlie measurement result of fluorescellt X-ray intellsit): by
using the calibration curve.
100481
Further~llore, for zinc oxide, it is necessary that the deposition amount of
5 zinc in ally colltinuous 1 lnm2 region does not exceed 1.5 g/m2. If the deposition
alllount of zinc oxide in any continuous 1 mm2 lilicro region exceeds 1.5 gh2, an
imbalatlce is caused it1 current distribution at the time of spot welding, \vhich
deteriorates weldability. Note that tlie value of the deposition amount of zinc in any
cotltinuous 1 m2 region is not particularly prescribed as lotig as it is 1.5 g/ni2 or less.
10 The lower limit value of the deposition amount of zinc in any continuous 1 mm2
region is not particularly prescribed, but is preferably, for example, 0.3 g/m2 or
greater.
[0049]
Here, tlie deposition amount of zinc it1 any continuous 1 mm 2 t.e gl.o n means
15 the maximum deposition amount of zinc in a measured area, obtained by performing
mapping analysis of zinc distribution on the surface of the Al-plated steel sheet by
using an electron probe microanalyser (EPMA). The measurement area is set to 1
mm2 or greater, and the Ineasurenlellt pitch is set to equal to or greater than 10 pm
and equal to or less than 100 pm. Performing mapping analysis with the
20 measurenlent area and measurement pitch decided makes it possible to find whether
the zinc compound is uniforn~lyd eposited on the surface of the Al-plated steel sheet.
[OOSO]
Such a surface coating layer containing zinc oxide can improve the lubricity
of tlie plated steel sheet. In particular, tlie surface coating layer containitlg zinc
25 oxide can fi~rtlierim prove lubricity and further improve forn~abilitya nd productivit):
as compared with the coating made of an inorganic compound containing at least one
of Si, Zr, Ti, and P, an organic compound, or a complex conlpound thereof disclosed
in Patent Literature 2.
[OOSl]
30 An average length RSm of a profile element in a roughness cunTe of the
surface of the Al-plated steel sheet used for hot pressilig according to the present
enlbodinlent after the forrlnation of the surface coating is preferably 500 pm or less.
When RSm is 500 pm or less, current distribution at the time of spot welding is
further dispersed and i~nprovement in weldability can be expected, which is
preferable. To make RSm 500 pm or less, any method may be used; for exan~ple,
5 the Al-plated steel sheet before the fomlation of the coating may be s~tbjectedto skin
pass depression. This RSm is further preferably equal to or greater than 50 prn and
equal to or less than 300 pm. The value of RSm is preferably as small as possible,
but RSm of less t11a11 50 prn requires a large cost to achieve the value, while
producing little effect. Note that since A1 plating melts during hot pressing, it
10 seems meaningless to control RSm to improve spot weldability after heating;
however, during heating, the binder component in the surface coating burns to leave
only zinc oxide before the plating melts, and zinc oxide fornls the coating with a
shape reflecting the unevenness of the base (that is, the A1 plating surface).
Accordingly, it is effective to control RSm of the A1 plating surface. Note that the
15 average length RSm of the profile element in the roughness curve of the surface after
the fortnation of the surface coating can be measured in accordance with JIS B0633
and JIS B 0601, by using a known surface roughness measuring machine.
[0052]
The reason for the surface coating layer containing zinc oxide being able to
20 in~prove lubricity is presumably the melting point of zinc oxide. That is, the
melting point of zinc oxide is approximately 1975"C, \vhic11 is higher than the
melting point (approximately 660°C) of the aluminu~n plating layer, Therefore,
even if the steel sheet is heated to SOO°C or higher, for example, when the plated
steel sheet is subjected to work by the hot pressing method, the surface coating layer
25 containing zinc oxide does not melt. This suppresses adl~esiont o dies, t11us cutting
the process to remove Al-Fe powder adhered to the dies and improving productivity,
[0053]
The surface coating layer containing zinc oxide has excellent corrosion
resistance after paint coating presumably because zinc oxide is partly dissol\red in a
30 chemical conversion liqnid, raises the pH of the plating surface, and for~ns a
chen~icacl onversion coating.
[0054]
Detailed description has been given above on tlie zinc compound contained
in tlic surface coating layel; taking zinc oxide as an example. The above-described
effects produced by containing zinc oxide are produced si~nilarlya lso in the case
5 where a zinc compound other than zinc oxide is contained.
[OOS5]
(Method for manufacturing plated steel sheet used for hot pressing)
Next, a method for manufacturing a plated steel sheet used for hot pressing
according to an embodiment of the present invention will be described.
10 [0056]
In the method for manufacturing a plated steel sheet used for hot pressing
according to the present embodiment, an Al-plated steel sheet including a steel sheet
and an A1 plating layer formed on one surface or both surfaces of the steel sheet is
used. Then, onto the A1 plating layer of this Al-plated steel sheet, an application
15 liquid containing a zinc compound and having a contact angle of equal to or greater
than 20" and equal to or less than 50" with respect to the Al-plated steel sheet is
applied. After that, the Al-plated steel sheet onto which the application liquid has
been applied is dried at a peak metal temperature of equal to or higher than 60°C and
equal to or lower than 200°C, so that a surface coating layer containing the zinc
20 compound is formed on the A1 plating layer.
[0057]
As the application liquid used for forming the surface coating layer
containing the zinc compound, a mixture of a sol containing the zinc compound and
a predetermined organic binder can be used as described above.
25 [0058]
Here, the contact angle of tlie application liquid wit21 respect to the plated
steel sheet having the Al plating layer is set to equal to or greater than 20" and equal
to or less than 50"; thus, the application liquid can be ulliformly deposited on the A1
plating layer, which inlproves the lubricity in hot state, corrosion resistance after
30 paint coating, and spot weldability of the formed surface coating layer. If the
contact angle is less that1 20°, the application liquid easily nlns by gravity, and is
affected by unevenness due to the surface ro~~gl~noefs tsh e steel sheet having the A1
plating layer, the application liquid becoming thin at convex parts and thick at
concave parts. This inhibits uniform deposition of the zinc compound. I f the
contact angle exceeds 50°, the application liquid is repelled by the steel sheet having
5 the A1 plating layer, which inhibits unifor~nd eposition of the zinc compound. The
contact angle of the application liquid is further preferably equal to or greater than
25" and equal to or less tlian 40".
[0059]
Note that the contact angle herein refers to an angle that is inside the liquid
10 and is formed between the liquid surface and the solid surface, at a place where the
free surface of a still liquid is in contact with a solid wvall. This contact angle can be
measured by, as a known method, using Young's formula and using a contact angle
gauge (e.g., DM-901, available from Kyowa Interface Science Co., LTD.), for
example.
16 [0060]
The contact angle of the application liquid can be controlled by, for example,
making the above application liquid contain a solvent, such as water or ethyl alcohol,
or an additive (e.g., Surfynol 104E available from Nissin Chenlical Indust~yC o., Ltd.
or Aron B500 available from TOAGOSEI CO., LTD.) as appropriate.
20 [0061]
The surface tension and viscosity of the application liquid are not
particularly limited, but the surface tension of the application liquid is preferably
equal to or greater than 20 rnnNI111 and equal to or less than 60 mN/nl, and the
viscosity of the applicatio~li quid is pseferably equal to or greater than 2 mPas and
25 equal to or less tlian 20 mPas. The surface tension and viscosity of the application
liquid in the above ranges allow the application liquid to be deposited further
uniformly, ~vhichfu rther improves lubricity in hot state, corrosion resistance after
paint coating, and spot weldability. The surface tension of the application liquid is
fi~rther preferably equal to or greater than 30 mN/m and equal to or less than 50
30 mN1111, and the viscosity of the application liquid is further preferably equal to or
greater than 2.5 mPas and equal to or less than 10 mPa.s.
[0062]
Here, the above surface tension is a value obtained by adjusting the
temperature of the application liqoid to 2S°C and perfornling ~lleasurernent by a
Wilhelmy inethod (e.g., CBVP-A3, available from Kyowa Interface Science Co.,
5 LTD.). The above viscosity is a value obtained by adjusting the tetllperature of the
application liquid to 25°C and performing measurement with a B-type viscorneter
(e.g., VDA2-L, available fiom Shibaura Systems Co., Ltd.).
[0063]
The surface tension and viscosity of the application liquid can be controlled
10 by, for example, making the above application liquid contain a solvent, such as water
or ethyl alcohol, or an additive (Surfynol 104E available fro111 Nissin Chemical
I~ldustry Co., Ltd. or Ason BS00 available fiom TOAGOSEI CO., LTD.) as
appropriate.
[0064]
15 In applying the application liquid onto the A1 plating layer, it is preferable to
use a roll coater. Application conditions with the roll coater are not particularly
limited, but for example, a method may be used in which a pick up roll and an
applicator roll are used for application. When the application liquid is applied onto
the A1 plating layer by this tnethod, the application liquid can be uniformly deposited
20 on the A1 plating layer, which fi~rther improves lubricity in hot state, corrosioti
resistance after paint coating, and spot weldability.
[0065]
As a tnetliod for baking and d~yingaf ter the application, methods including,
for example, an air-heating furnace, an induction heating fi~rnacea, near infrared ray
25 fi~rnacea, nd the like may be used, or a combination of any of these methods illay be
used. Here, the peak metal temperature of the steel sheet in baking and drying is set
to equal to or higher than 60°C and equal to or lower than 200°C. The peak metal
tenlperature (PMT) of the steel sheet of lower than 60°C is riot preferable because the
surface coating layer may come off. The peak metal temperature of the steel sheet
30 exceeding 200°C is not preferable because a crack may occur in the surface coating
layer and the surface coating layer may come off. The peak metal temperature of
the steel sheet is preferably equal to or higher than 70°C and equal to or lower than
150°C.
[0066]
Other hardening treatments may be carried out, depending on the type of
5 binder to be used for the applicatioll liquid, by using, for example, ultraviolet ray,
electron beam, or the like instead of baking and drying after the application.
Examples of an organic binder that can be subjected to such hardening treat~nellts
include polyurethane, polyester, acrylic, and a silane coupling agent.
[0067]
10 The method for forniing the surface coating layer containing the zinc
compound is not linlited to these examples, and the surface coating layer can be
formed by various methods. When the binder is not used, the adhesion after applied
onto the A1 plating is slightly low and the surface coating layer may partially come
o f f when rubbed strongly. However, once heated by going through a hot pressing
15 process, the surface coating layer exhibits strong adhesion.
[0068]
The method for manufacturing the Al-plated steel sheet used for hot
pressing according to the present embodiment has been described above.
20 [Exaniples]
[0069]
The Al-plated steel sheet used for hot pressing and the nlethod for
manufacturing the Al-plated steel sheet used for hot pressing according to the present
invention will now be described specifically by showing the examples and
25 conlparative examples. Note that the Examples of the Al-plated steel sheet used for
hot pressing and the method for manufacturing the Al-plated steel sheet used for hot
pressing according to the present invention, which are described below, are merely
exemplary, and the Al-plated steel sheet used for hot pressing and the method for
manufacturing the AI-plated steel sheet used for hot pressing according to the present
30 invention are not limited to such examples as described below.
[0070]
(Example 1)
In Example 1 described below, a cold-rolled steel sheet (sheet thickness of
1.4 mnl) having steel components as shown in Table 1 was used, and the surfaces of
the cold-rolled steel sheet were Al-plated by a Sendzimir method. The annealing
5 temperature used was approximately 800°C. Si of 9 % was contained in the A1
plating bath and also Fe that had been eluted fiom steel strips was contained therein.
The deposition amount after plating was adjusted to 80 gltn2 for both surfaces, by a
gas wiping method, and after cooling, an application liquid containing any one of
zinc oxide, zinc hydroxide, and zinc acetate was applied with a roll coater, and dried
10 at a peak metal temperature sho\vn in Table 2.
[0071]
The contact angle, surface tension, and viscosity of the used application
liquid were as shown in Table 1 below. Here, the contact angle, surface tension,
and viscosity of the application liquid were adjusted by adding water, and Surfin01
15 104E and/or Aron B500 to the application liquid.
[0072]
Each prepared steel sheet had a final sheet thickness of 1.6 mm. The
lubricity in hot state, corrosio~rl esistance after paint coating, and spot weldability of
the prepared steel sheet were evaluated. In the evaluation, an Al-plated steel sheet
20 without a coating was used for comparison. Some sanlples were subjected to skin
pass depression before formation of the coating, so that a surface form (RSm) of the
A1 plating was cllanged.
[0073]
A zinc deposition amount was determined by fluorescent X-ray
25 spectroscopy, as the average deposition amount in a 30 mm x 30 mm region.
Furthennore, mapping analysis using an electron probe microallalyser (JEOL JXA-
8530F) was performed on a portion where zinc was deposited in a 5 mm x 5 nun
region, and the presencelabsence of a portion having a zinc deposition amount
exceeding 1.5 g/m2 in anjr continuous 1 mm2 region in an analyzed region was
30 evaluated. Here, the measurement area was 5 mm x 5 mm, the measure~neut pitch
was 20 pm, the acceleratioll voltage of an electron bean1 was 15 kV, ir~adiation
current was 50 nA, the beam diameter was 20 ~ I I ~ Ia,n d the measurement time for
each measuretnent pitch was 50 ms. RSm wvas nleasured with a surface roughness
measuring machine (SE3500, available from Kosaka Laboratory Ltd.). The grain
size in a sol of the zinc co~llpoutid was tneasured by a dynamic light scattering
5 method using Nanotrac Wave available fi.0111 NIKKISO CO., LTD.
[0074]
[Table I]
10 Lubricity in hot state was evaluated with a Bowden tester shown in FIG. 1.
Table 1 Steel member components (unit: mass%)
A 150 x 200 nnn steel sheet was heated to 900°C, then steel ball was pressed from
C
above at 700°C, pressing load and pull-out load were measured, and a value was
calculated from pull-out load/ pressing load, as a coefficient of dynamic friction.
[0076]
15 For corrosion resistance after paint coating, a method prescribed in JASO
M609 established by Society of Automotive Engineers of Japan, Inc. was carried out.
First, each prepared Al-plated steel sheet was placed in a heating filmace
and heated at 900°C for 6 minutes. Itntiiediately aAer taken out, the Al-plated steel
Si I Mn I P
sheet was held between stainless steel dies and rapidly cooled. The cooling rate
20 was approximately 150°C per second. Next, the cooled Al-plated steel sheet was
0.2 1 0.1
sheared into 70 x 150 mm, subjected to chemical conversion treatment using a
chemical con\~ersionl iquid (PB-SX35T) available from Nihon Parkerizing Co., Ltd.,
and then coated with an electrodeposition paint (POWERNIX 110) available from
NIPPONPAINT Co., Ltd. with a target thickness of 20 prn, and baking wvas
25 performed at 170°C. After that, the paint coat was cross-cut with a cutter and was
subjected to a corrosion test of 180 cycles (60 days). The width of blistering of the
S
1.2 1 0.02 1 0.01 1 0.02 1 0.003 1 0.04
paint coat from the cross-cut (maximum value on one side) was then measured.
[0077]
Spot weldability was evaluated as follo\trs.
[0075]
Ti B Al
Each prepared Al-plated steel sheet was placed in a heating furnace and
heated at 900°C for 6 minutes. Immediately after taken out, the Al-plated steel
sheet was held between stainless steel dies and rapidly cooled. The cooli~~ragt e
was approximately 150°C per second. Next, the cooled Al-plated steel sheet was
5 sheared into 30 x 50 nml, and a suitable spot welding cunent range (upper litnit
current-lower limit current) was measured. Measurement conditions are listed
below. The lower limit current was defined as a cussent value at which a nugget
diameter became 4 x (sheet t l ~ c k n e s ~ )sp~e-c~ifi,c ally at which the nugget diameter
became 5.1 mm, and the upper limit current was defined as a current at \vhich
10 expulsion occurred.
[0078]
-Current: direct current
-Electrode: made of copper-cl~romiuma lloy, DR (40R with 6 tnm in tip diameter)
- Pressure application: 400 kgf (1 kgf is approximately 9.8 N)
15 -Cunent applying time: 240 msec
[0079]
Table 2 below shows the obtained evaluation results. In Table 2, "organic
acid zinc" means zinc citrate.
[OOSO]
20 [Table 21
u
1
5 w
.~ ~ .&+-
c-9 C 0 p:e g :S
LO" 3 2 C ga 0
.G s .5! - rue E .e B p %" ",bp, Cms .p$
3=33
E E 2 7
2%
"P<%b.; L 5 0 3 e:, a *, E
& O Sig
-c- 0 - .. Z..C" .: t g c .shax E u
L6;:
.d'" S5\ ~ 3 .
m $ "
- L - C 3 Z . G . ~ E $ $ E ~ . O
prz.="2 Z-. B O E II
.- " - b n u 3%
0;- " " E II= >"g % C -
8 :g,'t
f .; '2
u1 $ 8' E
- 0 3"" C -n"
6%" =a"a & r --E o
O " 9 '3 *
.A e " c -c-
-ij= h, .S- c c - 2.c
5 0 0 "
- ; g z
ii t-"
a
-a
D 0 c Z
Nos. 2 to 6 having a coating layer containing a zinc co~npound on the Al
plating surface had excellent lubricity and corrosion resistance after paint coating,
and spot weldability on a similar level, as compared with No. 1 not having a coating,
5 exhibiting favorable pcrfornlance. Especially, No. 2, whose zinc compound was
zinc oxide, was found to have better perfonnance than other satnples.
[0082]
No. 7 and No. 17, which had a coating layer containing zinc oxide but had a
deposition amount of zinc oxide (zinc amour~tb asis) falling outside the range of the
10 present invention, exhibited poor corrosion resistance or poor spot weldability. In
tie case where the deposition amount of zinc oxide (zinc amount basis) was equal to
or greater than 0.6 g/m2 and equal to or less than 1.0 g/m2 (No. 2, No. 9, and No. lo),
particularly favorable performance was exhibited. In the case where the deposition
amount of zinc oxide was within the range of the present invention but there existed
16 a portion having a zinc deposition amount in any continuous 1 nnn2 l.e gi.o n exceeding
1.5 g/m2 (No. 18), poor spot weldability was exhibited. It was also found that, if the
deposition amount (Zn amount basis) is the same, RSm of 500 pm or less is
preferable because favorable spot weldability is exhibited.
[0083]
20 The preferred embodiment(s) of the present invention haslhave been
described above with reference to the accompanying drawings, whilst the present
invention is not limited to the above examples. Aperson skilled in the art rnay find
various alterations and tnodifications withiil the scope of the appended claims, and it
should be understood that they will naturally come under tile technical scope of the
25 present invention.
Industrial Applicability
[0084]
As described above, by uniformly depositing a zinc compound, particularly
30 zinc oxide, with a deposition amount within a certain range on a surface of an Alplated
steel sheet, it is possible to significantly improve spot weldability while
securing lnbricity in hot state and corrosion resistance after paint coating.
Accordingly, the present invention can enhance the applicability of an aluminumplated
steel member to autonlobiles and industrial machines, which are end-uses,
thus being highly usefill in industry.
5
CLAIMS
Claim 1
An Al-plated steel shcct used for hot pressing, comprising:
a steel sheet;
an A1 plating layer formed on one surface or both surfaces of the steel sheet;
and
a surface coating layer formed on tlie A1 plating layer,
wherein tlie surface coating layer contains a zinc compound, and the zinc
compound is deposited on the Al-plated steel sheet in an amount of equal to or
10 greater than 0.3 g/m2 and equal to or less than 1.5 g/111~a s an amount of metallic zinc
per one surface, and a zinc deposition amount in any continuous 1 mm2 region is 1.5
g/~io~r 2le ss.
Claim 2
15 The Al-plated steel sheet used for hot pressing according to claim 1,
wherein the zinc compound in tlie surface coating layer is zinc oxide.
Claim 3
The Al-plated steel sheet used for hot pressitig according to claim 1 or 2,
20 wherein an average length RSm of a profile element in a roughness curve of
a surface of tlie Al-plated steel sheet used for hot pressing after the formation of the
surface coating layer is 500 pm or less.
Claim 4
25 A method for manufacturing an Al-plated steel sheet used for hot pressing,
comprising the steps of :
al)plying an applicatio~i liquid containing a zinc compound and having a
contact angle of equal to or greater than 20' and equal to or less than 50" with
respect to the Al-plated steel sheet onto the Al-plated steel sheet iticluding a steel
30 sleet and tlie A1 plating layer formed 011 one surface or.both surfaces of the steel
sheet; and
drying the Al-plated steel sheet onto which the application liquid has been
applied, at a peak metal temperature of equal to or higher than 60°C and equal to or
lower than 200°C, to form a surface coating layer containing the zinc con~pound bn
the A1 plating layer.
6
Claim 5
The method for manufacturing an Al-plated steel sheet used for hot pressing
according to claim 4,
wherein a surface tension of the application liquid at 25°C is equal to or
10 greater than 20 n1N1m and equal to or less than 60 mN/m.
Claim 6
The method for manufacturing an Al-plated steel sheet used for hot pressing
according to claim 4 or 5,
15 wherein a viscosity of the application liquid at 25OC is equal to or greater
than 2 mPas and equal to or less than 20 &as. .
Claim 7
The method for manufacturing an Al-plated steel sheet used for hot pressing
20 according to any one of claims 4 to 6,
wherein the application liquid is applied onto the A1 plating layer wit11 a roll
coater.
Claini 8
25 The method for matiufacturing an Al-plated steel sheet used for hot pressing
according to any one of claims 4 to 7,
wherein the zinc colnpound is zinc oxide.
Claim 9
30 The method for manufacturing an Al-plated steel sheet used for hot pressing
accordillg to any one of claims 4 to 8,
\\111erein in the manufactured surface coating layel; the zinc conipound is
deposited on the Al-plated steel sheet i11 an amomit of equal to or greater than 0.3
g/m2 axid eq11al to or lcss than 1.5 glm2 as an amount of nietallic zinc per one surface,
and a zi~icd eposition amount it1 any continuous 1 nuii2 l.eg l.o ll is 1.5 g/n12 or less.
5
Claim 10
Tlie method ibr ~iiariufachl~inagn Al-plated steel sheet used for hot pressing
according to ally one of claims 4 to 9,
~~lrereain avclage length RSnl of a profile element in a roughness curve of
10 a suiface of the tnanufachi~ed Al-plated steel sheet used for hot plessing afler the
fom~ationo f the surface coating laycl is 500 prn or less.