[Document Type] Specification
[Title of the Invention] AI-PLATED STEEL SHEET, METHOD FOR HOTPRESSING
AI-PLATED STEEL SHEET, AND AUTOMOTIVE PART
[Technical Field of the Invention]
[OOO 11
The present invention relates to an Al-plated steel sheet, a method for hotpressing
an Al-plated steel sheet, and an automotive part.
Priority is claimed on Japanese Patent Application No. 2012-095014, filed on
April 18,2012 and Japanese Patent Application No. 2012-100266, filed on April 25,
2012, the contents of which are incorporated herein by reference.
[Related Art]
[0002]
In recent years, there have been increasing demands for cutbacks on chemical
fuel consunlption in order to protect the environment and prevent global warming, and
these demands have had various effects on the manufactnring industly. For example,
even the automobile, an indispensable means of transportation in daily life and
activities, is no exception, and improved file1 efficiency and the like through vehicle
body weight reduction and other means are required. In the case of automobiles,
however, mere realization of vehicle body weight reduction is not acceptable kom the
viewpoint of product quality, and adequate safety also has to be secured.
[0003]
The components of an autonlobile are formed largely of steel, particularly a
steel sheet, and reducing the weight of the steel sheet is important for vehicle body
weight reduction. As just pointed out, I~owveerm, ere reduction of steel sheet weight
is not acceptable since the n~echanicasl trength of the steel sheet has to be secured.
Such demands for the steel sheet are not limited to the auto-making industty but also
applied similarly to various other manufacturing industries.
[0004]
Research and development has therefore been conducted with regard to the
steel sheet that, by i~nprovingth e mechanical strength of the steel sheet, is capable of
maintaining or increasing tnechanical strength even when made thinner than the steel
sheet used heretofore.
[OOOS]
Generally, a material having high mechanical strength tends to deteriorate in
shape fixability in bending and other forming, so that the working itself becomes
difficult in the case of formation into a complicated shape. One means available for
overcoming this formability problem is the so-called "hot pressing method" (also
referred to as a hot-stamping method, a hot press method, a die-quenching method, or
press-hardening).
[0006]
In the hot pressing method, the material to be formed is heated once to a high
temperature, softened, pressed and then cooled. Since the hot pressing method
softens the material by heating the steel once to a high temperature, the material can be
easily pressed, wvl~ile,i n additiot~t, he mechanical strength of the material can be
increased by the quenching through the cooling after the fonning. The hot pressing
method therefore makes it possible to obtain a formed product that simultaneously
achieves satisfactory shape fixability and high mechanical strength.
[0007]
However, when the hot pressing method is applied to a steel sheet, the heating
to a high temperature of, for example, 800°C or higher oxidizes iron and the like on the
surface, thereby producing scale (oxide). Accordingly, a process of removing the
scale (a descaling process) is therefore required after conducting the hot pressing,
which decreases productivity. In addition, in the case of a member wlvhich requires
corrosion resistance, it is necessaly to corrosion-proof or metal clad the member
surface after working, which makes a surface cleaning process and a surface treating
process necessary and also decreases productivity.
[OOOS]
As a method for suppressing such a decrease in productivity, a method of
providing coating on the steel sheet in advance is used. Generally, various materials
such as organic materials and inorganic materials are used as a material for coating the
steel sheet. Among the materials, a steel sheet having a zinc (Zn)-based plating that
provides the steel sheet wit11 a sacrificial protection effect is widely used for
automotive steel sheets and the like, from the viewpoints of anticorrosion performance
and steel sheet production technology.
[0009]
However, a heating temperature (700°C to 1000°C) in the hot pressing is
higher than decomposition temperatures of organic materials or melting points of metal
materials such as a Zn-based material, and a plating layer of the surface is melted and
evaporated during heating. Thus, the surface properties deteriorate significantly in
some cases.
[OOlO]
Accordingly, as a steel sheet which is hot-pressed, for example, a steel sheet
having an alutninutn (A1)-based metal coating, which has a higher melting point than
that of an organic material coating or a Zn-based metal coating, that is, a so-called AIplated
steel sheet is desirably used.
roo1 11
An Al-based tnetal coating of the steel sheet prevents scale from being
generated on the surface of the steel sheet and improves productivity by making a
descaling or other such process unnecessary. In addition, Al-based n~etacl oating has
a corrosion resistance effect and thus, post-painting corrosion resistance is improved.
[OO 121
Patent Document 1 describes a method which performs hot pressing using an
Al-plated steel sheet, obtained by coatiug a steel sheet having a predetermined
component composition with an Al-based metal coating.
[0013]
However, when the steel sheet is coated with an Al-based metal, depending on
heating conditions before hot pressing, the Al-based metal coating nlelts, and is
changed to an Al-Fe componnd by the combination of iron (Fe) diffusing from the steel
sheet and aluminum, and the AI-Fe compound (hereinaftel; also referred to as an "AIFe
alloy layer") is laminated on the surface of the steel sheet in some cases. Since the
AI-Fe alloy layer is hard, the AI-Fe alloy layer is brought into contact with a die, and
thus, processing scratches may be generated on the surface of the steel sheet during
pressing in some cases.
[0014]
The surface of the Al-Fe alloy layer is by nature relatively resistant to slipping
and poor in lubricity. In addition, the AI-Fe alloy layer is hard and thus relatively
susceptible to cracking, so that forn~abilitym ay decrease owing to cracking, powdering
and the like of the plating layer. Moreover, when the AI-Fe alloy powder exfoliated
fiom the AI-Fe alloy layer adheres to the die, the quality of the pressed-forn~edp roduct
is degraded. In addition, when the surface of the AI-Fe alloy layer is strongly
scratched and the AI-Fe alloy powder adheres to the die, the quality of the pressedformed
product is degraded.
[0015]
Therefore, it is necessary to remove the AI-Fe alloy powder adhering to the
die during repair, which decreases productivity and increases costs.
[0016]
In addition, since the AI-Fe compound is low in reactivity in phosphate
treatment, a coating (a phosphate coating) is not formed in chemical conversion, which
is an electrocoating pretreatment. The A1-Fe alloy layer has satisfactory coating
adhesion even without formation of the chemical conversion coating, and as long as a
deposition amount of the AI-Fe alloy layer is sufficient, post-painting corrosion
resistance also becomes satisfactory. However, when the deposition amount of the
AI-Fe alloy layer is increased, the aforementioned die adhesion is caused.
[0017]
As described above, the die adhesion is sometimes caused by exfoliation of
the Al-Fe alloy layer, surface scratches of the AI-Fe alloy layer, and the like.
Although the latter die adhesion (scratch adhesion) is improved by improving the
lubricity of the surface coating, the former die adhesion (exfoliation adhesion) is
relatively slightly improved. The most effective way to improve the former die
adhesion is reduction of the deposition amonnt of the A1-Fe alloy layer. However,
when the deposition amount of the Al-Fe alloy layer is reduced, the corrosion
resistance deteriorates.
[0018]
In contrast, in Patent Document 2, for the purpose of preventing processing
scratches, it is disclosed that a steel sheet having a predetermined component
con~positionis coated with an AI-based metal coating, and an inorganic compound
coating including at least one of silicon (Si), zirconium (Zr), titanium (Ti), and
phosphorus (P), an organic compound coating, or a complex compound coating thereof
is further formed on the Al-based metal coating.
[0019]
With the steel sheet on which such a surface coating is formed, the surface
coating remains during the pressing after heating, so that formation of processing
scratches during the pressing can be prevented. In Patent Document 2, it is disclosed
that since the surface coating functions as a lubricant during the pressing, formability is
improved. However, actually, sufficient lubricity cannot be obtained and another
lubricant or alternative means is required.
[0020]
In Patent Document 3, a method for solving a problem of surface deterioration
by evaporation of a galvanized layer in hot pressing of the galvanized steel sheet is
disclosed. Specifically, a zinc oxide (ZnO) layer having a high melting point is
formed on the surface of the galvanized layer to function as a barrier layer for
preventing the evaporation of the underlying galvanized layer.
[0021]
However, the teclmique in Patent Docutnent 3 assumes a galvanized layer.
The amount of Al in the galvanized layer is allowable up to 0.4%. However, a lower
Al concentration is preferable and actually, the technique is a teclmique not essentially
assumed on Al-based metal coating. Since a probleni of Patent Document 3 is Zn
evaporation of the galvanized layer, naturally, the above problem cannot arise in the
case of the Al-based metal coating having a high melting point.
[0022]
In Patent Document 4, a technique of coating a surface of an Al-plated steel
sheet with a Wurtzite-type compound to improve hot lubricity and chemical conversion
properties is disclosed. This technique is effective in improving lubricity and also
improves post-painting corrosion resistance. However, in order to improve lubricity,
the necessary deposition amount of the coating is about 2 g/m2 in terms of Zn, and in
order to obtain Inore stable lubricity, the necessary deposition amount thereof is about
3 g/m2. However, when the deposition amount of the coating of 3 g/n12 is converted
into the thickness of the coating, the thickness is about 1.5 pm. With such the
thickness of the coating (when the deposition amount of the coating is more than 3
&), the coating resistance increases and spot weldability deteriorates. As a result,
there is a concern of deterioration in cross tensile strength. Therefore, there have
been demands for a technique of improving lubricity with a smaller deposition amount
of the coating.
[Prior Art Document]
[Patent Document]
lo0231
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2000-38640
[Patent Doculllent 21 Japanese Unexamined Patent Application, First
Publication No. 2004-21 1151
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2003-129209
[Patent Document 41 PCT International Publication No. W02009113 1233
[Disclosore of the Invention]
[Problems to be Solved by the Invention]
[0024]
As described above, a plated steel sheet provided with Al-based plating
having a high melting point (an AI-plated steel sheet) is expected to be used as an
automotive steel sheet which requires corrosion resistance, and various suggestions
have been made for application to hot pressing. However, in a conventional Al-plated
steel sheet, satisfactory lubricity of an Al-Fe alloy layer formed on the surface thereof
was not able to be obtained. Thus, press formability was poor during hot pressing
and the steel sheet was not easily applied to hot pressing into a colnplicated shape.
[0025]
The present invention has been made in consideration of the aforementioned
circumstances, and an object thereof is to provide an Al-plated steel sheet having more
satisfactory lubricity than the conventional one, and capable of improving formability
during hot pressing and productivity, a method for hot-pressing the Al-plated steel
sheet, and an automotive part produced by the hot pressing method.
Measures for Solving the Problem]
[0026]
The present invention adopts the following measures to solve the above
problems and achieve the object.
(1) An Al-plated steel sheet according to an aspect of the present illvention
includes: a steel sheet; an Al plating layer which is formed on one surface or both
surfaces of the steel sheet and contains at least 85% or more of Al by mass%; and a
surface coating layer which is laminated on the surface of the Al plating layer and
contains ZnO and one or more lubricity improving conlpounds.
[0027]
(2) In the Al-plated steel sheet according to (I), the lubricity improving
compound may be a cotnpound including one or more transition metal elenlents.
[0028]
(3) In the Al-plated steel sheet according to (2), the transition metal element
may be any one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Mo, W, La, and Ce.
[0029]
(4) In the Al-plated steel sheet according to (2), an amount of the lubricity
improving compound including the transition metal element in the surface coating
layer may be 1% to 40% with respect to a total sunount of ZnO by mass ratio.
[0030]
(5) In the Al-plated steel sheet according to (I), the lubricity improving
cotnpound tnay be a colnpound including one or more typical elemel~ts.
[003 11
(6) In the AI-plated steel sheet according to (5), the typical element may be
any one or more of Mg, Ca, SI; Ba, P, Sn, and Ge.
[0032]
(7) In the Al-plated steel sheet according to (51, an amount of the lubricity
improving compound including the typical element in the surface coating layer tnay be
5% to 30% with respect to a total amount of ZnO by mass ratio.
[0033]
(8) In the Al-plated steel sheet according to any one of (1) to (7), the surface
coating layer may contain 0.3 g/m2 to 7 g/n12 of ZnO in terms of Zn.
[0034]
(9) In the Al-plated steel sheet according to any one of (1) to (8), the surface
coating layer may further contain 5% to 30% of an organic compound with respect to
the total amount of ZnO by mass ratio.
[0035]
(10) A method for hot-pressing an Al-plated steel sheet according to another
aspect of the present invention includes blanking and then heating tlie AI-plated steel
sheet according to any one of (I) to (9), and press-forming the steel sheet.
[0036]
(1 1) In the method for hot-pressing an Al-plated steel sheet according to
(lo), an average temperature rising rate until the temperature of the Al-plated steel
sheet feaches from 50°C to a temperature 10°C lower than a maximuni reaching
temperature may be 10 "Clsec to 300 "Clsec in the heating before the press-forming of
the steel sheet.
[0037]
(12) In the method for hot-pressing an Al-plated steel sheet according to
(11), the heating before the press-forming of the steel sheet may be performed by
electrical heating or induction heating.
[003 81
(13) An automotive part according to still another aspect of the present
invention is produced by the method for hot-pressing an Al-plated steel sheet according
to any one of (1 0) to (12).
[Effects of the Invention]
[0039]
According to the present invention, it is possible to provide an Al-plated steel
sheet which has more satisfactory lubricity than that of tlie conventional one and is
capable of inlproving forlnability and productivity duritig hot pressing, a method for
hot-pressing the Al-plated steel sheet, and an automotive part produced by the hot
pressiflg method.
[Brief Description of the Drawing]
[0040]
FIG. lAis a layer strncture view of an Al-plated steel sheet according to a first
embodiment of the present invention.
FIG. 1B is a layer structure view of an Al-plated steel sheet according to a
second embodiment of the present invention.
FIG. 2 is an external view showing a center pillar reinforcing material as at1
exan~pleo f an automotive part of the present invention.
FIG. 3 is a view showing a form of an apparatus for evaluating hot lubricity of
the Al-plated steel sheet.
FIG. 4 is a view showing a relationship between a zinc oxide content (amount
of zinc oxide in terms of Zn) of a surface coating layer and a coating exfoliation rate.
FIG. 5 is a view showing a relationship between a zinc oxide content (amount
of zinc oxide in terms of Zn) of a surface coating layer and hot lubricity (friction
coeficient).
FIG. 6 is a view showing a relationship behveen a zinc oxide content (amount
of zinc oxide in terms of Zn) of a surface coating layer and strength of a spot joint.
[Embodiments of the Invention]
[0041]
Hereinafter, an embodiment of the present invention will be described in
detail with reference to drawings.
[First Embodiment]
First, a first embodiment of the present inver~tionw ill be described. FIG. 1A
is a view showing a layer structure of an Al-plated steel sheet 10 according to the first
embodiment. As sho\vn in FIG. 1 A, the Al-plated steel sheet 10 according to the first
embodiment includes a steel sheet 11, an A1 plating layer 12 which is formed on one
surface (for example, an npper surface) of the steel sheet 11, and a surface coating
layer 13 wvl~ich is formed on the surface of the AI plating layer 12.
In FIG. 112, an example of a case in which the A1 plating layer 12 and the
surface coating layer 13 are formed on the one surface of the steel sheet 11 is shown.
However, the A1 plating layer 12 and the surface coating layer 13 may be formed on
both surfaces of the steel sheet 11.
[0042]
The steel slieet 11 is a substrate of the Al-plated steel sheet 10 and has
required mechanical properties (tensile strength, yield point, elongation, reduction,
hardness, impact value, fatigue strength, creep strength, and other such properties
related to mechanical deformation and fracture). For example, the steel sheet 11
contains, by mass%, 0.1% to 0.4% of carbon (C), 0.01% to 0.6% of silicon (S), 0.5%
to 3% of manganese (Mn), 0.01% to 0.1% of titanium (Ti), 0.0001% to 0.1% of boron
(B), and a balance consisting of iron (Fe) and unavoidable impurities.
[0043]
Hereinafter, the role of each of the aforementioned elements contained in the
steel sheet 11 will be described. In the following description, "%" refers to mass%.
[0044]
Carbon is an element for securing the mechanical strength of the steel sheet 11.
When the carbon content in the steel sheet 11 is less than 0.1%, sufficient mechanical
strength cannot be obtained. On tlie other hand, when the carbon content in the steel
sheet 11 is more than 0.4%, the hardness (nlechanical strength) of the steel sheet 11
increases, but melting cracks are easily generated in the steel sheet 11. Accordingly,
the carbon content in the steel sheet 11 is desirably 0.1% to 0.4%.
[0045]
Silicon is an element for securing the mechanical strength of the steel sheet 11,
siniilar to carbon. When the silicon content in the steel sheet 11 is less than 0.01%, a
strength improving effect cannot be exhibited and st~ficienmt echanical strength
cannot be obtained. On the other hand, silicon is also an easily oxidizable element,
and thus, when the silicon content in the steel sheet 11 is more than 0.6%, wettability
deteriorates during hot-dip plating of the steel sheet 11 and unplating occurs.
Accordingly, the silicon content in the steel sheet 11 is desirably 0.01% to 0.6%.
[0046]
Manganese is an element for increasing the hardenability of the steel sheet 11
to increase the strength of the steel sheet 11. In addition, manganese combines with
sulfur (S) which is an unavoidable impurity in the steel sheet 11 to form manganese
sulfide (MnS) and prevents hot embrittlement of the steel sheet 11 by sulfur. When
the manganese content in the steel sheet 11 is less that1 0.5%, the addition effect is not
exhibited. On the other hand, when the manganese content in the steel sheet 11 is
more than 3%: the residual y phase in the steel sheet 11 increases and the strength
decreases. Accordingly, the manganese content in the steel sheet 11 is desirably 0.5%
to 3%.
[0047]
Titanium is a strength improving element and also an element for improving
the heat resistance of the Al plating layer 12. When the titanium content in the steel
sheet 11 is less than 0.01%, a strength improving effect and heat resistance improving
effect cannot be obtained. On the other hand, when the titanium content in the steel
sheet 11 is more than 0.1%, for example, carbides and nitrides are formed and the steel
sheet 11 is likely to be softened. Thus, the desired n~echanicasl trength cannot be
obtained. Accordingly, the titanium content in the steel sheet 11 is desirably 0.01% to
0.1%.
[0048]
Boron is an element for increasing the hardenability of the steel sheet 11 to
improve the strength of the steel sheet 11. When the boron content in the steel sheet
l l is less than 0.0001%, a strength improving effect cannot be exhibited. On the
other hand, when the boron content in the steel sheet 11 is more than 0.1%, the fatigue
strength of the steel sheet 11 decreases owing to formation of inclusions. Accordingly,
the boron content in the steel sheet 11 is desirably 0.0001% to 0.1%.
[0049]
The steel sheet 11 may contain unavoidable impurities mixed in other
production processes within a range not impairing the mechanical strength of the steel
sheet 1 1.
[OOSO]
In the steel sheet 11 having the above component composition, the mechanical
strength is increased to about 1500 MPa or more by quenching in the hot pressing
method. In the hot pressing method, since the steel sheet 11 is press-formed in a state
in which the steel sheet is softened, forming is easy. In addition, when the steel sheet
11 is made thin to reduce the weight, high mechanical strength is maintained.
[005 I]
The A1 plating layer 12 is formed on one surface (for exan~plea, n upper
surface) of the steel sheet 11 and contains at least 85% or more of aluminum (Al) by
mass%. For example, the Al plating layer 12 is desirably formed by a hot-dip plating
method. However, the forming method thereof is not limited to the hot-dip plating
method.
[0052]
As described above, the A1 plating layer 12 preferably contains 85% or more
of aluminum and may contain conlponents other than aluminum. Components other
than aluminum are not particularly limited, but silicon is preferable for the following
reasons.
[0053]
Silicon is an element having a fi~nctiono f suppressing forniation of an alloy
layer of iron and aluminum (hereinafter, referred to as a Fe-AI alloy layer) during the
hot-dip plating of the steel sheet 11. When the silicon content of the A1 plating layer
12 is less than 3%, the Fe-AI alloy layer grows thickly during the hot-dip plating of the
steel sheet 11, and thus, cracking of the plating layer is facilitated during the process
and corrosion resistance may deteriorate. On the other hand, when the silicon content
of the AI plating layer 12 is more than 15%, the workability and the corrosion
resistance of the Al plating layer 12 deteriorate. Thus, the silicon content of the A1
plating layer 12 is desirably 3% to 15%.
[0054]
The A1 plating layer 12 having the above component composition has a
function of preventing corrosion of the steel sheet 11 and preventing scales (oxides of
iron) from being formed on the surface of the steel sheet 11 due to heating before hot
pressing.
[0055]
Accordingly, a scale re~novingp rocess, a surface cleaning process, a surface
treating process, and the like can be omitted by forming the Al plating layer 12 on the
steel sheet 11, and thus, the productivity of the Al-plated steel sheet 10 is improved.
In addition, the Al plating layer 12 has a higher melting point than those of coatings
made of organic materials and coatings made of other metal nlaterials (for example,
Zn-based material) and thus can be processed at a high temperature during hot pressing.
[0056]
Some of alunlinutn contained in the Al plating layer 12 is alloyed with iron in
the steel sheet 11 during the hot-dip plating or the hot pressing. Accordingly, the A1
plating layer 12 is not necessarily formed with a single layer with a fixed component
conlposition and may include apartially alloyed layer (an alloy layer).
Further, when the deposition amount of the Al plating layer 12 with respect to
the steel sheet 11 is 80 g/m2per surface, the thickness of the Al plating layer 12 per
surface is about 15 pnl.
[0057]
The surface coating layer 13 is laminated on the sn~faceo f the Al plating layer
12 and contains at least zinc oxide (ZnO). For example, the surface coating layer 13
is formed by applying a solution in which fine zinc oxide particles are suspended to the
surface of them plating layer 12 by a roll coater or the like. The surface coating
layer 13 has an effect of improving lubricity during the hot pressing of the Al-plated
steel sheet 10 and reactivity with a chemical conversion solution.
[OOSX]
The surface coating layer 13 further contains one or more lubricity improving
compounds, in addition to the aforementioned zinc oxide. The lubricity improving
compound is preferably a compound including one or more transition tnetal elements
(elements belonging to the third group to the eleventh group in the periodic table).
[0059]
Further, the aforementioned transition metal element is preferably any one or
more of titanium (Ti), vanadium (V), chromic (Cr), manganese (Mn), iron (Fe), cobalt
(Co), nickel (Ni), copper (Cu), zirconit~ln( Zr), molybdenum (Mo), tungsten (W),
lanthanum (La) and cerium (Ce).
Particularly, a compound including any one or more of nickel, titanium,
zirconium, manganese, and copper has a remarkable lubricity improving effect.
Compounds of these elements are preferably oxides, nitrides, sulfides, or phosphates.
[0060]
Although the reason why the compound of the aforementioned transition
metal element contributes to improving lubricity during the hot pressing is not clear, it
is considered that the lubricity improving effect cannot be obtained only with the
conlpound of the aforementioned transition metal element, and thus, the lubricity is
improved due to a composite effect or by a synergistic effect wit11 zinc oxide.
[0061]
While zinc oxide has the aforementioned lubricity improving effect essentially,
with the addition of the compound of the transition metal element, zinc oxide and the
compound of the aforementioned transition metal element are reacted ~vithe ach other
durit~gh ot pressing to form a composite oxide. As a result, it is assumed that
lubricity is improved.
[0062]
For example, the surface coating layer 13 containing zinc oxide and the
compound of the aforetnentioned transition metal element (lubricity improving
compound) is formed on the surface of the Al plating layer 12 by applying a coating
containing zinc oxide and the lubricity improving compound to the surface of t11eAl
plating layer, and baking and drying the coating after the applying of the coating.
[0063]
As a method for applying zinc oxide and the lubricity improving compound,
for example, a method of forming a coating by mixing a si~spensionc ontaining zinc
oxide and the lubricity improving con~poundc ontaining the transition metal element
with an organic binder, and applying the coating to the surface of the Al plating layer
12, and a method of application by powder coating can be used.
100641
As the lubricity improving compound containing the aforementioned
transition metal element, for example, nickel sulfate, manganese sulfate, titanium
fluoride, zirconium nitrate, copper sulfate, or the like is preferably used. As the
organic binder, for example, polyurethane resins, polyester resins, acrylic resins, silane
coupling agents, or the like are preferably used.
[0065]
The aforementioned lubricity improving compound and organic binder are
preferably aqueous to be mixed with the suspension containing zinc oxide respectively.
In this mamer, the coating obtained by mixing the suspension containing zinc oxide
and the lubricity improving compound and the organic binder is applied to the surface
of the Al-plated steel sheet 12.
[0066]
The particle size of zinc oxide is not particularly limited and the diameter
thereof is desirably 50 nm to 1000 nm. The particle size of zinc oxide is a particle
size after heat treatment is performed. Specifically, the particle size of zinc oxide is
determined by observing the zinc oxide with a SEM or the like after zinc oxide is
retained in a fi~rnacea t 900°C for 5 ininutes to 6 minutes and rapidly cooled with a die.
[0067]
In the surface coating layer 13, the amount of the lubricity improving
compound containing the transition metal element is preferably 1% to 40% with
respect to a total amount of zinc oxide by mass ratio. When the amount of the
lubricity improving compound is less than 1%, a sufficient lubricity improving effect
cannot be obtained during the hot pressing. On the other hand, wvhen the amount of
the lubricity improving compound is more than 40%, the adhesion of the surface
coating layer 13 deteriorates after heating.
[0068]
The amount of the organic binder (organic compound) such as a resin
component or a silane coupling agent in the surface coating layer 13 is desirably 5% to
30% wit11 respect to the total atnount of zinc oxide by mass ratio. When the amount
of the organic binder is less than 5%, a sufficient binder effect cannot be obtained and
the coating before heating is easily exfoliated. In order to stably obtain the binder
effect, the amount of the organic binder is preferably 10% or more with respect to the
total amount of zinc oxide by mass ratio. When the amount of the organic binder is
more than 30%, odor emission becomes significant during heating, aid thus, the
amount of more than 30% is not preferable.
[0069]
The present inventors have confirmed that the surface coating layer 13
according to the embodiment has higher lubricity compared to an inorganic compound
coating containing at least one of silicon, zirconium, titanium, aid phosphorus
described in Patent Document 2, an organic compoond coating, or a complex
compound coating thereof. Tlms, according to the AI-plated steel sheet 10 of the
embodiment, more satisfactory lubricity than that of the cot~ventionaol ne can be
achieved and improvement of formability and productivity can be realized during hot
pressing.
[0070]
The surface coating layer 13 preferably contains 0.3 g/m2 to 7 g/m2 of zinc
oxide in temls of zinc.
[0071]
FIG. 4 is a view showing a relationship behveen the amount of zinc oxide
(amount of zinc oxide in terms of zinc) in the surface coating layer 13 and a coating
exfoliation rate. The coating exfoliation rate is a valne obtained by dividing an
amount of zinc exfoliated from the surface coating layer 13 by a scratch test by an
amount of zinc contained in the surface coating layer 13 before the scratch test, and is
used as an index for evaluating lubricity.
[0072]
As shown in FIG. 4, when the amount of zinc oxide in the surface coating
layer 13 is 0.3 g/m2 to 7 g/m2, the coating exfoliation rate is 15% or less, and the
lubricity of the surface coating layer 13 is satisfactosy. When the amount of the zinc
oxide in the surface coating layer 13 is more than 7 g/m2, the thickness of the surface
coating layer 13 increases and the coating exfoliation rate rapidly increases. Thus,
weldability and coating adhesion deteriorate,
[0073]
Accordingly, the amount of zinc oxide in the surface coating layer 13 on one
surface of the steel sheet 11 is desirably 0.3 g/m2 to 7 gh2in t erms of zinc. Further,
the amount of zinc oxide in the surface coating layer 13 is more desirably 0.5 g/m2 to 2
g'rn2, and in addition to lubricity, weldability and coating adhesion become more
satisfactosy during hot pressing.
[0074]
As a method for baking and drying the coating containing zinc oxide, the
lubricity improving compound, and the organic binder after the applying of the coating,
for example, a method using an hot air furnace, an induction heating furnace, a nearinfrared
fnrnace, or the like, or a method in which these furnaces are combined may be
used. Depending on the type of the organic binder, instead of baking and drying after
the applying of the coating, a curing method using ultraviolet rays or electron beams
may be adopted. The method for forming the surface coating layer 13 is not limited
to the above method and various coating forming methods can be adopted.
[0075]
When the organic binder is not used, the surface coating layer 13 has slightly
low adhesion with the Al plating layer 12 before heating and is partially exfoliated
when being rubbed with great force. In this manner, since the surface coating layer
13 of the Al-plated steel sheet 10 exhibits satisfactory lubricity during hot pressing, the
formability of the Al-plated steel sheet 10 is in~provedd uring the hot pressing and the
corrosion resistance of the AI-plated steel sheet 10 after the hot pressing is also
improved.
[0076]
In addition, the surface coating layer 13 of the Al-plated steel sheet 10 has an
effect of suppressing adhesion of the Al-plated steel sheet 10 to a die. If the Al
plating layer 12 is powdered, the surface coating layer 13 containing zinc oxide on the
surface prevents adhesion of powder (AI-Fe powder or the like) to the following die.
Thus, a process of removing the AI-Fe alloy powder adhering to the die is not required
and thus, productivity is improved.
[0077]
The surface coating layer 13 functions as a protective coating for preventing
scratches of the steel sheet 11 andlor the A1 plating layer 12 or the like from being
generating during hot pressing, and thus, the formability of the Al-plated steel sheet 10
is further improved. Further, the surface coating layer 13 has an effect of suppressing
deterioration in the spot weldability of the Al-plated steel sheet 10 and the coating
adhesion or the like. By forming the chemical conversion coating on the Al-plated
steel sheet 10, tlie post-paining corrosion resistance of the Al-plated steel sheet 10 is
significantly improved, and thus, the deposition amount of the surface coating layer 13
can be reduced. As a result, when tile Al-plated steel sheet 10 is rapidly pressed, the
adhesion of the AI-Fe alloy powder exfoliated fiorn the AI-plated steel sheet 10 to the
die is suppressed and productivity is further improved.
When the deposition amount of the surface coating layer 13 with respect to
the steel sheet 11 (A1 plating layer 12) is 1 g/m2 in terms of Zn, the thickness of the
surface coating layer 13 is about 0.5 ptn.
[0078]
The Al-plated steel sheet 10 of the embodiment can be processed and formed
by various methods but is suitable for forming by a hot pressing method. Hereinafter,
a method for hot-pressing the Al-plated steel sheet 10 in the embodiment will be
described.
[0079]
In the method for hot-pressing tlie Al-plated steel sheet 10, first, the Al-plated
steel sheet 10 is blanked and then, heated and softened. The softened Al-plated steel
sheet 10 is press-formed into a desired shape, and then, cooled. Since the Al-plated
steel sheet 10 is softened once, the following press forming can be easily performed.
Tllen, the Al-plated steel sheet 10 is quenched by heating and cooling to obtain a steel
sheet having a high n~ecl~anicsatlr ength of 1500 MPa or more.
[0080]
As a heating method in the hot pressing tnethod, typical heating methods
using an electric furnace, a radiant tube furnace, inkared rays, or the like can be
adopted. When the Al-plated steel sheet 10 is heated to a melting point of aluminuln
or higher, the Al plating layer 12 is melted and aluminum and iron diffuse mutnally to
form an alloy layer of alulninum and iron (AI-Fe alloy layer) or an alloy layer of
aluminum, iron, and silicon (Al-Fe-Si alloy layer). Both the A-Fe alloy layer and Al-
Fe-Si alloy layer respectively have a high melting point and the melting point is about
11 50°C.
[0081]
In the Al-plated steel sheet 10, plural compounds which form the AI-Fe alloy
layer and the AI-Fe-Si alloy layer are present. However, these compoonds change to
compounds having high iron concentration by heating these compounds at a high
temperature or heating these compounds for a long period of time. A final product of
the Al-plated steel sheet 10 desirably has a surface state in which alloying extends to
the surface and the iron conce~~tratioisn n ot high in the alloy layer.
[0082]
When aluminum which is not alloyed remains, only a portion in wl~ich
aluminum remains is rapidly corroded and thus, the coating easily blisters atter
painting. Conversely, even when the iron concentration becomes excessively high in
the AI-Fe alloy layer, the corrosion resistance of the Al-Fe alloy layer itself deteriorates,
and the coating easily blisters after painting. The reason is that the corrosion
resistance of the AI-Fe alloy layer is dependent on the aluminum concentration in the
alloy layer.
[0083]
An alloying state therefore exists that is preferable for securing post-painting
corrosion resistance and the alloying state is determined by the deposition amount of
the plating and the heating condition.
[0084]
In the hot pressing method of the embodiment, an average temperature rising
rate until the temperature of the Al-plated steel sheet 10 reaches fro111 50°C to a
temperature 10°C lower than the maximum reaching temperature can be set to
10 "Clsec to 300 "Clsec. The productivity of the Al-plated steel sheet 10 is affected
by the average temperature rising rate. However, a general average temperature
rising rate is about 5 'Clsec in a case of atmospheric heating at a high temperature.
An average temperature rising rate of 100 "Clsec or higher can be achieved by
electrical heating or high frequency induction heating.
[OOSS]
When the aforementioned high average temperature rising rate is realized,
productivity is improved. In addition, the average temperature rising rate affects the
composition and the thickness of the AI-Fe alloy layer and is thus an important factor
of controlling product quality. In case of the Al-plated steel sheet 10 of tlie
ernbodinlent, since the temperature rising rate can increase to 300 OC/sec; productivity
is improved and product quality can be controlled over a broader range.
[0086]
Regarding heating before hot pressing, the heating needs to be performed in
the austenite region based on the principle of hot pressing. The peak temperature
(maximnum reaching temperature) in the heating is generally 900°C to 950°C.
Although the maximum reaching temperature is not particularly limited in the hot
pressing method of the embodiment, a temperature of lower than 850°C is not
preferable since sufficient hardness cannot be obtained. In addition, tlie A1 plating
layer 12 needs to change to an AI-Fe alloy layer, and thus, a temperature of lower than
850°C is not preferable.
100871
When the heating temperature is higher than 1000°C, the alloying advances
excessively, the iron coucentration ia the AI-Fe alloy layer increases to cause
deterioration in post-painting corrosion resistance. Although nothing absolute can be
said, the post-painting corrosiou resistance is dependent on the temperature rising rate
and the deposition amount of the Al plating layer 12, and thus, heating at a temperature
of higher than 1000°C is not desirable in terms of economic eficiency.
[0088]
As described above, in the Al-plated steel sheet 10 according to the first
embodiment, the surface coating layer 13 coutaining zinc oxide and the lubricity
improving compound including the transition metal element is formed on the surface
of the Al plating layer 12, and thus, more satisfactoly lubricity can be obtained than
that of the conventional one and improvement of formability and productivity can be
realized during hot pressing.
Further; in the Al-plated steel sheet 10 according to the first embodiment,
improvement of chemical co~iversionp roperties after hot pressing and post-painting
corrosion resistance can be realized.
[0089]
The reasoll why the chemical conversiou properties are improved by the
formation of the surface coating layer 13 (the adherence of the chernical conversion
coating is improved) is not clear. However, the chemical conversion reaction
progresses with an acid-etch reaction toward the substrate acting as a trigger, the zinc
oxide contained in the surface coating layer 13 is an amphoteric compound that
dissolves in an acid. Accordingly, it is considered that the reason why the chemical
conversion properties are irr~provedis that the zinc oxide contained in the surface
coating layer 13 easily reacts with a chemical conversion solutiot~.
[0090]
Further, the Al-plated steel sheet 10 according to the embodiment can be
fortned into machine parts having various shapes by the application of the
aforementioned hot pressing method and is particularly suitable for producing an
automotive part which requires weight reduction, high rigidity, and high corrosion
resistance. Examples of the automotive part include a door impact beam, a bumper
beam, and press-formed patts such as a center pillar reinforcing material.
FIG. 2 is an external view showing a center pillar reinforcing material as an
example of an automotive part. As sho\vn in FIG. 2, a center pillar reinforcing
material 100 is an autornotive part formed into a vertically long shape as seen fion~a
plan view, an upper end 11 1 is fixed to a roof side rail of an automobile, and a lower
end 112 is f ~ etdo a sill of a lower portion of the automobile. The collision safety of
automobiles cal be improved by forming the Al-plated steel sheet 10 of the
embodiment to produce the center pillar reinforcing material 100.
[0091]
[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG.
1B is a view showing a layer structure of an Al-plated steel sheet 20 according to the
second embodiment. As shown in FIG. lB, the Al-plated steel sheet 20 according to
tlie second embodiment includes a steel sheet 21, an A1 plating layer 22 wl~icliis
formed on one surface (for example, an upper surface) of the steel sheet 21, and a
surface coating layer 23 which is formed on the surface of the A1 plating layer 22.
In FIG. 19, a case in which the A1 plating layer 22 and the surface coating
layer 23 are formed on one surface of the steel sheet 21 is sho~vnb, ut the A1 plating
layer 22 and the surface coating layer 23 may be formed on both surfaces of the steel
sheet 21.
[0092]
The steel sheet 21 is a substrate of the Al-plated steel sheet 20 and has
required mechanical properties (tensile strength, yield point, elongation, reduction,
hardness, impact value, fatigue strength, creep strength, and other such properties
related to mechanical deformation and fracture). For example, the steel sheet 21
contains, by mass%, 0.1% to 0.4% of carbon (C), 0.01% to 0.6% of silicon (Si), 0.5%
to 3% of manganese (Mn), 0.01% to 0.1% of titaniutn (Ti), 0.0001% to 0.1% of boron
(B), and a balance consisting of iron (Fe) and unavoidable impurities.
[0093]
Hereinafter, the role of each of the aforementioned elements contained in the
steel sheet 21 will be described. In the following description, " % refers to mass%.
[0094]
Carbon is an elenlent for securing the tnechar~icasl trengtli of the steel sheet
21. When the carbon content in the steel sheet 21 is less than 0.1%, sufficient
mechanical strength cannot be obtained. On the other hand, when the carbon content
in the steel sheet 21 is more than 0.4%, the hardness (mechanical strength) of the steel
sheet 21 increases, but melting cracks are easily generated in the steel sheet 21.
Accordingly, the carbon content in the steel sheet 21 is desirably 0.1% to 0.4%.
[0095]
Silicon is an element for securing the mechanical strength of the steel sheet 21,
similar to carbon. When the silicon content in the steel sheet 21 is less than 0.01%,
strength improving effect cannot be exhibited and suficient mechanical strength
cannot be obtained. On the other hand, silicon is also an easily oxidizable element,
and thus, when the silicon content in the steel sheet 21 is more than 0.6%, wvettability
deteriorates during hot-dip Al plating of the steel sheet 21 and unplating occurs.
Accordingly, the silicon content in the steel sheet 21 is desirably 0.01% to 0.6%.
[0096]
Manganese is an elenlent for increasing the hardenability of the steel sheet 21
to increase the strength of the steel sheet 21. In addition, manganese combines with
sulfur (S) which is an unavoidable impurity in the steel sheet 21 to form manganese
sulfide (MnS) and prevents hot embrittlement of the steel sheet 21 by sulfi~r. When
the manganese content in the steel sheet 21 is less than 0.5%, the addition effect is not
exhibited. On tlie other hand, when the manganese content in the steel sheet 21 is
more than 3%, residual y phase in the steel sheet 21 increases and the strength
decreases. Accordingly, the manganese content in the steel sheet 21 is desirably 0.5%
to 3%.
[0097]
Titanium is a strength strengthening element and also an element for
improving the heat resistance of the A1 plating layer 22. When the titanium content in
the steel sheet 21 is less than 0.01%, a strength improving effect and oxidation
resistance improving effect cannot be obtained. On the other hand, wvl~en the titanium
content in the steel sheet 21 is more than 0.1%, for example, carbides and nitrides are
formed and the steel sheet 21 is likely to be softened. Thus, tlie desired mechanical
strength cannot be obtained. Accordingly, the titaniun content in the steel sheet 21 is
desirably 0.01% to 0.1%.
[0098]
Boron is an element for increasing the hardenability of the steel sheet 21 to
improve the strengtl~o f the steel sheet 21. When the boron content in the steel sheet
21 is less than 0.0001%, a strength improving effect cannot be exhibited. On the
other hand, wllen the boron content in the steel sheet 21 is more than 0.1%, the fatigue
strength of the steel sheet 21 decreases owing to fonnation of inclusions.
Accordingly, the boron content in the steel sheet 21 is desirably 0.0001% to 0.1%.
[0099]
The steel sheet 21 may contain unavoidable impnrities mixed in other
production processes within a range not impairing the mechanical strength of the steel
sheet 21.
[OlOO]
In the steel sheet 21 having the above component composition, the mechanical
strength is increased to about 1500 MPa or more by quenching in the hot pressing
method. In the hot pressing method, since the steel sheet 21 is press-formed in a state
in which the steel sheet is softened, forming is easy. In addition, ~vl~tehne steel sheet
21 is made thin to reduce the weight, high mecl~anicals trength is maintained.
[OlOl]
The A1 plating layer 22 is fonned on one surface (for example, an upper
surface) of tile steel sheet 21 and contains at least 85% or more of alutninum (Al) by
mass%. For example, the A1 plating layer 22 is desirably formed by a hot-dip plating
method. However, the forming method thereof is not limited to the hot-dip plating
method.
[0102]
As described above, the A1 plating layer 22 preferably contains 85% or more
of aluminum and may colltain components other than alutninum. Components other
than alutninum are not particularly limited, but silicon is preferable for the following
reasons.
[0103]
Silicon is an element having a function of suppressing formation of an alloy
layer of iron and aluminum (hereinafter, referred to as a Fe-A1 alloy layer) during the
hot-dip plating of the steel sheet 21. When the silicon content of the Al plating layer
22 is less than 3%, the Fe-A1 alloy layer grows thickly during the hot-dip plating of the
steel sheet 21, and thus, cracking of the plating layer is facilitated dnring the process
and corrosion resistance may deteriorate. On the other hand, wile11 the silicon content
of the A1 plating layer 22 is inore than 15%, the workability and the corrosion
resistance of the Al plating layer 22 deteriorate. Thus, the silico~ci ontent of the Al
plating layer 22 is desirably 3% to 15%.
[0104]
The Al plating layer 22 having the above component conlposition has a
function of preventing corrosion of the steel sheet 21 and preventing scales (oxides of
iron) from being formed on the surface of the steel sheet 21 due to heating before hot
pressing.
[0105]
Accordingly, a scale removing process, a surface cleaning process, a surface
treating process, and tlie like can be omitted by forming tlie A1 plating layer 22 on tlie
steel sheet 21, and thus, the productivity of the Al-plated steel sheet 20 is in~proved.
In addition, the Al plating layer 22 has a higher melting point than those of coatings
made of organic materials and coatings made of other metal materials (for example,
211-based material) and thus can be processed at a high temperature during hot pressing.
[0106]
Some of aluminum contained in the A1 plating layer 22 is alloyed ~vithir on in
the steel sheet 21 during tlie hot-dip plating or the hot pressing. Accordingly, the Al
plating layer 22 is not necessarily formed with a single layer wit11 a fixed component
co~npositiona nd may include a partially alloyed layer (an alloy layer).
Further, wvl~en the deposition amount of the Al plating layer 22 with respect to
the steel sheet 21 is 80 g/m2 per surface, the thickness of tlie Al plating layer 22 per
surface is about 15 pm.
[0107]
The surface coating layer 23 is laminated on the surface of tlie Al plating layer
22 and is a surface coating layer containing at least zinc oxide (ZnO). For example,
the surface coating layer 23 is formed on the surface of the A1 plating layer 22 by
applying a solution in which fine zinc oxide particles are suspended to the surface of
the A1 plating layer 22 by a roll coater or the like. The surface coating layer 23 has an
effect of improving lubricity during the hot pressing of the Al-plated steel sheet 20 and
reactivity with a chemical conversion solution.
[0108]
The surface coating layer 23 fi~rtherc ontains one or niore lubricity improving
compounds, in addition to the aforementioned zinc oxide. In tlie first embodiment, a
compound including the transition metal element is used as a lubricity improving
con~pound. However, in the second embodiment, as a lubricity improving compound,
a compound including one or tnore typical elements (elements belonging to the first
group and the second group and the twelfth group to tlie eighteenth group in the
periodic table) having a temperature rising property improving effect (temperature
rising rate improving effect) of tlie Al-plated steel sheet 20 during heating, in addition
to a lubricity itliproving effect, is used.
Particularly, the aforementioned typical element is preferably any one or niore
of magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), phosphorus (P), tin
(Sn), and germanium (Ge).
Compounds of these typical eletnents are desirably oxides, and for example,
magnesia, calcia, or tile like is preferable. It is considered that these compounds have
high emissivity and effectively absorb heat in the heating fi~rnace. Particularly, when
an emitting body emitting far-infrared rays is used in the heating furnace, the
temperature properties are significantly improved. The reason thereof is assumed that
the oxide including zinc oxide has high emissivity with respect to a ~vavelengtlio f a
far-infrared region.
[O 1091
For example, the surface coating layer 23 including zinc oxide and the
compound of the aforementiol~edty pical element (lubricity improving compound) is
formed on the surface of the Al plating layer 22 by applying a coating containing zinc
oxide and the lubricity improving compound to the surface of the A1 plating layer, and
baking and drying the coating after applying the coating.
[OllO]
As a method for applying zinc oxide and the lubricity improving compound,
for example, a metliod of forming a coating by mixing a suspension containing zinc
oxide and the lubricity improving compound containing the typical metal element with
an organic binder, and applying the coating to the surface of the A1 plating layer 22,
and a method of application by powder coating can be used.
[Olll]
As the lubricity iinproving cotnpound including the aforementioned typical
element, for example, magnesia (MgO), calcia (CaO), forsterite (MgzSiOd), or the like
is preferably used. Further, as the organic binder, for example, polyurethane resins,
polyester resins, acrylic resins, silane coupling agents, or the like are preferably used.
The aforementioned lubricity improving compound and organic binder are preferably
aqueous to be mixed with the suspension containing zinc oxide respectively. In this
manner, the coating obtained by mixing the suspension containing zinc oxide and the
lubricity improving compound and the organic binder is applied to the surface of the
Al-plated steel sheet 22.
[0112]
The particle size of zinc oxide is not particularly limited and the diameter
thereof is desirably 50 nm to 1000 nm. The particle size of zinc oxide is a particle
size after heat treatment is performed. Specifically, the particle size of zinc oxide is
determined by observing the zinc oxide with a SEM or the like after zinc oxide is
retained in a furnace at 900°C for 5 minutes to 6 minutes and rapidly cooled with a die.
[0113]
The particle size of the lubricity improving compound including the typical
element in the surface coating layer 23 is not particularly limited. However, the
particle size thereof is desirably the same as the particle size of zinc oxide.
[0114]
The amount of the lubricity improving compoutld including the typical
element in the surface coating layer 23 is preferably 5% to 30% with respect to the
total amount of zinc oxide by mass ratio. When the amount of the lubricity
improving compound is less than 5%, a sufficient temperature rising property
improving effect (temperature rising rate inlproving effect) cannot be obtained in the
heating furnace. On the other hand, when the amount of the lubricity improving
conlpound is more than 30%, reactivity with a chemical co~lversiotsi olution is easily
impaired after heating.
[0115]
The amount of the organic binder (organic compound) such as a resin
comporient or a silane coupling agent in tlie surface coating layer 23 is desirably 5% to
30% with respect to tlie total amount of zinc oxide by mass ratio. When tlie amount
of the organic binder is less than 5%, a sufficient binder effect cannot be obtained and
the coating before heating is easily exfoliated. In order to stably obtain the binder
effect, the amount of the organic binder is preferably 10% or Inore with respect to the
total amount of zinc oxide by mass ratio. When the amount of the organic binder is
inore than 30%, odor emission becomes significant during heating, aid tl~ust,l ie
amount of more than 30% is not preferable.
[0116]
The present inventors have confirmed that the surface coating layer 23
according to the embodiment has higher lubricity compared to an inorganic compound
coating containing at least one of silicon, zirconium, titanium, and phosphorus
described in Patent Document 2, a1 orgatiic cotiipout~dc oating, or a conlplex
compound coating thereof. Thus, according to the Al-plated steel sheet 20 of the
embodiment, more satisfactory lubricity than that of the conventional one can be
achieved and improvement of formability and productivity can be realized during hot
pressing.
[0117]
The surface coating layer 23 preferably contains 0.3 g/m2 to 7 g/m2 of zinc
oxide in ternis of zinc.
[0118]
When the amount of zinc oxide is 0.3 &n2 or more in the surface coating
layer 23 in terms of zinc, a lubricity improving effect is exhibited. On the other hand,
~vl~ethne amount of zinc oxide in the surface coating layer 23 is more than 7 g/m2 in
terms of zinc, the tl~icknesseso f the A1 platiug layer 22 and the surface coating layer
23 are increased and thus, weldability and coating adhesion deteriorate.
[0119]
Accordingly, the amount of zinc oxide in the surface coating layer 23 is
desirably 0.3 g/m2 to 7 g/nxZ on one surface of the surface coating layer 23 in terms of
zinc. From the viewpoint of securing lubricity during the hot pressing and further
securing satisfactory weldability and coating adhesion, the anlout~ot f zinc oxide in the
surface coating layer 13 is particularly desirably 0.5 g/m2 to 2 g/m2.
[OI 201
As a method for baking and drying the coating containing zinc oxide, tlie
lubricity improving compound, and the organic binder after the applying of the coating,
for example, a method using an hot air furnace, an induction heating funlace, a nearinfrared
furnace, or tlie like, or a method in \vliich these furnaces are combined may be
used. Depending on the type of the organic binder, instead of baking and drying after
the applying of the coating, a curing method using ultraviolet rays or electron beams
tnay be adopted. The method for forming the surface coating layer 23 is not limited
to the above method and various coating forming methods can be adopted.
[0121]
When the organic binder is not used, the surface coating layer 23 has sligl~tly
low adhesion with the Al plating layer 22 before heating and partially exfoliated when
being rubbed wit11 great force. In this manner, since the surface coating layer 23 of
the AI-plated steel sheet 20 exhibits satisfactory lubricity during hot pressing, the
formability of the Al-plated steel sheet 20 is improved during the hot pressing and the
corrosion resistance of the Al-plated steel sheet 20 after the hot pressing is also
improved.
[O 1221
In addition, the surface coating layer 23 of the Al-plated steel sheet 20 has an
effect of suppressing adhesion of the Al-plated steel sheet 20 to a die. If t11eAl
plating layer 22 is powdered, the surface coating layer 23 containing zinc oxide on the
surface prevents adhesion of powder (Al-Fe powder or the like) to the following die.
Thus, a process of removing the AI-Fe alloy powder adhering to the die is not required
and thus, productivity is improved.
[0123]
The surface coating layer 23 functions as a protective coating for preventing
scratches of tlie steel sheet 21 and/or t11eAl plating layer 22 or the like from being
generating during hot pressing, and thus, the formability of the Al-plated steel sheet 20
is further improved. Further, the surface coating layer 23 has an effect of suppressing
deterioration in the spot weldability of the Al-plated steel sheet 20 and the coating
adhesion or the like. By forming tlie cliemical conversion coating on the Al-plated
steel sheet 20, the post-painting corrosion resistance of the Al-plated steel sheet 20 is
significantly improved, and thus, the deposition amount of the surface coating layer 23
can be reduced. As a result, when the Al-plated steel sheet 20 is rapidly pressed, the
adhesion of the Al-Fe alloy powder exfoliated from the Al-plated steel sheet 20 to tlie
die is suppressed and productivity is fi~rtherim proved.
When the deposition amount of the surface coating layer 23 with respect to
the steel sheet 21 (A1 plating layer 22) is 1 g/m2 in terms of Z11, the thickness of the
surface coating layer 23 is about 0.5 pm.
[0 1241
The Al-plated steel sheet 20 of the en~bodi~necnatn be processed and fornied
by various methods but is suitable for forming by a hot pressing method. Hereinafter,
a method for hot-pressing the Al-plated steel sheet 20 in the embodiment will be
described.
[0125]
In the tnethod for hot-pressing the Al-plated steel sheet 20, first, the Al-plated
steel sheet 20 is blanked and then, heated and softened. The softened Al-plated steel
sheet 20 is press-formed into a desired shape, and then, cooled. Since the Al-plated
steel sheet 20 is softened once, the following press forming can be easily performed.
Then, the Al-plated steel sheet 20 is quenched by heating and cooling to obtain a steel
sheet having high mechanical strength of 1500 MPa or more.
[0126]
As a heating method in the hot pressing method, typical heating methods
using an electric furnace, a radiant tube furnace, infrared rays, or the like can be
adopted. When the Al-plated steel sheet 20 is heated to the melting point of
aluminum or higher, the Al plating layer 22 is melted and alttminu~na nd iron diffi~se
mutually to form an alloy layer of aluminum and iron (AI-Fe alloy layer) or an alloy
layer of aluminum, iron, and silicon (Al-Fe-Si alloy layer). Both the AI-Fe alloy layer
and Al-Fe-Si alloy layer respectively have a high melting point and the melting point is
about 1150°C.
[0 1271
In the Al-plated steel sheet 20, plural compounds which form the A1-Fe alloy
layer and the Al-Fe-Si alloy layer are present. However, these compounds change to
compoul~dsh aving high iron con cent ratio^^ by heating these compounds at a high
temperature or heating these compounds for a long period of time. A final product of
the Al-plated steel sheet 20 desirably has a surface state in which alloying extends to
the surface atld the iron concentration is not high in the alloy layer.
[0128]
When aluminum wvl~ich is not alloyed remains, only a portion in wvliicli
aluminum remains is rapidly corroded and thus, the coating easily blisters after
painting. Conversely, even when the iron concentration becomes excessively high in
the alloy layer, the corrosion resistance of the alloy layer itself deteriorates, and the
coating easily blisters after painting. The reason is that the corrosion resistance of the
alloy layer is dependent on the aluminum concentration in the alloy layer.
[0129]
An alloying state therefore exists that is preferable for securing post-painting
corrosion resistance and the alloying state is determined by the deposition amount of
the plating and the heating condition.
[0130]
In the hot pressing method of tlie emnbodiment, an average temperature rising
rate until the temperature of the Al-plated steel sheet 20 reaches from 50°C to a
temperature 10°C lower than the maximum reaching temperature can be set to
10 "Clsec to 300 "Clsec. The productivity of the AI-plated steel sheet 20 is affected
by the average temperature rising rate. However, a general average temperature
rising rate is about 5 "Clsec in a case of atnlospheric heating at a liigll tetnperature.
An average teniperatnre rising rate of 100 "Clsec or higher can be achieved by
electrical lieating or high frequency induction lieating.
[0131]
When tlie aforementioned high average temperature rising rate is realized,
productivity is improved. In addition, the average tetnperature rising rate affects the
composition and tlie thickness of the alloy layer and is thus an itnportant factor
co~ltrollingp roduct quality. In case of the Al-plated steel sheet 20 of the embodiment,
since the temperature rising rate can increase to 300 OC/sec, productivity is improved
and product quality can be controlled over a broader range.
[0132]
Regarding heating before hot pressing, the heating needs to be performed in
the austenite region based on the principle of hot pressing. The peak temperature
(maximum reachiug temperature) in the heating is generally 900°C to 950°C.
Although the maximum reaching temperature is not particularly limited in the hot
pressing method of the embodiment, a temperature of lower than 850°C is not
preferable since sufficient hardness cannot be obtained. In addition, the Al plating
layer 22 needs to change to an alloy layer, and thus, a temperature of lower than 850°C
is not preferable.
[0133]
When the heating temperature is higher than 1000°C, the alloying advances
excessively, the iron concentratiou in the alloy layer increases to cause deterioration in
post-painting corrosion resistance. Although nothing absolute can be said, the postpainting
corrosion resistance is dependent on the temperature rising rate and the
deposition amount of the A1 plating layer 22, and thus, heating at a temperature of
1100°C or higher is not desirable in terms of economic efficiency.
[0134]
As described above, in the Al-plated steel sheet 20 according to the second
embodiment, the surface coating layer 23 containing zinc oxide and the lubricant
compound i~lcludingth e typical element is formed on the surface of the Al plating
layer 22, and thus, more satisfactoty lubricity can be obtained than that of the
conventional one and improvetnetlt of formability and productivity can be realized
during hot pressing.
In addition, according to the Al-plated steel sheet 20 of the second
embodiment, iniprovement of clie~nicacl onversion properties after hot pressing and
post-painting corrosion resistance can be realized.
Further, according to the Al-plated steel sheet 20 of the second embodiment,
in addition to the aforementioned effect, a temperature rising property improving effect
(temperature rising rate improving effect) during heating can be obtained.
[0135]
The Al-plated steel sheet 20 accordi~igto the second embodiment cai be
formed into machine parts having various shapes by the application of the
aforemeutioned hot pressing method, similar to the first e~nbodimenta, id is
particularly suitable for producing an automotive part which requires weight reduction,
high rigidity, and high corrosion resistance (for example, a door impact beam, a
bumper beam, and press-formed parts such as a center pillar reinforcing material).
[Examples]
[0136]
Although examples of the present invention will be described below,
conditions employed in the examples are merely one condition example employed so
as to confirm the operability and effect of the present invention, and the present
invention is not limited to the one condition example. The present invention can
employ various conditions as long as tile object of the present invention is achieved
\vitl~out departing from the scope of the present invention.
[0137]

A cold-rolled steel sheet having the component coniposition shown in Table 1
(1.4 Inn1 thickness) was plated with A1 by the Sendzimir method. The annealing
temperature was set to about 800°C, and the Al plating bath contained 85% or more of
Al and 9% of Si %by mass% and additionally, contained Fe eluted from the steel sheet.
[Table 11
(~nass%)
After tl~est eel sheet was hot-dip plated, the deposition amount of the plating
was adjusted to 160 g/~no2n both surfaces of the steel sheet by the gas wiping method.
The steel sheet in which the Al plating layer was formed in this manner was cooled,
and then, a coating treatment solution shown in Tables 2 and 3 (the continuation of
Table 2) was applied to the surface of the A1 plating layer with a roll coater and baked
at about 80°C. Thus, a surface coating layer including zinc oxide and the lubricity
improving cotnponnd (particularly, the compound of the transition metal element
described in the first embodiment) was formed. All coating treatment solutions
shown in Tables 2 and 3 were suspensions or aqueous solutions in which a reagent and
distilled water are mixed.
[0140]
The properties of the steel sheet in which t11eAl plating layer and tlie surface
coating layer including the compound of the transition metal element were formed in
this manner (that is, the Al-plated steel sheet corresponding to the first embodiment:
hereinafter, referred to as a first Al-plated steel sheet) were evaluated in the following
metl~ods.
[0141]
(1) Hot Lubricity
The hot lubricity of the first Al-plated steel sheet was evaluated using the
apparatus shown in FIG 3. A test piece 1 having a size of 150 nun x 200 mm
collected from tlie first Al-plated steel sheet was placed on a test piece stand 4 of a
furnace 3 provided on the upper portion of a heater 2, and then, the test piece 1 was
heated to 900°C. Then, in a state in which a load P (pressing load) was applied to the
test piece 1 using a load applying apparatus 5 under the temperature condition of about
700°C, the furllace 3 was driven by a furnace driving apparatus 6 along a ball way 7 to
measure the drawing load of the test piece 1.
[0142]
The drawing load of the test piece 1 was measured by a load cell 8 connected
to the load applying apparatus 5. A dynamic friction coefficient was calculated by
dividing tlie drawing load by the pressing load.
[0143]
(2) Coating Adhesion after Heating
A test piece collected from the first Al-plated steel sheet was placed in an
atmospheric fi~rnacea nd was heated at 900°C for 6 minutes. Then, the test piece was
taken out from the atmosplieric fi~rnace,i mmediately clamped by a stainless steel die,
and rapidly cooled. The temperature rising rate of the test piece was about 5 'Clsec,
and the cooling rate was about 150 "C/sec. Next, tlie test piece was cut to have a size
of 50 nnn x 50 mm and was provided for a n~bbingte st. In the rubbing test, a gauze
was rubbed on the surface of tlie test piece 10 times with a load of 1.5 kgf within a
width of 30 mm, and the Zn deposition amounts of the gauze before and after the test
were measured to calculate tlie reduction rate (%) of Zn.
[0144]
(3) Strength of Spot Welded Joint
Atest piece collected from the first Al-plated steel sheet was placed it1 the
atmospheric fitrnace and was heated at 90OoC for 6 minutes. Then, the test piece was
taken out from the atmospheric firrt~acei, mmediately clamped by a stainless steel die,
and rapidly cooled. The temperature rising rate of the test piece was about 5 "Clsec,
and the cooling rate was about 150 "Clsec. Next, the cross tensile strength of the test
piece was lneasured according to JIS 23137 (1990). At tliis time, the welding
conditions are as follo~is. In the test, at1 average value of joint strength ~vas
calculated by setting the number N of samples of the test piece to 3 and measuring the
strength of each of 3 samples.
[0145]
Electrode: chromium-copper, DR (8 mm+ tip of 40 R)
Applied pressure: 880 kgf
Energizing duration: up-slope 3 cycles - 22 cycles energized (60 Hz)
Weld current: 9.5 kA
(4) Post-painting Corrosion Resistance
A test piece collected from the first Al-plated steel sheet was placed in the
atmospheric furnace and was heated at 900°C for 6 minutes. Tlien, the test piece was
taken out from the atmospheric fi~rnace,i mmediately clamped by a stainless steel die,
and rapidly cooled.
[0146]
The temperattire rising rate of the test piece was about 5 'Clsec, and the
cooling rate was about 150 "Clsec. Next, the test piece was cut to have a size of 70
mm x 150 mm and was subjected to chemical collversion using a chetnical conversion
solutiot~(P B-SX35) manufactured by Nihon Parkerizing Co., Ltd. Then, the test
piece was painted with an electrodeposition coating (Powemics 110) manufactured by
Nippon Paint Co., Ltd. to have a coating thickness of 20 pm, and baked at 170°C.
[0147]
The post-painting corrosion resistance of the test piece was evaluated by the
method defined by JASO M609 (1991) defined by the Society ofAi~tomotive
Engineers of Japan. A cutter was used to make a crosscut in the coating in advance,
and the width (maximum value on one side) of the coating blister from the crosscut
after a corrosion test was performed for 180 cycles (60 days) was n~easured. As for a
comparison example, a galvannealed steel sheet with a deposition anlonnt of 45 g/m2
on one surface was evaluated. At this time, the blister width thereof was 7 nlm.
[0148]
Each evaluation result is summarized in Tables 2 and 3 (the continuation of
Table 2). In addition, for comparison, a case in which the surface coating layer was
not formed was also evaluated in the same manner. The evaluation results are shown
in Table 4.
The hot lubricity indicates the measured dynamic friction coefficient, the
[Table 41
coating adhesion indicates Zn exfoliation rates before and after the test, the spot joint
Hot lubricity
0.95
strength indicates the cross tensile strength, and the post-painting corrosion resistance
indicates the maximum blister width on one surface from the crosscut. Exaniples
Coating
adhesion
-
(Nos. 1 to 23 in Tables 2 and 3) having the surface coating layer containing ZnO
exhibit i~nprovedh ot lubricity compared to Comparative Examples (Nos. 24 to 29 in
Strength of spot
welded joint
7.3 kA
Table 3) in which the surface coating layer is not formed
[0153]
Post-painting
corrosion resistance
6.5 mnm
However, if the deposition amount of the surface coating layer is relatively
small, sufficient hot lubricity cannot be obtained. When the deposition amount of the
surface coating layer is large, the strength of the spot joint, particularly, the cross
tensile strength, decreases. A decrease in the strength of the spot joint and decrease in
the cross tensile strength are not desirable in terms of stability of automotive part
quality. The reason why the strength of the spot joint decreases when the deposition
amount of the surface coating layer is large is not clear. However, there is a
possibility that oxides may remain in a wvelding nogget and stress may be concentrated
on the portion in ~vhichth e oxides remain during drawing.
[0154]
Contrarily, the surface coating layer (Nos. 7 to 13 in Table 2) including a
compound of one or two or more transition metal ele~nents(p articularly, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zr, Mo, W, La, and Ce) exhibits excellent hot lubricity in a region with
a snlall deposition amount, and whet] the amount is 2 g/m2 or less in terms of Zn, a
decrease in the strength of the spot joint can be suppressed.
[0155]
Although ~vl~ethtehre aforementioned compound affects post-painting
corrosion resistance is not clear, compared to the steel sheets without the surface
coating layer, all steel sheets exhibit excellent post-painting corrosion resistance.
This is because the chemical conversion properties are improved.
[0156]
The influence of the deposition amount of the surface coating layer on the
properties was investigated. For Nos. 7 to 13 in Table 2 (Exaniples), Nos. 24 to 29 in
Table 3 (Comparative Examples), and a case in which the surface coating layer is not
fonned of Table 4, the coating exfoliation rate, the hot lubricity (friction coefficient),
and the strength of the spot joint are respectively shown in FIGS. 4, 5, and 6.
[0157]
Compared to a system not including a compound of one or two or more
transition metal eletnents (particularly, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Mo, W, La,
and Ce), a system including these con~poundse xhibits improved hot lubricity with a
stnaller deposition amount.
LO1581
When the deposition amount of the surface coating layer increases, the
strength of the spot welded joint and the coating adhesion decrease. It is found that in
order to increase the strength of the spot welded joint, the deposition amount of the
surface coating layer is preferably 2 dm2 or less (refer to FIG 6), and in order to
increase the coating adhesion, the deposition amount is preferably 7 g/m2 or less (refer
to FIG. 4).
[0159]
The test piece 1 of Example 1 was rapidly heated with near-infrared rays.
The temperature rising rate was set to 21 "Clsec. The quenching of the test piece was
performed in the same manner as in Example 1, and the following evaluation was also
performed in the same manner as in Example 1. The evaluation results are shown in
Table 5. It was found that altlio~ought he post-painting corrosion resistance was
improved, there was no change in properties other than the post-painting corrosion
resistance. From the results, it is possible to expect further improvement in the
properties by rapid heating.
An aqueous solution in which 20% of urethane resin was added to ZnO was
[Table 51
used as a base, and 10% of sodium vanadate, sodium dichromate, ferrous sulfate,
Hot lubricity
0.62
cobalt chloride, sodium molybdate, sodiu~ntu ngstate, cerium nitrate were added to tlie
solution.
[0 1621
Coating
adhesion
8%
1 g/~no2f the thus-prepared treatnient solutiou in terms of Zn was applied to
the first Al-plated steel sheet used in Example 1, and the hot lubricity was measured by
Strength of spot
welded joint
7.3 kA
tlie nlethods described in Example 1. As a result, a nu~nericavl alue in a range of 0.60
Post-painting
corrosion resistance
1.2 mm
to 0.65 was obtained in all the methods. Thus, it is found that all of the
aforementioned compounds contribute to improving hot lubricity.
[0 1631
A cold-rolled steel sheet having the component composition shown in Table 6
(1.4 mni thickness) was plated with Al by the Sendzimir method. The annealing
temperature was set to about SOO°C, and tlie Al plating bath contained 85% or more of
Al and 9% of Si %by mass% and additionally, contained Fe eluted from the steel sheet.
[Table 61
After the steel sheet was hot-dip plated, the deposition amount of the plating
was adjusted to 160 g1111~o n both surfaces of tlie steel sheet by the gas wiping method.
The steel sheet in whicli the Al plating layer was formed in this manner was cooled,
and then, a coating treatment solution shown in Table 7 was applied to the surface of
the A1 plating layer with a roll coater and baked at about 80°C. Thus, a surface
coating layer including zinc oxide and the lubricity iniproving compound (particularly,
the compound of the typical element described in the second embodiment) was formed.
All coating treatment solutions shown in Table 7 were suspensions or aqueous
solutions in whicli a reagent and distilled water are mixed.
[0166]
As described above, tlie propel-ties of the steel sheet including the A1 plating
layer and the surface coating layer containing the compound of the typical elemeut
(that is, corresponding to the Al-plated steel sheet of the second embodiment:
hereinafter, referred to as a second Al-plated steel sheet) were evaluated by the
following methods.
[0 1671
(1) Hot Lubricity
The hot lubricity of the second Al-plated steel sheet was evaluated using the
apparahls shown in FIG. 3. A test piece 1 having a size of 150 Inn1 x 200 mm
collected from the secolld Al-plated steel sheet was placed on a test piece stand 4 of a
furnace 3 provided on the upper portion of a heater 2, and then, the test piece 1 was
heated to 900°C. Then, in a state in which a load P (pressing load) was applied to the
test piece 1 using a load applying apparatus 5 under the temperature condition of about
70OoC, the furnace 3 was driven by a furnace driving apparatus 6 along a ball way 7 to
measure the drawing load of the test piece 1. The drawing load of the test piece 1
was measured by a load cell 8 connected to the load applying apparatus 5. A dynamic
friction coefficient was calculated by dividing the drawing load hy the pressing load.
[0 1 681
(2) Temperature Rising Properties during Heating
After a thermocouple was welded to a test piece having a size of 70 mm x 150
Inn1 collected from the second Al-plated steel sheet, the test piece was placed in an
atmospheric furnace with a temperature set to 900°C, and a time until the temperature
of the test piece reached from 50°C to 890°C was measured to calculate an average
temperature rising rate.
[0 1691
(3) Strength of Spot Welded Joint
A test piece collected from the second Al-plated steel sheet was placed in the
atmospheric furnace and was heated at 900°C for 6 minutes. Then, the test piece was
taken out from the atmospheric furnace, immediately clamped by a stainless steel die,
and rapidly cooled. The cooling rate of the test piece was about 150 "C/sec. Next,
the cross tensile strength of the test piece was measured according to JIS 23137. At
this time, the welding conditions are as follows. 111 the test, an average value of joint
strength was calculated by setting the number of samples of the test piece to 3 and
measuring the strength of each of 3 samples.
[0170]
Electrode: chromium-copper, DR (8 inm@ tip of 40 R)
Applied pressure: 880 kgf
Energizing duration: up-slope 3 cycles - 22 cycles energized (60 Hz)
Weld current: 9.5 kA
(4) Post-painting Corrosion Resistance
A test piece collected from the second Al-plated steel sheet was placed in the
atmospheric furnace and was heated at 900°C for 6 minutes. Then, the test piece was
taken out from the atmospl~ericfu rnace, immediately clamped by a stainless steel die,
and rapidly cooled. The cooling rate of the test piece was about 150 OCIsec. Next,
the test piece was cut to have a size of 70 mm x 150 mm and was subjected to
chemical conversion using a cliemical conversion solutiotl (PB-SX35) manufactured
by Nihon Parkerizing Co., Ltd. Then, the test piece was painted with an
electrodeposition coating (Powernics 110) manufactured by Nippon Paint Co., Ltd. to
have a thickness of 20 pm, and baked at 170°C.
[0171]
The post-painting corrosion resistance of the test piece was evaluated by the
method defiued by JASO M609 defined by the Society of Automotive Et~git~eeorsf
Japan. A cutter was used to make a crosscut in the coating in advance, and the width
(maximum value on one side) of the coating blister from the crosscut after a corrosion
test was performed for 180 cycles (60 days) was measured. As a compariso~m~a terial,
a galvannealed steel sheet with a deposition amount of 45 g/m2 on one surface was
evaluated. At this time, the blister width thereof was 7 mm.
[0172]
Each evaluation result is sun~marizedin Table 7. In addition, for comparison,
a case in which the surface coating layer was not fanned was also evaluated in the
same manner. The evaluatiol~re sults are shown in Table 8.
[0175]
The hot lubricity indicates the measured dynamic f'rictiori coefficient, the
temperature rising rate indicates a value obtained by measuring a temperature rising
rate, the strength of the spot welded joint indicates the cross tensile strength, and the
post-painting corrosion resistance indicates the maximum blister width 011 one side
from the crosscut. Examples (refer to Table 7) having the surface coating layer
containing ZnO exhibit improved hot lubricity and post-painting corrosion resistance
Hot lubricity
0.93
compared to Comparative Exar~~pl(erse fer to Table 8) in ~vhichth e surface coating
layer is not formed.
[0176]
On the other hand, regarding the temperature rising rate, it is found that
Examples (Nos. 1 to 7 in Table 7) containing a compound of one or lsvo or nlore
typical elements (particularly, Mg, Ca, Sr, Ba, P, Sn, and Ge) have excellent
tetnperature rising properties compared to steel sheets without the surface coating layer
Temperature rising
rate
4.5 OCls
or Comparative Example (No. 8 in Table 7) in which the surface coating layer was
only con~posedo f ZnO and a binder.
[0177]
Nos. 9 to 14 in Table 7 are Examples in ~vl~itchhe deposition amount of the
surface coating layer is changed, andNos. 15 to 18 are Examples in which the amount
of the Ca compound in the surface coating layer is changed. It is found that when the
surface coating layer is thin, the hot lubricity is lightly low, and when the surface
Strength of spot
joint
7.4 kA
Post-painting
corrosion resistance
6.8 mm
coating layer is thick, the strength of the spot welded joint slightly decreases. Further,
it is also found that when the amount of tlie Ca compoulld is smrill, the temperature
rising rate is slightly low, and when the anioont of the Ca compound is large, the postpainting
corrosion resistance slightly deteriorates.
[0178]

Test pieces Nos. 1 to 8 in Example 4 were heated wit11 far-infrared rays. At
this time, a furnace having a tenlperature rising furnace and a holding furnace was used
and the movement between the furnaces was performed manually. While the
tenlperatnre of the temperature rising furnace was set to 1150°C and the temperature of
the holding furnace was set to 900°C, a thermocouple was welded to each test piece
having a size of 70 mm x 150 mm. When the temperature of the test piece reached
850°C in the temperature rising furnace, the test piece was moved to tlie holding
furnace.
[0179]
Similar to Example 4, an average temperature rising rate was calculated at
50°C to 890°C. Quenching was perfonned in the sanle manner as in Example 4 and
the evaluation after the quenching was also perfonned in the same manner as in
Exanlple 4. The evaluatio~rle sults are shown in Table 9. When the temperature
rising rate was high, it was recognized that the post-painting corrosion resistance was
improved. A higher temperature rising rate was obtained in No. 1 compared to No. 8.
-
No. Hot lubricity Temperature
rising rate
("C/s)
Strength of
spot welded
joint
Post-painting
corrosion
resistance
[0181]

To an aqueous solution in \vliich 20% of urethane resin was added to ZnO,
and 1) 5% of MgO and CaO were respectively added, 2) 5% of CaO and SrO were
respectively added, and 3) 5% of Sn02 and GeO2 were respectively added to prepare a
treatment solution. 2 g/m2 of the treatment solution in terms of Zn was applied to the
second Al-plated steel sheet used in Example 4, and the temperature rising rate was
measured by the methods described in Example 4. As a result, the temperature rising
rate in a range of 8 "C/sec to 8.5 "Clsec was obtained in all the methods. As seen
from the result, it is found that all of the aforementioned compounds contribute to
improving hot lubricity.
[Industrial Applicability]
[0182]
According to the present invention, it is possible to improve formability and
productivity in the hot pressing by securing the lubricity of the A1 plating layer in the
hot pressing of the Al-plated steel sheet. Further, according to the present invention,
it is possible to improve the chemical conversion properties of the Al-plated steel sheet
after the hot pressing, the post-painting corrosion resistance of the Al-plated steel sheet,
and the temperature rising properties of the Al-plated steel sheet in a heating furnace.
Thus, the present invention expands the range of application to hot pressing of the Alplated
steel sheet and enhances the applicability of the Al-plated steel sheet to the
automobiles and industrial equipment that are the final applications, md thns, the
industrial applicability is high.
[Brief Description of the Reference Symbols]
[0183]
lO,20: Al-plated steel sheet
11,211 Steel sheet
12,22: Al plating layer
13,23: Surface coating layer
1: Test piece
2: Heater
3: Furnace
4: Test piece stand
5: Load applying apparatus
6: Furnace driving apparatus
7: Ball way
8: Load cell
[Document Type] CLAIMS
[Claim 11
An Al-plated steel sheet comprising:
a steel sheet;
a11 Al plating layer which is fonned on one surface or both surfaces of the
steel sheet and contains at least 85% or tnore ofAl by mass%; and
a surface coating layer ~vlvhich is lalninated on the surface of the A1 plating
layer and contains ZnO and one or more lubricity improving compounds.
[Claim 21
The AI-plated steel sheet according to claim 1,
wherein the lubricity improving compound is a compound including one or
more transition metal elements.
[Clairn 31
The Al-plated steel sheet according to claim 2,
wherein the transition metal element is any one or more of Ti, V, Cr, Ivhl, Fe,
Co, Ni, Cu, Zr, Mo, W, La, and Ce.
[Claim 41
The Al-plated steel sheet according to claim 2,
wherein axamount of the lubricity itnproving compound including the
transition metal element in the surface coating layer is 1% to 40% with respect to a
total amount of ZnO by mass ratio.
[Claim 51
The Al-plated steel sheet according to claitn 1,
wherein the lubricity improving co~llpound is a compound including one or
more typical elements.
[Clairn 61
The AI-plated steel sheet according to claim 5,
wherein the typical element is any one or more of Mg, Ca, Sr, Ba, P, Sn, and
Ge.
[Claim 71
The Al-plated steel sheet according to clai~n5 ,
wherein an amount of the lubricity improving compound including the typical
element in the surface coating layer is 5% to 30% with respect to a total amount of
ZuO by mass ratio.
[Claim 81
The Al-plated steel sheet according to any one of claims 1 to 7,
wherein the surface coating layer contains 0.3 g/111* to 7 g/m2 of ZnO in terms
of Zn.
[Claim 91
The Al-plated steel sheet according to any one of claims 1 to 8,
~vl~eretihne surface coating layer further contains 5% to 30% of an organic
compound with respect to the total amount of ZnO by mass ratio.
[Claim 101
Amethod for hot-pressing an Al-plated steel sheet comprising:
blanking and then heating the Al-plated steel sheet according to any one of
claims 1 to 9; and
press-forming the steel sheet.
[Claim 11]
The method for hot-pressing an Al-plated steel sheet according to claim 10,
wherein an average temperature rising rate until the ten~peratureo f the Alplated
steel sheet reaches from 50°C to a temperature 10°C lower
reaching temperature is 10°C sec to 300°C sec in the heating before the press-forming
of the steel sheet.
[Claim 12]
The method for hot-pressing an Al-plated steel sheet according to claim 11,
wherein the heating before the press-forming of the steel
by electrical heating or induction heating.
[Claim 131]
An automotive part prodoced by the method for hot-pressing an Al-plated
steel sheet according to any one of claims 10 to 12.
Dated this 15.09.2014
sn a illaxiinu~i~
: the press-fonning
ling to claim 11,
eet is perfonned
TJNA MErITA-DU'lT]
: &WRY & SAGAR
TI-IE APPLICANT[S]