DESCRIPTION
Title of Invention: PLATED STEEL PLATE FOR HOT PRESSING
AND HOT PRESSING METHOD OF PLATED STEEL PLATE
5
Technical Field
[OOOl] The present invention relates to plated steel
sheet for hot press use which is coated by an A1 plating
which is mainly comprised of A1 and which is excellent in
10 hot lubricity, coating adhesion, spot weldability, and
coated corrosion resistance and a method of hot pressing
such plated steel sheet.
Background Art
[0002] In recent years, to protect the environment and
15 prevent global warming, demand has been rising for
keeping down the consumption of fossil fuels. This demand
has had an impact on various manufacturing industries.
For example, even for automobiles, which are essential
means of transport for daily life and activities, are no
20 exceptions. Improvement of fuel economy etc. by
lightening of the weight of the chasses are being sought.
However, in automobiles, just realizing lighter weight of
a chassis is not allowed in terms of product performance.
It is necessary to secure suitable safety.
25 [0003] Much of the structure of an automobile is
formed by an iron-based material, in particular steel
sheet. Reduction of the weight of this steel sheet is
important in lightening the weight of the chassis.
However, as explained above, just reducing the weight of
30 the steel sheet is not allowed. Securing mechanical
strength of the steel sheet is simultaneously sought.
Similar demands are made on steel sheet in various other
manufacturing industries in addition to the automobile
manufacturing industry. Accordingly, steel sheet which is
35 raised in mechanical strength so as to enable the
thickness to be reduced compared with the conventionally
used steel sheet while maintaining or improving the
mechanical strength is being researched and developed.
[00041 In general, a material which has a high
mechanical strength tends to fall in shape freezability
after bending or other shaping and is difficult to form
5 into a complicated shape. As one means for solving this
problem with shapeability, the so-called "hot press
method (also called the hot stamp method, hot pressing
method, or the die quench method)" may be mentioned. With
this hot press method, the material to be shaped is
10 heated once to a high temperature to soften the steel
sheet by heating, then the steel sheet is press formed
to shape it, then is cooled. According to this hot press
method, the material is heated once to a high temperature
to make it soften, so the material can be easily press
15 formed. Furthermore, due to the hardening effect caused
by the cooling after shaping, the material can be raised
in mechanical strength. Therefore, the hot press method
enables a shaped product to be obtained which achieves
both good shape freezability and high mechanical
2 0 strength.
[OOOS] However, if applying this hot press method to
steel sheet, heating the steel sheet to an 800°C or more
high temperature causes the surface of the steel sheet to
oxidize and scale (oxides) to form. Therefore, after
25 performing hot press forming, a step of removing this
scale (descaling step) becomes necessary and the
productivity falls. Further, in members which require
corrosion resistance etc., the surfaces of the members
have to be treated to make them rustproof or covered by
30 metal after being worked. A surface cleaning step and
surface treatment step become necessary, so the
productivity further falls.
[0006] As a method for suppressing such a drop in
productivity, the method of providing the steel sheet
3 5 with a covering may be mentioned. As the covering of the
steel sheet, in general an organic material or inorganic
material or other various materials are used. Among
these, galvannealed steel sheet, which has a sacrificial
corrosion action against the steel sheet, is being widely
used for automobile steel sheet etc. from the viewpoint
of the anticorrosion performance and steel sheet
5 production technology. However, the heating temperature
(700 to 1000°C) in hot press forming is higher than the
decomposition temperature of the organic material or the
melting point and boiling point of the Zn or other metal.
When using a hot press for heating, the surface coating
10 and plating layer evaporate causing remarkable
deterioration of the surface properties.
[00071 Therefore, as the steel sheet to which the hot
press method which is accompanied with high temperature
heating is applied, it is desirable to use steel sheet
15 which is provided with an Al-based metal covering, which
has a higher boiling point than an organic material
covering or a Zn-based metal covering, or an A1 plated
steel sheet. Here, an "A1 plated steel sheet" includes
sheets to which elements other than A1 have been added to
2 0 improve the characteristics of the plating layer. The A1
of the plating layer should be, by mass%, 50% or more.
[OOOS] By providing the Al-based metal covering, it is
possible to prevent scale from forming on the surface of
the steel sheet and therefore descaling and other steps
25 become unnecessary, so the shaped product is improved in
productivity. Further, an Al-based metal covering also
has a rustproofing effect, so the corrosion resistance is
also improved. The method of hot pressing steel sheet
which comprises steel sheet which has a predetermined
3 0 chemical composition and is provided with an Al-based
metal covering is disclosed in PLT 1.
[OOOS] However, when providing an Al-based metal
covering, depending on the conditions of the preheating
before the hot press forming, the A1 covering will melt,
3 5 then Fe will diffuse from the steel sheet and cause the
formation of an A1-Fe alloy layer and, further, growth of
the Al-Fe alloy layer until the surface of the steel
sheet becomes an Al-Fe alloy layer. This A1-Fe alloy
layer is extremely hard, so there was the problem that
contact with the die at the time of press forming caused
work marks on the shaped product.
5 [OOlOl An A1-Fe alloy layer is lower in slip at its
surface and is poorer in lubricity. Furthermore, this Al-
Fe alloy layer is hard and easily fractures. The plating
layer suffers from cracks and powdering etc., so the
shapeability falls. Further, any peeled off A1-Fe alloy
10 layer sticks to the die or the surface of A1-Fe alloy
layer of the steel sheet, is strongly rubbed against, and
sticks to the die or Al-Fe intermetallic compounds
derived from the Al-Fe alloy layer to adhere to the die
and cause the shaped product to decline in quality. For
15 this reason, it is necessary to periodically remove the
A1-Fe intermetallic compounds which have adhered to the
die. This becomes one cause of a drop in productivity of
the shaped product or an increase in the production
costs.
2 0 [OOll] Furthermore, an Al-Fe alloy layer is low in
reactivity with the usual phosphate treatment. Therefore,
the surface of the A1-Fe alloy layer cannot be formed
with a chemically converted coating (phosphate coating)
as prctrcatment for electrodeposition painting. Even when
25 a chemically converted coating is not formed, if making
the material good in coating adhesion then making the
amount of deposition of A1 sufficient, the coated
corrosion resistance also will become excellent, but if
increasing the amount of deposition of Al, adhesion of
3 0 A1-Fe intermetallic compounds to the die will increase.
[00121 Adhesion of A1-Fe intermetallic compounds
include the case where peeled off parts of the A1-Fe
alloy layer deposit and the case where the A1-Fe alloy
layer layer surface is strongly rubbed against and
3 5 deposits. When hot press forming steel sheet which has a
surface coating, if improving the lubricity, the strong
rubbing and adhesion by the surface of the Al-Fe alloy
layer are eased. However, improvement of the lubricity is
not effective for alleviating the deposition of peeled
off parts of the Al-Fe alloy layer on the die. To
alleviate the deposition of peeled off parts of the Al-Fe
5 alloy layer on the die, it is most effective to reduce
the amount of deposition of A1 on the A1 plating.
However, if reducing the amount of deposition of Al, the
corrosion resistance deteriorates.
[0013] Therefore, steel sheet which prevents the
10 shaped product from being formed with work marks is
disclosed in PLT 2. The steel sheet which is disclosed in
PLT 2 is steel sheet which has a predetermined chemical
composition on the surface of which an Al-based metal
covering is provided and, furthermore, on the surface of
15 that Al-based metal covering an inorganic compound
coating, organic compound coating, or composite compound
coating of the same which contains at least one of Si,
Zr, Ti, or P is formed. In the steel sheet which is
formed with such a surface coating like that disclosed in
20 PLT 2, even at the time of the press forming after
heating, the surface coating will never peel off and
therefore it is possible to prevent the formation of work
marks at the time of press forming. However, with the
surface coating which is described in PLT 2, a sufficient
2 5 lubricity cannot be obtained at the time of press
forming, so improvement etc. in the lubricant are sought.
[0014] PLT 3 discloses a method of solving the problem
of surface deterioration of galvanized steel sheet due to
evaporation of the galvanization layer in hot pressing of
3 0 galvanized steel sheet. That is, it causes the formation
of a high melting point zinc oxide (ZnO) layer as a
barrier layer on the surface of the galvanization layer
to thereby prevent the evaporation of Zn in the Zn
plating layer at the bottom layer. However, the method
35 which is disclosed in PLT 3 is predicated on the steel
sheet having a galvanization layer. The A1 content in the
galvanization layer is allowed to be up to 0.4%. However,
the content of A1 is desirably small. The method which is
disclosed in. PLT 3 is for preventing evaporation of Zn
from the Zn plating layer. A1 is included only
incidentally. However, with incidental inclusion of A1 in
5 the Zn plating layer, it is not possible to completely
prevent the evaporation of Zn in the Zn plating layer.
Therefore, the general practice is to use A1 plated steel
sheet which has high boiling point A1 as a main
component.
10 [0015] PLT 4 discloses a method of applying a wurtzite
type compound to the surface of an A1 plated steel sheet.
The method which is disclosed in PLT 4 improves the hot
lubricity and the chemical convertability and secures
adhesion of the surface coating before hot press forming
15 by adding a binder ingredient to the surface coating.
However, the binder of the method which is disclosed in
PLT 4 ends up breaking down due to heat at the time of
hot press forming and therefore there was the problem
that the wurtzite type compound fell in coating adhesion
20 from the steel sheet at the time of shaping.
[0016] PLT 5 discloses galvannealed steel sheet which
is formed with a surface coating layer which contains Zn
hydroxide and Zn sulfate. However, the steel sheet which
is disclosed in PLT 5 forms a surface coating layer on
25 the galvannealed steel sheet, so while it is excellent in
corrosion resistance, there was the problem that the zinc
in the galvannealed layer ended up evaporating at the
time of hot pressing. Further, both surfaces of the steel
sheet which is disclosed in PLT 5 are formed with an
30 oxide layer which has 3Zn (OH) .ZnS04 .nH20 (n=O to 5) . ZnS04
dissolves the A1 plating layer, so an Al plated steel
sheet could not be used.
[0017] PLT 6 discloses steel sheet which is comprised
of an A1 plated steel sheet which is formed with a
35 surface coating layer which contains a Zn compound which
is selected from Zn sulfate, Zn nitrate, and Zn chloride.
However, an aqueous solution of Zn sulfate, Zn nitrate,
or Zn chloride is high in pH, so when coating the
treatment solution when forming the surface coating
layer, acts to dissolve the A1 plated steel sheet. As a
result, there was the problem that the coated corrosion
5 resistance was degraded. Further, while the cause is not
certain, there was the problem that the weldability was
also degraded. This problem was particularly remarkable
when including Zn sulfate and Zn nitrate as the Zn
compound.
10 [00181 PLT 7 discloses steel sheet which is comprised
of an A1 plated steel sheet which is formed with a
surface coating layer which contains a vanadium compound,
a phosphoric acid compound, and at least one type of
metal compound which is selected from Al, Mg, and Zn.
15 However, the surface coating layer of the steel sheet
which is disclosed in PLT 7 contains a vanadium compound,
so the valence of the vanadium compound causes various
colors to be formed and therefore there was the problem
of an uneven appearance.
20 Citations List
Patent Literature
[0019] PLT 1: Japanese Patent Publication No. 2000-
3864011
PLT 2: Japanese Patent Publication No. 2004-211151A
25 PLT 3: Japanese Patent Publication No. 2003-129209A
PLT 4: W02009/131233A
PLT 5: Japanese Patent Publication No. 2010-077498A
PLT 6: Japanese Patent Publication No. 2007-302982A
PLT 7: Japanese Patent Publication No. 2005-048200A
30 Summary of Invention
Technical Problem
[00201 A1 has a high boiling point and a high melting
point, so A1 plated steel sheet is considered promising
as steel sheet which is used for a member for which
35 corrosion resistance is demanded such as automobile steel
sheet. Therefore, various proposals have been made
regarding application of A1 plated steel sheet to hot
pressing. However, an A1-Fe alloy layer cannot give a
good lubricity in hot pressing and the press formability
is inferior etc., so when using hot pressing to obtain a
complicated shape of a shaped product, an A1 plated steel
5 sheet is not being used. Further, in recent years, for
automobile use, steel sheet is mostly being coated after
being shaped. A1 plated steel sheet is further being
required to offer chemical convertability (coatability)
after hot press forming and coated corrosion resistance.
10 Further, steel sheet which is used for the chasses of
automobiles is also being required to have spot
weldability.
[0021] The present invention was made in consideration
of the above situation. The object of the present
15 invention is to provide A1 plated steel sheet for hot
press use which is excellent in hot lubricity, coating
adhesion, spot weldability, and coated corrosion
resistance and a method of hot pressing A1 plated steel
sheet.
2 0 Solution to Problem
[0022] To solve the above problem, the inventors etc.
engaged in intensive studies and as a result discovered
that by forming a surface coating layer which contains a
compound which contains Zn on the A1 plating layer which
25 is formed on one or both surfaces of steel sheet, the
lubricity at the time of hot press forming becomes better
and the chemical convertability also is greatly improved.
Further, they discovered that by not including a vanadium
compound in the surface coated layer, it is possible to
30 prevent the valence of the vanadium compound from causing
various colors to be formed and possible to solve the
problem of the uneven appearance of the steel sheet.
Further, they discovered that if including a
predetermined amount of a Zn compound with a high water
3 5 solubility such as Zn sulfate or Zn nitrate, the
deposition ability at the time of application and the
coating adhesion and spot weldability become inferior.
hot press use of the present invention is comprised of
steel sheet on one or both surfaces of which an A1
plating layer is formed and on the surface of which A1
plating layer a surface coating layer which contains a
5 compound of Zn is further formed.
to0351 Steel Sheet Before Plating
As the steel sheet before plating, steel sheet which has
a high mechanical strength (meaning tensile strength,
yield point, elongation, drawability, hardness, impact
10 value, fatigue strength, creep strength, and other
properties relating to mechanical deformation and
fracture) is desirably used. One example of the steel
sheet before plating which is used for the steel sheet
for hot press use of the present invention is shown next.
15 [0036] First, the chemical composition will be
explained. Note that, the symbols "%" mean mass% unless
otherwise indicated. The chemical composition of the
steel sheet before plating preferably contains, by mass%,
C: 0.1 to 0.4%, Si: 0.01 to 0.6%, and Mn: 0.5 to 3%.
20 Furthermore, it preferably contains 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%. Further, the balance is
comprised of Fe and unavoidable impurities.
[00371 C is included to secure the desired mechanical
25 strength. If C is less than 0.1%, a sufficient mechanical
strength cannot be obtained. On the other hand, if C
exceeds 0.4%, the steel sheet can be hardened, but melt
fracture easily occurs. Therefore, the content of C is
preferably 0.1 to 0.4%.
3 0 [0038] Si is an element which improves the mechanical
strength. Like C, it is included to secure the desired
mechanical strength. If Si is less than 0.01%, the effect
of improvement of strength is difficult to secure and a
sufficient improvement in mechanical strength cannot be
3 5 obtained. On the other hand, Si is an easily oxidizable
element. Accordingly, if Si exceeds 0.6%, when performing
hot dip A1 coating, the wettability falls and nonplated
parts are liable to be formed. Therefore, the content of
Si is preferably made 0.01 to 0.6%.
[00391 Mn is an element which improves the mechanical
strength and is also an element which improves the
hardenability. Furthermore, Mn is effective for
preventing hot ernbrittlement due to the unavoidable
impurity S. If Mn is less than 0.5%, these effects cannot
be obtained. On the other hand, if Mn exceeds 3%, the
residual y-phases become too great and the strength is
10 liable to fall. Therefore, the content of Mn is
preferably 0.5 to 3%.
[0040] Cr, V, and Mo are elements which improve the
mechanical properties and are also elements which
suppress the formation of pearlite at the time of cooling
from the annealing temperature. These effects cannot be
obtained if Cr is less than 0.05%, V is less than 0.01%,
or Mo is less than 0.01%. On the other hand, if exceeding
Cr: 3.0%, V: 1.0%, or Mo: 0.3%, the surface area rate of
the hard phases becomes excessive and the shapeability
deteriorates.
[0041] Ti is an element which improves the mechanical
strength and is an element which improves the heat
resistance of the A1 plating layer. When Ti is less than
0.01%, the eftect of improvement of the mechanical
strength and oxidation resistance cannot be obtained. On
the other hand, if excessively including Ti, carbides and
nitrides are formed and the steel is liable to be
softened. In particular, when Ti exceeds 0.1%, the
desired mechanical strength cannot be obtained.
Therefore, the content of Ti is preferably made 0.01 to
0.1%.
[0042] B is an element which acts to improve the
strength at the time of hardening. If B is less than
0.0001%, such an effect of improvement of strength cannot
be obtained. On the other hand, if B exceeds 0.1%,
inclusions are formed in the steel sheet causing
embrittlement and the fatigue strength is liable to be
lowered. Therefore, the content of B is preferably made
0.0001 to 0.1%.
[00431 Note that, the above-mentioned chemical
composition of the steel sheet before plating is an
5 example. Other chemical compositions are also possible.
For example, as a deoxidizing element, Al: 0.001 to 0.08%
may also be contained. Further, impurities which end up
unavoidably entering in the manufacturing process etc.
may also be included.
10 200441 The steel sheet before plating which has such a
chemical composition may be hardened by heating by the
hot press method etc. even after plating so as to be
given an approximately 1500 MPa or more tensile strength.
Even steel sheet which has such a high tensile strength
15 can be easily shaped by the hot press method in the state
softened by heating. Further, the shaped product can
realize high mechanical strength and, even when made thin
for lightening the weight, can be maintained or be
improved in mechanical strength.
20 100451 A1 Plating Layer
An A1 plating layer is formed on one or both surfaces of
the steel sheet before plating. The A1 plating layer is,
for example, formed on one or both surfaces of the steel
sheet by the hot dip method, but the invention is not
2 5 limited to this.
[0046] Further, the chemical composition of the A1
plating layer should contain Al: 50% or more. The
elements other than A1 are not particularly limited, but
Si may be proactively included for the following reasons.
3 0 LO0471 If Si is included, an Al-Fe-Si alloy layer is
formed at the interface between the plating and the base
iron and therefore it is possible to suppress the
formation of the brittle A1-Fe alloy layer which is
formed at the time of hot dip coating. If Si is less than
35 3%, the Al-Fe alloy layer will grow thick at the stage of
performing A1 plating, cracking of the plating layer will
be assisted at the time of working, and the corrosion
resistance may be detrimentally affected. On the other
hand, if Si exceeds 15%, conversely the volume rate of
the layer which contains Si will increase and the
workability of the plating layer or corrosion resistance
5 is liable to fall. Therefore, the Si content in the A1
plating layer is preferably made 3 to 15%.
[0048] The Al plating layer prevents corrosion of the
steel sheet for hot press use of the present invention.
Further, when working the steel sheet for hot press use
10 of the present invention by the hot press method, even if
heated to a high temperature, the surface will never
oxidize and scale (oxides of iron) will never be formed.
By using the A1 plating layer to prevent the formation of
scale, it is possible to eliminate the step of removal of
15 scale, the step of cleaning the surface, the step of
treating the surface, etc. and possible to improve the
productivity of the shaped product. Further, the A1
plating layer is higher in boiling point and melting
point than a plating covering comprised of an organic
20 material or a plating covering comprised of another
metal-based material (for example, Zn-based material).
Therefore, when using the hot press method to shape it,
the covering will not evaporate, so shaping at a high
temperature becomes possible, the shapeability in hot
2 5 press forming is further raised, and easy shaping becomes
possible.
[00491 The heating at the time of hot dip coating and
hot pressing can cause the A1 plating layer to alloy with
the Fe in the steel sheet. Accordingly, the A1 plating
30 layer is not necessarily formed in a single layer with a
constant chemical composition and will include partially
alloyed layers (alloy layers).
[0050] Surface Coating Layer
The surface coating layer is formed on the surface of the
3 5 A1 plating layer. The surface coating layer includes one
or more Zn compounds which are selected from the group
comprised of Zn hydroxide, Zn phosphate, and Zn organic
acid. As the Zn compound, Zn hydroxide and Zn phosphate
are particularly preferred. As the Zn organic acid, Zn
acetate, Zn citrate, Zn oxalate, Zn oleate, and other
such Zn salts of carboxylic acids and Zn salts of
5 hydroxyl acids, zinc gluconate, etc. may be mentioned.
These compounds have the effect of improving the
lubricity at the hot press or reactivity with the
chemical conversion solution. Zn hydroxide and Zn
phosphate have a small solubility in water, so are used
10 as suspensions, while Zn acetate, which has a large
solubility in water, is preferably used as an aqueous
solution. Note that, these Zn compounds may contain one
or both of Zn sulfate and Zn nitrate, but if exceeding a
mass% of lo%, as explained above, the coated corrosion
15 resistance and the weldability are degraded. Therefore,
the allowable values of the contents of Zn sulfate and Zn
nitrate are preferably 10% or less.
[0051] Next, the case where Zn hydroxide is contained
in the surface coating layer will be used as an example
20 for the explanation. Zn hydroxide breaks down upon
heating to form a smooth coating and results in a better
coated corrosion resistance than even with the case of
use of ZnO. Note that, even when using a Zn compound
other than Zn hydroxide, a surface coating layer is
2 5 formed in the same way as the case of Zn hydroxide and a
similar effect can be obtained.
[0052] The surface coating layer which contains Zn
hydroxide can be formed, for example, by applying a
coating which contains Zn hydroxide and by baking and
30 drying it to harden it after application so as to thereby
form a coating film on the A1 plating layer. As the
method of applying the Zn hydroxide, for example, the
method of mixing a suspension which contains Zn hydroxide
and a predetermined organic binder and applying it to the
35 surface of the A1 plating layer and the method of coating
by powder coating etc. may be mentioned. As the
predetermined organic binder, for example, a
polyurethane-based resin, polyester-based resin, acrylbased
resin, silane coupling agent, silica, etc. may be
mentioned. These organic binders are made water soluble
so as to enable mixing with the suspension of Zn
5 hydroxide. The thus obtained treatment solution is coated
on the surface of the A1 plated steel sheet.
100531 The Zn hydroxide is not particularly limited in
particle size, but is desirably a size of 50 to 1000 nm
or so. The particle size of the Zn hydroxide is made the
10 particle size after heat treatment. That is, the particle
size after holding at 900°C in the furnace for 5 to 6
minutes, then rapid cooling in the die is made the one
determined by observation by a scan type electron
microscope (SEM) etc.
15 [0054] The contents of the resin ingredient, silane
coupling agent, silica, and other binder ingredients in
the surface coating are preferably, by mass ratio to the
Zn hydroxide, together 5 to 30% or so. If the contents of
the binder ingredients is less than 5%, the deposition
20 effect is not sufficiently obtained and the coating
easily peels off. To stably obtain the deposition effect,
the binder ingredient is more preferably made, by mass
ratio, 10% or more. On the other hand, even if the
content of the binder ingredient exceeds 30%, the effect
25 of deposition becomes saturated and the odor which is
produced at the time of heating becomes remarkable, so
this is not preferable. The upper limit of the content of
the binder ingredient is more preferably made 16%.
[0055] The surface coating layer which contains the Zn
30 compound of the present invention is confirmed to have a
higher lubricity compared with even the inorganic
compound coating, organic compound coating, or composite
compound coating which contain at least one of Si, Zr,
Ti, and P which are described in PLT 2. For this reason,
3 5 the shapeability is further improved.
[0056] The amount of deposition of Zn hydroxide at the
surface coating layer which is formed on the Al plated
the surface coating layer is somewhat low in adhesion
before curing treatment. If rubbed by a strong force, it
may partially peel off.
100601 If the surface coating layer is heated once at
5 the time of hot press forming, it exhibits extremely
strong adhesion. PLT 4 discloses to improve the adhesion
before hot press forming, but the present invention
improves the adhesion after hot press forming.
Improvement of adhesion after hot press forming cannot be
10 obtained if including the wurtzite type compound which is
disclosed in PLT 4 in the surface coating and is an
important characteristic of the present invention. Due to
the Zn hydroxide being heated, it is expected that it
will be dehydrogenated and partially become Zn oxide etc.
15 and the crystal structure will change. At this time, it
is believed that fine particles easily proceed to sinter.
In the same way, Zn phosphate and Zn organic acid also
are believed to break down upon being heated. A compound
with a low solubility in water such as Zn hydroxide and
20 Zn phosphate can be applied to an A1 plated steel sheet
in a solution state. Further, Zn hydroxide, Zn phosphate,
and Zn organic acid are believed to precipitate as
compounds in the baking step after application or the
heating step at the time of hat stamping, but compared
25 with a solution dispersed in water, there is no secondary
aggregation in water and precipitation occurs in a finer
form. Therefore, it is believed that the precipitated
particles sinter and therefore the strength as a coating
can be easily maintained.
30 [0061] The surface coating layer improves the
lubricity, so even with an A1 plated steel sheet which is
inferior in shapeability, the shapeability at the time of
hot press forming can be improved. Further, it is
possible to enjoy the excellent corrosion resistance of
35 the A1 plated steel sheet. Further, the excellent
lubricity of the surface coating layer suppresses the
adhesion of the A1-Fe intermetallic compounds on the die.
Even if the A1 plating layer powderizes, the surface
coating layer which contains the Zn compound can prevent
the powder (powder of A1-Fe intermetallic compound) from
adhering to the die which is used for the later hot press
5 forming. Accordingly, a step of removing the powder of
the A1-Fe intermetallic compound which adheres to the die
etc. become unnecessary, so the productivity of the
shaped product can be further improved.
100621 Further, the surface coating layer can play the
10 role of a protective layer which protects damage to the
A1 plating layer which can occur at the time of hot press
forming and can improve the shapeability. Furthermore,
the surface coating layer does not lower the spot
weldability and coating adhesion or other aspects of
15 performance either. If the treatment solution when
forming the surface coating layer in high in water
solubility, the spot weldability and coating adhesion
deteriorate. If the treatment solution is high in water
solubility, the applied treatment solution easily runs
20 off from the steel sheet and the deposition ability
deteriorates.
100631 Furthermore, the surface coating layer can
greatly improve the coated corrosion resistance and can
reduce the amount of deposition of A1 of the A1 plating
2 5 layer compared with the past. As a result, even when
rapidly performing hot press forming, adhesion can be
reduced and the productivity of the shaped product is
further raised.
[0064] Hot Press Method
30 Next, the method of hot pressing the steel sheet for hot
press use of the present invention will be explained.
100651 In the hot press method of the present
invention, first, the plated steel sheet for hot press
use is blanked as required, then heated to a high
3 5 temperature to make the plated steel sheet for hot press
use soften. Further, the softened plated steel sheet for
hot press use is press formed to shape it, then is
cooled. By softening the plated steel sheet for hot press
use once in this way, it is possible to easily perform
the subsequent press forming. Further, the plated steel
sheet for hot press use of the present invention can be
5 hardened by heating and cooling and realize an
approximately 1500 MPa or more high tensile strength.
[0066] As the heating method, in addition to the usual
electrical furnace, a radiant tube furnace, infrared
furnace, etc. may be employed.
10 100671 The A1 plated steel sheet melts if heated to
the melting point or more and simultaneously diffuses
with Fe whereby the A1 phase changes to the A1-Fe alloy
phase and Al-Fe-Si alloy phase. The A1-Fe alloy phase and
Al-Fe-Si alloy phase have high melting points of 1150°C or
15 so. The A1-Fe phase and Al-Fe-Si phase come in a
plurality of types and if heated at a high temperature or
heated for a long time, change to the higher Fe
concentration alloy phase.
[0068] The surface state which is desirable for the
20 final shaped product is a state alloyed up to the surface
and a state where the concentration of Fe in the alloy
phase is not that high. If unalloyed A1 remains, only
this portion rapidly corrodes, the coated corrosion
resistance deteriorates, and blisters occur extremely
25 easily, so this is not desirable. On the other hand, if
the concentration of Fe in the alloy phase becomes too
high, the alloy phase itself falls in corrosion
resistance, the coated corrosion resistance deteriorates,
and blisters easily occur. That is, the corrosion
3 0 resistance of the alloy phase depends on the
concentration of A1 in the alloy phase. Therefore, to
improve the coated corrosion resistance, the state of
alloying is controlled by the amount of deposition of A1
and the heating conditions.
35 [0069] In the present invention, the average
temperature elevation rate in the temperature region from
50°C to a temperature 10°C lower than the maximum peak
temperature is preferably made 10 to 300°C/sec. The
average temperature elevation rate governs the
productivity in press forming plated steel sheet for hot
press use. If the average temperature elevation rate is
5 less than 10°C/sec, softening of the steel sheet for hot
press use requires time. On the other hand, if over 30OoC,
the softening is rapid, but the alloying of the plating
layer becomes remarkable and causes powdering. The
general average temperature elevation rate is, in the
10 case of heating in the atmosphere, about 5OC/sec. An
average temperature elevation rate of 100°C/sec or more
can be achieved by ohmic heating or high frequency
induction heating.
[0070] The plated steel sheet for hot press use of the
15 present invention can realize a high average temperature
elevation rate, so the productivity of the shaped product
can be improved. Further, the average temperature
elevation rate has an effect on the chemical composition
and thickness of the A1-Fe alloy phase, so is one of the
20 important factors in control of the quality in plated
steel sheet for hot press use. In the case of the plated
steel sheet for hot press use of the present invention,
the temperature elevation rate can be raised to 300°C/sec,
so a broader range of control of quality becomes
25 possible.
[0071] Regarding the maximum peak temperature, due to
the principle of the hot press method, heating is
necessary in the austenite region, so usually a
temperature of 900 to 950°C or so is employed. In the hot
3 0 press method of the present invention, the maximum peak
temperature is not particularly limited, but if less than
850°C, sufficient quenching hardness is not obtained, so
this is not preferable. Further, the A1 plating layer has
to be made an Al-Fe alloy phase. From this viewpoint, it
3 5 is not preferable to make the maximum peak temperature
less than 850°C. On the other hand, if the maximum peak
temperature exceeds 1000°C, the alloying will proceed too
far, the concentration of Fe in the A1-Fe alloy phases
will rise, and a drop in the coated corrosion resistance
will be invited. The upper limit of the maximum peak
5 temperature cannot be defined across the board since it
depends also on the temperature elevation rate and amount
of deposition of Al, but even if considering economy, it
is not preferable to make the maximum peak temperature is
1100°C or more.
10 [0072] Advantageous Effects of Plated Steel Sheet for
Hot Press Use and Hot Press Method of Present Invention
The plated steel sheet for hot press use of the present
invention has a surface coating layer which contains a
compound which contains Zn, in particular which contains
15 Zn hydroxide, so a high lubricity is realized and the
chemical convertability is improved. Further, the plated
steel sheet for hot press use of the present invention is
resistant to peeling of the coating after shaping. As a
result, adhesion of the A1-Fe intermetallic compounds to
20 the die is prevented, the shapeability and productivity
at the time of hot press forming are improved, and the
chemical convertability after hot press forming is also
improved. Furthermore, the steel sheet for hot press use
of the present invention is excellent in adhesion of the
25 A1 plating layer and surface coating layer after shaping
and also excellent in corrosion resistance of the shaped
product, that is, the coated corrosion resistance.
[0073] The reason the chemically converted coating
forms due to a Zn compound such as Zn hydroxide is
3 0 unclear at the present stage, but the chemical conversion
reaction proceeds while triggering an etching reaction by
acid of the material. The surface of the A1-Fe
intermetallic compound is extremely inert to acid, so it
is guessed that the reaction has difficulty proceeding. A
35 Zn compound is an amphoteric compound and dissolves in an
acid, so is believed to react with a chemical conversion
solution.
Examples
[0074] Next, examples will be shown while further
explaining the present invention. Note that, the present
invention is not limited to the examples which are shown
5 below.
[0075] Example 1
A cold rolled steel sheet of the chemical composition
which is shown in Table 1 (sheet thickness 1.4 mm) was
used. This cold rolled steel sheet was plated with Al by
10 the Sendzimir process. The annealing temperature was made
about 800°C, the A1 plating bath contained Si: 9%, and Fe
which was eluted from the cold rolled steel sheet was
contained. The amount of deposition of A1 after plating
was adjusted by the gas wiping method to 160 g/m2 at both
15 surfaces. After cooling, a suspension or aqueous solution
which was shown in Table 2 was coated by a roll coater
and was baked on at about 80°C to produce a test material.
Note that, each solution which is shown in Table 2 was
obtained by using reagents and mixing them with distilled
20 water to form a suspension or aqueous solution.
100761 The characteristics of the thus produced test
material were evaluated by the following methods. Note
that, the average temperature elevation rate when heating
to 900°C was made !i0C/sec.
25 100771 (1) Hot Lubricity
The apparatus which is shown in FIG. 1 was used to
evaluate the hot lubricity. A 150 x 200 mm test material
was heated to 90OoC, then a steel ball was pushed against
it from the top at 700°C to measure the push-in load and
30 pull-out load. The (pull-out load) / (push-in load) was
made the dynamic coefficient of friction.
[0078] (2) Coating Adhesion
The test material was inserted into an atmosphere
furnace, heated at 900°C for 6 minutes, taken out, then
35 immediately clamped in a stainless steel die and rapidly
cooled. The cooling rate at this time was made 150°C/sec.
Next, the test material was cut to 50 x 50 mm and used
for a wrapping test. The method was to run gauze to which
2.0 kgf (I kgf is 9.8N) of load was applied back and
forth 10 times over a 30 mm length, measure the amount of
5 deposition of Zn before and after the test, and calculate
the amount of reduction %.
[00791 (3) Spot Weldability
The test material was inserted into an atmosphere
furnace, heated at 90OoC for 6 minutes, taken out, then
10 immediately clamped in a stainless steel die and rapidly
cooled. The cooling rate at this time was made 150°C/sec.
Next, the test material was cut to 30 x 50 mm and the
range of suitable current for spot welding (difference of
upper limit current and lower limit current) was
15 measured. The measurement conditions were as follows: The
lower limit current was made the current value when the
nugget size 4t1" (t: sheet thickness) was 4.4 mm, while
the upper limit current was made the dust generating
current.
20 Electrode: made of chromium copper, DR type (tip size 6
mm, 40R radius shape)
Applied voltage: 400 kgf (1 kgf is 9.8N)
Electrification time: 12 cycles (60 Hz)
[0080] (4) Coated Corrosion Resistance
25 The test material was inserted into an atmosphere
furnace, heated at 900°C for 6 minutes, taken out, then
immediately clamped in a stainless steel die and rapidly
cooled. The cooling rate at this time was made 150°C/sec.
Next, the test material was cut to 70 x 150 mm and was
30 chemically converted using a chemical conversion solution
(PB-SX35) made by Japan Parkerizing, then was given an
electrodeposition coating (Powernix 110) made by Nippon
Paint and was baked at 170°C to form a 20 pm coating.
[00811 The coated corrosion resistance was evaluated
3 5 based on the JASO M609 of the Society of Automotive
Engineers of Japan. The coating was cross-cut by a cutter
in advance and the width of blisters from the cross-cuts
after an 180 cycle (60 day) corrosion test (maximum value
of one side) were measured. The reference material was a
general corrosion-proof steel sheet comprised of hot dip
5 galvannealed steel sheet with zinc deposited to 45 g/m2
per surface. If the coated corrosion resistance is better
than the reference material, use as corrosion-proof steel
sheet is possible. Note that the width of the blisters of
the reference material was 7 mrn.
10 [00821 Table 1. Chemical Composition of Test Material
(mass%)
[00831 Table 2
*l. Inclusion, by mass%, of 20% of urethane resin with respect to Zn
compounds in addition to Zn compounds.
*2. Amount when all Zn.
[0084] Table 3
Note 1) "A+D" indicates inclusion of A and D in equal amounts. The
amount of deposition of the surface coating bras made 1 g/mZ by total
amount of Zn.
5 Note 2) "A+5 to 15%G1' indicates inclusion of G in 5 to 15% by mass%
with respect to A. The amount of deposition of the surface coating
was made 1 g/m2 by total amount of Zn.
Note 3) "A+5 to 15%H1' indicates inclusion of H in 5 to 15% by mass%
with respect to A. The amount of deposition of the surface coating
10 was made 1 g/mZ by total amount of Zn.
[OOSS] The results of evaluation are shown in Table 3.
The hot lubricity is shown by the measured dynamic
coefficient of friction, the coating adhesion is shown by
15 the amount of reduction of Zn% before and after heating,
the spot weldability is shown by the suitable range of
current, and the coated corrosion resistance is shown by
the width of blisters. Note that, No. 7 was A1 plated
steel sheet as is without formation of a surface coating
20 layer.
[0086] From Table 3, it was confirmed that by forming
surface coating layers which contain Zn compounds of A to
E, it is possible to improve the hot lubricity, coating
adhesion, and coated corrosion resistance without causing
25 the deterioration of the spot weldability.
[0087] Here, No. 6 is a comparative example where a
treatment solution comprising a suspension of ZnO and a
urethane-based binder mixed together is coated. While the
hot lubricity and coated corrosion resistance were
excellent, the coating adhesion was 25% or remarkably
5 inferior compared with the invention examples.
[0088] Further, the comparative examples where surface
coating layers which contain Zn compounds of G and H are
formed (Nos. 11 and 12) were inferior in coating adhesion
and spot weldability. This is because the treatment
10 solutions which contain the compounds G and H are high in
water solubility, easily run off when coated on A1 plated
steel sheet, and are inferior in deposition ability.
However, as shown in Nos. 8 and 10, if the contents in
the surface coating layers of G and H are, by mass%, 10%
15 or less, it was confirmed that the effect on
deterioration of the coating adhesion and spot
weldability was small.
[0089] Next, to what extent a surface coating which
contains a Zn compound should be formed was determined by
20 changing the amount of deposition of surface coating
layer and evaluating the hot lubricity. The amount of
deposition of the surface coating was evaluated by the
amount of deposition of Zn in the surface coating. The
treatment solution used was one which contained the Zn
2 5 compound of A of Table 2. The results are shown in FIG.
2.
[OOSO] As clear from FIG. 2, it was confirmed that by
an amount of deposition of Zn of 0.5 g/m2 or more, more
preferably 1 g/m2 or more, the hot lubricity can be
3 0 improved. The various values in FIG. 2 are shown in Table
4. As clear from Table 4, it was confirmed that by an
amount of deposition of Zn of 2 g/m2, the value of the
coefficient of hot friction became saturated.
[00921 Example 2
A treatment solution was prepared by changing the ratio
5 of addition ( % ) of a urethane resin to a suspension which
contains the Zn compound of A of Table 2 with respect to
the Zn(OH)2. This was applied to the A1 plated steel sheet
of Example 1 to form a surface coating layer and prepare
a test material. The baking conditions were the same as
10 in Example 1. Further, the adhesion of this test material
was evaluated. The methods of evaluation were the same as
Example 1 except for the evaluations being performed
before the heating. That is, the test material was cut to
[00911 Table 4. Coefficient of Hot Friction
50 x 50 mm and subjected to a wrapping test. The method
15 was to run gauze to which 1.5 kgf (1 kgf is 9.8N) of load
was applied back and forth 10 times over a 30 mm length,
measure the amount of deposition of Zn before and after
the test, and calculate the amount of reduction %.
[0093] Table 5
Am't of
deposition
( g/m2)
Coefficient0.95
of hot
friction
7
0.84
2.9
0.64
0
[0094] The results are shown in Table 5. It could be
confirmed that the adhesion before heating was improved
by the addition of urethane resin. Further, it could be
25 confirmed that even if adding urethane resin in 16% or
more, the effect became saturated.
[0095l Example 3
A steel sheet for hot press use of the present invention
which was formed using a treatment solution which
3 0 contains the Zn compound of No. 1 in Example 1 was used.
5
0.610.6
20 Table 5. Coating Adhesion Before Heating
0.7
0.76
Resin ratio
Adhesion before
heating
1.1
0.71
0%
20%
1.5
0.65
5 %
5%
2
0.65
9 %
1 %
2.4
0.66
16%
0.2%
28%
0.2%
50%
0.2%
An infrared ray furnace was used to heat the steel sheet
by an average heating rate of 30°C/second to evaluate the
characteristics of the test material. The methods of
evaluation were similar to the methods which were shown
5 in Example 1 except for the heating method. The results
of evaluation are shown in Table 6. The coated corrosion
resistance was superior to the case of No. 1 as a result.
It could be confirmed that the rapid heating method was
effective.
10 [0096] Table 6. Results of Evaluation at Time of
Application of Rapid Heating
I Compound I Hot ( Coatinq 1 Spot ]coated corrosion)
[0097] Example 4
A cold rolled steel sheet of the chemical composition
15 which is shown in Table 1 (sheet thickness 1.4 rnm) was
used. This cold rolled steel sheet was plated with A1 by
the Sendzimir process. The A1 plating bath was changed in
Si concentration to 3, 6, 9, 13, 15, 18, and 21%. In
addition, it contained Fe which was eluted from the cold
20 rolled steel sheet. The amount of deposition of A1 after
plating was adjusted by the gas wiping method to 160 g/mZ
at both surfdces. After cooiing, a treatment solution
which contained the Zn compound which was shown by A in
Table 2 was coated by a roll coater and was baked on at
25 about 80°C to produce test materials. These test materials
were evaluated for characteristics by methods similar to
Example 1. Note that, the amount of deposition of Zn was
in each case about 1 g/m2. The results of evaluation are
shown in Table 7. As clear from Table 7, it could be
30 considered that when the Si concentration is 3 to 15%,
the coated corrosion resistance is particularly
excellent.
[0098] Table 7
100991 As explained above, while preferred embodiments
of the present invention were explained in detail, the
5 present invention is not limited to these embodiments.
Any embodiments which are within the scope described in
the claims shall be considered to be included in the
present invention.
Industrial Applicability
10 [OlOO] According to the present invention, when hot
pressing the A1 plated steel sheet, since the lubricity
is good and the workability is improved, a more
complicated shape of shaped product than the past can be
press formed. Furthermore, labor can be saved in the
15 maintenance and inspection of the hot press die and the
productivity of the shaped product can be improved. The
shaped product after hot press forming as well is good in
chemical convertability, so the painting ability and
corrosion resistance of the final shaped product can be
20 improved as well. In this way, the present invention
enables hot pressing of A1 plated steel sheet to be
expanded to the automobile industry etc. Therefore, the
present invention is high in value of application in

Amended CLAIMS under Art. 19
Claim 1. (Amended) Plated steel sheet for hot press use i!
characterized by being plated steel sheet for hot press use
which contains
steel sheet,
an A1 plating layer which is formed on one surface or both
surfaces of said steel sheet, and'
a surface coating layer which is formed on said A1 plating
layer,
said surface coating layer containing at least one Zn
compound which is selected from th-e group comprised of Zn
hydroxide and a Zn organic acid.
Claim 2. The plated steel sheet for hot press use according
to claim 1 characterized in that the amount of deposition of
the Zn compound in said surface coating layer is 0.5 to 7
g/mZ per surface as Zn.
Claim 3. The plated steel sheet for hot press use according
to claim 2 characterized in that sa