【DESCRIPTION】
【Invention Title】
HOT PRESS FORMED ARTICLE HAVING EXCELLENT DELAMINATION
RESISTANCE AND METHOD FOR MANUFACTURING SAME
【Technical Field】
The present invention relates to a method for
manufacturing a hot press formed (HPF) article having
excellent delamination resistance, and more particularly,
to a hot press formed (HPF) article having excellent
delamination resistance through control of a hardness ratio
between an alloyed layer and a soft diffusion layer formed
when alloy heat treating a hot dip aluminum plated steel
sheet having a hot dip aluminum plating layer on the
surface to a fixed value, and a method for manufacturing
the same.
【Background Art】
A steel sheet for aluminum plating HPF (Hot Press
Forming) is commonly manufactured by immersing a steel
sheet having high hardenability in a plating bath with Al
as a base, and a plated steel sheet having an Al plating
layer on the surface is subsequently heat treated to be
widely used in manufacturing articles for automobiles
having a complex shape and strength of 1300 MPa or greater.
However, during a HPF heat treatment process, the
plating layer may have a structure having an alloyed layer
including an intermetallic compound such as FeAl, Fe2Al5 or
the like as an upper layer, and a diffusion layer formed
with 80% by weight to 95% by weight of Fe (hereinafter,
compositions of steel components are all provided in % by
weight) as a lower layer. However, the alloyed layer on
the upper side in the plating layer may be brittle as
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compared to the diffusion layer, and is removed from the
plating layer during press forming and adsorbed on a
pressing surface, which may be disadvantageous, making
continuous press forming difficult.
In detail, in a hot press formed (HPF) article
prepared by forming a hot dip aluminum plating layer on a
substrate steel sheet and high temperature forming
thereafter, the plating layer is formed to have a soft
diffusion layer and a hard alloy layer, and a breakage may
occur at an interface of the diffusion layer and the alloy
layer during high temperature hot processing. Then the
broken plating layer may accumulate on a mold in fine
powder form, and in some cases, may adhere to the mold and
cause various issues such as size changes in the hot press
formed (HPF) article, surface dent occurrence, mold
modification and cutting work, and additional costs may be
incurred.
When such problems occur, companies receiving and hot
processing the material bear extra costs, and automobile
companies may also face significant losses since production
delays may inevitably occur. Consequently, there has been
demand for the development of hot press formed (HPF)
articles having excellent press formability in which
problems described above have been overcome.
【Disclosure】
【Technical Problem】
The present invention has been made in view of the
above, and an object of the present invention is to provide
a formed article for HPF having excellent delamination
resistance by managing a difference in hardnesses between
an alloyed layer and a soft layer forming a hot dip
aluminum alloy plating layer within a fixed value range.
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Another object of the present invention is to provide
a method for manufacturing the hot press formed (HPF)
article.
However, problems that the present invention is
intended to solve are not limited to the problems described
above, and other problems that have not been mentioned may
be clearly understood to those skilled in the art from the
descriptions made below.
【Technical Solution】
In view of the above, an embodiment of the present
invention provides a hot press formed (HPF) article having
excellent delamination resistance, a hot dip aluminum
plating layer being formed on a surface of a substrate
steel sheet, wherein the substrate steel sheet includes,
in % by weight, C: 0.18% to 0.25%, Si: 0.1% to 1.0%, Mn:
0.9% to 1.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.01%
to 0.05%, Cr: 0.05% to 0.5%, Ti: 0.01% to 0.05%, B: 0.001%
to 0.005, N: 0.009% or less, and a remainder of Fe and
other impurities;
the plating layer is formed to have a soft diffusion
layer and a hard alloy layer; and
a tau phase is irregularly and discontinuously
dispersed and distributed inside the alloy layer in an
amount of 10% or greater based on the total area fraction
so that a difference in hardnesses between the alloy layer
and the diffusion layer may be 400 (Hv) or less.
The substrate steel sheet may be a cold rolled steel
sheet or a hot rolled steel sheet.
In the present invention, the tau phase is preferably
present in a range of 10% by area to 20% by area inside the
alloy layer.
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The tau phase preferably has an aspect ratio in a
range of 1 to 4.
The tau phase having a size of 5 μm or less
preferably occupies an area of 50% or greater with respect
to the total tau phase fraction.
The substrate steel sheet preferably further includes
Mo+W in an amount of 0.001% to 0.5%.
In addition, the substrate steel sheet preferably
further includes a sum of one or more types of Nb, Zr or V
in a range of 0.001% to 0.4%.
Further, the substrate steel sheet preferably further
includes Cu+Ni in a range of 0.005% to 2.0%.
Moreover, the substrate steel sheet preferably
further includes one or more types of Sb, Sn or Bi in an
amount of 0.03% or less.
Another embodiment of the present invention relates
to a method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance, the
method including preparing a steel sheet having steel
components as above;
hot dip aluminum plating the steel sheet by, after
heating the steel sheet to a temperature of 550C to 850C,
immersing the steel sheet in a hot dip aluminum plating
bath maintained at a temperature of 640C to 680C and
composed of, in % by weight, Si: 7% to 13%, Fe: less than
3%, and the remainder of Al and other unavoidable
impurities;
skin pass milling (SPM) the hot dip aluminum plating
steel sheet with an elongation of 0.5% to 3% after cooling
the hot dip galvanized steel sheet;
alloying a hot dip aluminum plating layer on a
surface of the hot dip aluminum plated steel sheet by
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heating the hot dip aluminum plated steel sheet to a
temperature of 850C to 950C and maintaining the
temperature for a certain period of time; and
manufacturing a hot press formed (HPF) product by
rapidly cooling the alloyed hot dip aluminum plated steel
sheet to a temperature of 300C or lower while hot press
forming the alloyed hot dip aluminum plated steel sheet.
The alloyed hot dip aluminum plating layer is formed
to have a soft diffusion layer and a hard alloy layer; and
preferably, a tau phases is irregularly and discontinuously
dispersed and distributed inside the alloy layer in an
amount of 10% or greater, based on the total area fraction.
The substrate steel sheet may be a cold rolled steel
sheet or a hot rolled steel sheet.
The tau phase is preferably present in a range of 10%
by area to 20% by area inside the alloy layer.
The tau phase preferably has an aspect ratio in a
range of 1 to 4.
The tau phase having a size of 5 μm or less
preferably occupies an area of 50% or greater with respect
to the total tau phase fraction.
Another embodiment of the present invention relates
to a method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance, the
method including skin pass milling (SPM) a hot dip aluminum
plated steel sheet with an elongation of 0.5% to 3% on
which a hot dip aluminum plating layer is formed;
alloying a hot dip aluminum plating layer on a
surface of the hot dip aluminum plated steel sheet by
heating the hot dip aluminum plated steel sheet to a
temperature of 850C to 950C and maintaining the
temperature for a certain period of time; and
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manufacturing a hot press formed (HPF) product by
rapidly cooling the alloyed hot dip aluminum plated steel
sheet to a temperature of 300C or lower while hot press
forming the alloyed hot dip aluminum plated steel sheet.
The alloyed hot dip aluminum plating layer is formed
to have a soft diffusion layer and a hard alloy layer; and
preferably, a tau phases is irregularly and discontinuously
dispersed and distributed inside the alloy layer in an
amount of 10% or greater based on the total area fraction.
The substrate steel sheet may be a cold rolled steel
sheet or a hot rolled steel sheet.
The tau phase is preferably present in a range of 10%
by area to 20% by area inside the alloy layer.
The tau phase preferably has an aspect ratio in a
range of 1 to 4.
The tau phase having a size of 5 μm or less
preferably occupies an area of 50% or greater with respect
to the total tau phase fraction.
【Best Mode】
The present invention having a composition as described
above is capable of decreasing a difference in hardnesses
between an alloy layer and a soft layer by dispersing a
soft tau phase into a relatively hard alloy layer, and as a
result, a hot press formed (HPF) article having excellent
delamination resistance can be effectively provided.
【Description of Drawings】
FIG. 1 is a view of a longitudinal section
illustrating a plating layer of a plated steel sheet
according to an embodiment of the present invention.
FIG. 2 is a drawing illustrating the structural
picture of FIG. 1.
【Best Mode】
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Hereinafter, the present invention will be described.
When external pressure is applied to a hot dip
aluminum alloy plating layer during high temperature hot
processing, stress formed in the plating layer may act on
the hard alloy layer and the soft diffusion layer formed in
the plating layer during a transfer thereof to a substrate
steel sheet. Herein, the inventors of the present
invention have discovered from their research results that
the hard layer develops and spreads cracks since it does
not sufficiently overcome deformation stress, while the
soft diffusion layer prevents the spread of cracks by
absorbing deformation stress caused by the processing.
Further, the inventors of the present invention have taken
an interest in a tau phase present in the alloy layer and
have repeatedly carried out experiments thereon, and as a
result, have identified that a tau phase in the alloy layer
is relatively soft and suppresses the spreading of cracks,
and delays the cracks from reaching an interface of the
diffusion layer and the alloy layer.
Moreover, as a result of studies into methods for
reducing powdering during high temperature processing, the
inventors of the present invention have identified that the
generation of powdering in high temperature processing may
be suppressed when a difference in hardnesses between a
diffusion layer and an alloy layer is controlled to a
certain level or below, and moreover, the difference in
hardnesses is influenced by tau phase distribution and the
like in the hard layer, and is providing the present
invention.
Hereinafter, a hot press formed (HPF) article having
excellent delamination resistance of the present invention
will be described. The hot press formed (HPF) article of
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the present invention has a structure on which a hot dip
aluminum plating layer is formed on a surface of a
substrate steel sheet, and the substrate steel sheet
includes, in % by weight, C: 0.18% to 0.25%, Si: 0.1% to
1.0%, Mn: 0.9% to 1.5%, P: 0.03% or less, S: 0.01% or less,
Al: 0.01% to 0.05%, Cr: 0.05% to 0.5%, Ti: 0.01% to 0.05%,
B: 0.001% to 0.005%, N: 0.009% or less, and a remainder of
Fe and other impurities. Specific steel components and
reasons for limitation are as follows.
C: 0.18% to 0.25%
C is an essential element increasing the strength of
martensite. When the C content is lower than 0.18%,
sufficient strength for securing crashworthiness is
difficult to obtain. When the content is greater than
0.25%, impact toughness of a slab may be reduced, and
weldability of the hot press formed (HPF) article may be
reduced as well. Considering the above, the present
invention preferably limits the content of C to 0.18% by
weight to 0.25% by weight (hereinafter, simply referred to
as %).
Si: 0.1% to 1.0%
Si is effective in material homogenization of steel
after hot press forming (HPF), and may contribute to the
production of a tau phase in a plating layer by being
diffused to the plating layer during HPF heat treatment.
When the Si content is lower than 0.1%, sufficient effects
in the material homogenization and the diffusion to the
plating layer may not be obtained, and when the content is
greater than 1.0%, favorable hot dip aluminum plated
surface qualities may be difficult to secure due to Si
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oxides produced on the steel sheet surface during annealing,
and therefore, Si is added in an amount of 1.0% or less.
Mn: 0.9% to 1.5%
Like Cr, B and the like, Mn is added to secure the
hardenability of steel. When the Mn content is lower than
0.9%, sufficient hardenability is difficult to secure and
bainite may resultantly be produced, and as a result,
sufficient strength may be difficult to secure. When the
content is greater than 1.5%, steel sheet manufacturing
costs may be increased and bending properties of the hot
press formed (HPF) article may be significantly reduced as
well, since Mn is segregated inside the steel. Considering
the above, the present invention preferably limits the Mn
content to a range of 0.9% to 1.5%.
P: 0.03% or less (not including 0%)
P is an element inhibiting many properties of the hot
press formed (HPF) article as a grain boundary segregation
element, and is preferably added in as small an amount as
possible. When the P content is greater than 0.03%,
bending properties, impact properties, weldability and the
like of the formed article are degraded, and limiting the
upper limit to 0.03% is preferable.
S: 0.01% or less (not including 0%)
S is an element inhibiting bending properties and
weldability of the formed article as impurities present in
the steel, and is preferably added in as small an amount as
possible. When the S content is greater than 0.01%,
bending properties, weldability and the like of the formed
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article may be degraded, and the upper limit is preferably
limited to 0.01%.
Al: 0.01% to 0.05%
Al is added for deoxidation in steel manufacturing,
which is similar to Si. To realize this goal, Al needs to
be added in an amount of 0.01% or greater, however, when
the content is greater than 0.05%, a deoxidation effect may
be saturated and surface qualities of a plating material
may be inferior as well, and the upper limit is preferably
limited to 0.05%.
Cr: 0.05% to 0.5%
Like Mn, B and the like, Cr is added to secure the
hardenability of steel. When the Cr content is lower than
0.05%, sufficient hardenability is difficult to secure, and
when the content is greater than 0.5%, hardenability may be
sufficiently secured, however, the hardenability properties
may be saturated, and steel manufacturing costs may be
increased. Considering the above, the present invention
preferably limits the Cr content to a range of 0.05% to
0.5%.
Ti: 0.01% to 0.05%
Ti is added for retaining a solid solution B
essential to secure hardenability by bonding with nitrogen
remaining as an impurity in the steel to produce TiN. When
the Ti content is lower than 0.01%, the effect is difficult
to be sufficiently expected, and when the content is
greater than 0.05%, the hardenability properties may be
saturated, and steel manufacturing costs may be increased
as well. Considering the above, the present invention
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preferably limits the Ti content to a range of 0.01% to
0.05%.
B: 0.001% to 0.005%
Like Mn and Cr, B is added to secure hardenability in
the hot press formed (HPF) article. B needs to be added in
an amount of 0.001% or greater to realize the abovementioned
goal, and when the content is greater than 0.005%,
the hardenability effect is saturated, and a hot rolling
property significantly decreases as well. Accordingly, the
present invention preferably limits the B content to a
range of 0.001% to 0.005%.
N: 0.009% or less
N is present as impurities in the steel, and is
preferably added in as small an amount as possible. When
the N content is greater than 0.009%, steel surface defects
may be caused, and the upper limit is preferably limited to
0.009%.
Next, a substrate steel sheet forming the hot press
formed (HPF) article of the present invention may
additionally contain the following components.
Mo+W: 0.001% to 0.5%
Mo and W are elements for increasing hardenability
and precipitation, and are highly effective in further
securing high strength. When the added sum of Mo and W is
less than 0.001%, sufficient hardenability and
precipitation strengthening effects may not be obtained,
and when the sum of Mo and W is greater than 0.5%, the
hardenability and precipitation effects may be saturated,
and manufacturing costs may be increased as well.
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Accordingly, the present invention preferably limits the
Mo+W content to a range of 0.001% to 0.5%.
Sum of one or more types of Nb, Zr or V: 0.001% to
0.4%
Nb, Zr and V are elements enhancing steel sheet
strength, grain refinement and heat treatment properties.
When the content of one or more types of Nb, Zr and V is
less than 0.001%, the effects as above are difficult to be
expected, and when the content is greater than 0.4%,
manufacturing costs may be increased excessively.
Accordingly, the present invention preferably limits the
content of these elements to 0.001% to 0.4%.
Cu+Ni: 0.005% to 2.0%
Cu is an element enhancing strength by producing fine
Cu precipitates, and Ni is an element efficient for
increasing strength and enhancing heat treatment properties.
When the sum of these components is less than 0.005%,
target strength may not be sufficiently obtained, and when
the content is greater than 2.0%, operability may be poor
and manufacturing costs may be increased. Considering the
above, the present invention preferably limits Cu+Ni to
0.005% to 2.0%.
One or more types of Sb, Sn or Bi: 0.03% or less
Sb, Sn and Bi are grain boundary segregation elements,
and may be thickened at an interface of a plating layer and
substrate iron when undertaking HPF heating to enhance
adhesive strength of the plating layer. By enhancing
adhesive strength of the plating layer, Sb, Sn and Bi may
contribute to the prevention of plating layer elimination
during the hot press forming. Sb, Sn and Bi have similar
Page 14
properties, and therefore, these 3 elements may be mixed
and used, and herein, employing the sum of one or more
types in an amount of 0.03% or less is preferable. When a
sum of these components is greater than 0.03%, there may be
concern for degenerating brittleness in substrate iron
during hot press forming.
The hot press formed (HPF) article of the present
invention has a hot dip aluminum plating layer formed on a
surface of a substrate steel sheet having the abovedescribed
steel components, and as is commonly known in the
art, such a plating layer is formed to have a soft
diffusion layer and a hard alloy layer.
First, an alloy plating layer structure of the ironaluminum
alloy plated article is simply described. After
hot dip aluminum plating a steel sheet, the result is heat
treated to form an alloy plating layer. The alloy plating
layer is formed to have a diffusion layer and an alloy
layer, and the diffusion layer is present between a
substrate steel sheet and the alloy layer. The alloy layer
is formed of Fe2Al5, an intermetallic compound, and a tau
phase, and the tau phase is a Fe-Al-Si-based three-membered
alloy phase, and is distributed inside the alloy layer. In
other words, the alloyed layer is formed including a Fe2Al5
matrix phase having brittleness and a tau phase(Fe-Al-Sibased
alloy phase particle) having soft properties is also
formed. In detail, hardness of the diffusion layer is
normally a Vicker’s hardness (load 1 g) level of 500, which
is relatively soft, and the alloy layer is a Fe2Al5
intermetallic compound, and is very hard with a Vicker’s
hardness (load 1 g) level of 900 to 1100. Accordingly,
there is a large difference in hardnesses between the
diffusion layer and the alloy layer, when a difference in
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hardnesses increases as above and external force is applied,
cracks may develop and spread at an interface between the
diffusion layer and the alloy layer failing to overcome
deformation, and a problem of being delaminated to have a
fine powder form occurs.
The hot dip aluminum plating layer forming the hot
press formed (HPF) article of the present invention is
formed such that the diffusion layer and the alloy layer
have a difference in hardnesses of 400 (Hv) or less. When
a difference in hardnesses between the diffusion layer and
the alloy layer is greater than 400, deformation is not
absorbed during the high temperature processing, and
delamination at an interface between the diffusion layer
and the alloy layer occurs.
As a means of achieving the above, a method of
including components such as Si, Mn, Ti and W steel
components in the alloy layer in very small amounts may be
used in the present invention, and when components such as
above are included in a crystal lattice of an intermetallic
compound, the compound may be crystalline and hardness may
be decreased. Generally, when a specific element is
included in a metal element, hardness increases due to
crystal lattice distortion, however, the intermetallic
compound base itself has strong bonding and high hardness,
and hardness may be significantly decreased when including
a specific element.
As another means, surface tempering is carried out on
the hot dip aluminum plated steel sheet by 0.5% to 3% prior
to high temperature processing so as to facilitate alloy
phase formation during high temperature heating and to
facilitate the introduction of specific elements into the
alloy phase.
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In addition, the inventors of the present invention
have paid attention to a tau phase inside the alloy layer,
a hard layer, and have identified that shapes and
distribution of a tau phase have an influence on a
lamination resistance properties, and delamination
resistance is improved when a tau phase is irregularly and
discontinuously dispersed and distributed in an amount of
10% or greater based on the total area fraction of the
alloy layer. In other words, delamination ultimately
occurs between the diffusion layer and the alloy layer, and
when external force is applied causing the development and
spread of cracks from the surface, the tau phase in the
alloy layer first absorbs some of the external force.
Herein, the tau phase being dispersed into the alloy layer
is advantageous for stress dispersion, and therefore,
dispersion needs to be irregular and discontinuous.
The tau phase needs to be 10% or greater based on the
area fraction of the whole alloy layer to exhibit an
improvement effect, and when less than 10%, a stress
dispersion effect may be insignificant. Preferably, the
tau phase area fraction in the alloy layer is managed to be
within a 10% to 20% range.
In addition, the tau phase has a shape advantageous
for stress dispersion, and closer to a circular shape is
more advantageous. Considering the above, the present
invention preferably controls the tau phase aspect ratio to
1 to 4, and accordingly, delamination resistance may be
more improved. More preferably, the aspect ratio is
managed to be in a range of 1 to 2.
In addition, a tau phase having a smaller size is
more advantageous for stress dispersion. The tau phase is
distributed in the alloy layer in various sizes, and in the
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present invention, having a tau phase with a size of 5 μm
or less in 50% or greater with respect to the total tau
phase fraction is preferable, and more excellent
delamination resistance may be provided thereby.
Next, a method for manufacturing a hot press formed
(HPF) article having excellent delamination resistance will
be described.
The method for preparing a hot press formed (HPF)
article of the present invention includes preparing a steel
sheet having steel components as above; hot dip aluminum
plating the steel sheet by, after heating the steel sheet
to a temperature of 550C to 850C, immersing the steel
sheet in a hot dip aluminum plating bath maintained at a
temperature of 640C to 680C and composed of, in % by
weight, Si: 7% to 13%, Fe: less than 3%, and the remainder
of Al and other unavoidable impurities; skin pass milling
(SPM) the hot dip galvanized steel sheet with an elongation
of 0.5% to 3% after cooling the hot dip galvanized steel
sheet; alloying a hot dip aluminum plating layer on a
surface of the hot dip aluminum plated steel sheet by
heating the hot dip aluminum plated steel sheet to a
temperature of 850C to 950C and maintaining the
temperature for a certain period of time; and preparing a
hot press formed (HPF) product by rapidly cooling the
alloyed hot dip aluminum plated steel sheet to a
temperature of 300C or lower while hot press forming the
alloyed hot dip aluminum plated steel sheet.
First, in the present invention, a steel sheet having
components as described above is prepared. The steel sheet
in the present invention may be a hot rolled steel sheet or
a cold rolled steel sheet.
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Next, in the present invention, the steel sheet is
heated to a temperature of 550C to 850C, and then hot dip
aluminum plated by being immersed in a hot dip aluminum
plating bath maintained at a temperature of 640C to 680C
and composed of, in % by weight, Si:7% to 13%, Fe: less
than 3%, and the remainder of Al and other unavoidable
impurities.
First, the steel sheet is heated to a temperature of
550C to 850C in an annealing furnace. The goal of
annealing the steel sheet is to readily process a steel
sheet that has become hard from cold rolling, and is not to
secure final physical properties. When the annealing
temperature is lower than 550C, processing is difficult
with a hardened structure, which may cause changes in sizes
when a cutting process or a forming process is carried out.
On the other hand, the annealing temperature being higher
than 850C is not suitable since heating facility
degradation and waste of heat energy occur.
Then the heated steel sheet is hot dip aluminum
plated by being immersed in a hot dip aluminum plating bath,
and herein, the temperature of the plating bath is
preferably in a range of 640C to 680C. The temperature of
the plating bath is determined considering phase
transformation of the hot dip aluminum, and it is
preferable to maintain a molten state when plating and
rapidly phase transform a plating layer to a coagulated
state after hot dipping. When the temperature of the hot
dip aluminum plating bath is less than 640C, the aluminum
plating bath is locally coagulated in a pot causing early
coagulation of an aluminum plating layer plated on the
steel sheet, and plating qualities may be poor. On the
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other hand, the temperature of the plating bath being
greater than 680C is not suitable since the plating bath
may be quickly eroded.
Meanwhile, in the present invention, the hot dip
aluminum plating bath may have a composition of 7% by
weight to 13% by weight of Si, less than 3% by weight of Fe,
and the remainder of aluminum and other impurities. When
adding Si to the hot dip aluminum plating bath, Si
participates in a reaction between iron and aluminum, and a
Fe-Al-Si-based alloy layer is produced between the
substrate steel sheet and the plating layer. A Fe-Al-Si
alloy layer is known to suppress excessive production of a
Fe-Al-based alloy layer.
A diffusion reaction is involved when high
temperature heat treating the aluminum plated steel sheet
in a high temperature forming process, and therefore, Si
among the plating bath components affects a structure and
distribution of the alloy plating layer (alloy layer) after
the high temperature forming. In other words, when the Si
content is lower than 7% by weight in an aluminum plating
bath, plating layer formation is heterogeneous, and tau
phase formation inside the alloy layer may be insufficient
during the high temperature heat treatment as well. On the
other hand, when the Si content is greater than 13% by
weight, the plating bath has a higher melting point and an
alloy layer formation reaction is delayed in high
temperature heat treatment, and a tau phase having target
fractions, shapes and distribution may not be obtained.
The hot dip aluminum plating bath includes Fe in an
amount less than 3% by weight, and this is dissolved and
comes from the substrate steel sheet and is in equilibrium
within a solubility limit of iron in the aluminum. However,
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when Fe is included in a plating bath solution in an amount
of 3% by weight or greater, dross is formed in the plating
bath and adheres on a surface of the plated steel sheet,
which lowers surface plating qualities.
In addition, a thickness of the plating layer
obtained from the hot dip aluminum plating is preferably
managed to be within a range of 20 μm to 40 μm in the
present invention.
A thickness of the plating layer may be part of an
order or a request made by a customer and is not a
controlling factor; however, a plating thickness affects
alloy plating layer formation in high temperature forming.
When the plating layer has a thickness of less than 20 μm,
the alloy plating layer may be relatively thin after high
temperature forming, and an article may not be sufficiently
protected from corrosion, and when a plating thickness is
greater than 40 μm, defects such as a flow pattern
frequently occur when plating, and the alloy plating layer
is thickly formed in high temperature forming, which may
cause a problem of facilitating delamination of the alloy
plating layer.
In the present invention, the hot dip galvanized
steel sheet is skin pass milled (SPM) with an elongation of
0.5% to 3% after cooling the hot dip galvanized steel sheet.
In other words, the plated steel sheet that is aluminum
plated by passing through the hot dip aluminum plating bath
goes through an air knife controlling a coating amount of
plating and a cooling process coagulating the hot dip
aluminum plating layer. Subsequently, skin pass milling
(hereinafter, SPM) continuously providing a surface
function is carried out with an elongation of 0.5% to 3%.
Page 21
A main goal of the skin pass milling is to remove
yield point elongation, correct a shape, or provide surface
roughness. However, based on the research results, the
inventors of the present invention have identified that,
when forming an alloy plating layer during high temperature
heat treatment, a tau phase may be irregularly dispersed
and distributed inside the alloy plating layer by providing
deformation to the plating layer through controlling skin
pass elongation and rolling force. However, when the
elongation is less than 0.5%, deformation of the plating
layer is low, and an effect of dispersion and distribution
of the tau phase on the alloy plating layer may not be
obtained, and the elongation greater than 3% is not
desirable, since a part of the plating layer may be
destroyed due to excessive elongation, and coated on the
skin pass roll causing dents and the like. Skin pass
rolling force is a dependent variable of elongation, and is
normally within a range of 100 tons to 250 tons.
Subsequently, by heating the hot dip aluminum plated
steel sheet to a temperature between 850C to 950C, and
maintaining the sheet at the temperature for a certain
period of time, a hot dip aluminum plating layer on the
surface is alloyed.
In other words, the aluminum plated steel sheet is
inserted into a heating furnace having an atmospheric
temperature in a range of 850C to 950C and heat treated at
a temperature in a range of 850C to 950C. The atmospheric
temperature of the heating furnace may not be the same as
the temperature of the inserted steel sheet, and a core
factor is the temperature of the steel sheet. However,
controlling the atmospheric temperature is more convenient
than directly controlling the temperature of the steel
Page 22
sheet. When the temperature of the steel sheet is less
than 850C, austenite homogenization of the steel structure
may be insufficient. On the other hand, a temperature of
the steel sheet greater than 950C is advantageous for
heating the plated steel sheet, but has a problem such as a
waste of energy, and degradation of heating furnace
facilities.
The hot dip aluminum plating layer formed on the
surface of the steel sheet is alloyed from high temperature
heat treatment as described above. In other words, a hot
dip aluminum plating layer formed with a soft diffusion
layer and a hard alloy layer may be obtained, and the alloy
layer may be formed to include a Fe2Al5 matrix phase having
brittleness and a tau phase(Fe-Al-Si-based alloy phase)
having soft properties.
In addition, the tau phases may be irregularly and
discontinuously dispersed and distributed inside the alloy
layer in an amount of 10% or greater, based on the total
area fraction, and accordingly, a difference in hardnesses
between the alloy layer and the soft layer may be
controlled to 400 or less. More preferably, the tau phase
is present in a range of 10% by area to 20% by area inside
the alloy layer.
In addition, the tau phase preferably has an aspect
ratio in a range of 1 to 4.
Moreover, the tau phase having a size of 5 μm or less
preferably occupies an area of 50% or greater with respect
to the total tau phase fraction.
Meanwhile, in the present invention, total heating
time is preferably within 30 minutes in the heat treatment.
When the total heating time is a maximum of 30 minutes in
the above-mentioned temperature range, austenite structure
Page 23
homogenization is saturated, and when total heating time is
longer than 30 minutes, a problem of a reduction in
productivity may occur.
In addition, time taken for the heated steel sheet to
be transported from a furnace to a mold is preferably
within 20 seconds. When the transport time is longer than
20 seconds, the steel sheet temperature decreases to a
ferrite transformation initiation temperature or lower, and
target strength may not be obtained. Preferably, the
transport time is within 12 seconds.
Subsequently, in the present invention, a hot press
formed (HPF) product is manufactured by rapidly cooling the
alloyed hot dip aluminum plated steel sheet to a
temperature of 300C or lower while hot press forming the
alloyed hot dip aluminum plated steel sheet. In other
words, the alloyed steel sheet is formed with a hot press
mold of which inside is water-cooled, and the HPF
processing is finished by removing the processed article
from the mold after the temperature of the steel sheet is
300C or lower. Removing the formed article from the mold
at a steel sheet temperature of 300C or higher after the
hot dip press may cause deformation due to thermal stress.
【Mode for Invention】
Hereinafter, the present disclosure will be described
more specifically according to examples. However, the
following examples are for illustrative purposes only, and
should not be seen as limiting the scope of the present
disclosure. The scope of the present disclosure should be
determined by the claims and information reasonably
inferable therefrom.
Page 24
(Examples)
After preparing a cold rolled steel sheet having a
thickness of 1.5 mm formed with, in % by weight, C: 0.22%,
Si: 0.25%, Mn: 1.18%, P: 0.014%, S: 0.0022%, Al: 0.033%,
Cr: 0.181%, Ti: 0.034%, B: 0.0023%, N: 0.0050%, and a
remainder of Fe and other impurities, the cold rolled steel
sheet was cut into fragments having a size of 120 mm x 200
mm to prepare specimens. Then, the prepared samples were
degreased with kerosene and ultrasonic acetone to remove
surface rolling oil and contaminants. Thereafter, the
prepared samples were heated to a temperature of 780C for
a total heating time of 6 minutes, and then plated by being
immersed in a hot dip aluminum plating bath at a
temperature of 660C, and herein, the coating amount of the
plating layer was controlled to be 20 μm to 40 μm.
Moreover, the hot dip aluminum plating bath composition
herein was formed to have 0% by weight to 13% by weight of
Si, and a remainder of aluminum, Fe being inevitably eluted
from the steel sheet, and other impurities, and skin pass
elongation was controlled to be within a range of 0% to 3%.
The following Table 1 lists specific hot dip aluminum
plating conditions for the plated steel sheet specimens
used in the tests.
【Table 1】
Plated
Steel
Sheet No.
Heating
Temperature
(C)
Plating
Bath
Temperature
(C)
Plating
Bath Si
Amount
(%)
Plating
Thickness
(μm)
Skin Pass
Elongation
(%)
1 780 660 13 30 0.5
2 780 660 9 35 3
3 780 660 9 30 1.2
Page 25
4 780 660 7 40 0.5
5 780 660 3 25 3
6 780 660 0 30 0
7 780 660 9 30 10
8 780 660 9 30 0
When the hot dip aluminum plating was carried out as
in Table 1, the plated steel sheets 1 to 4 exhibited
favorable plating qualities; however, in the plated steel
sheets 5 to 8, problems such as dross adherence after the
hot dipping occurred. Particularly, the plated steel sheet
7 had poor plating surface quality due to excessive plating
layer deformation.
Thereafter, plated steel sheets 1 to 6 and 8 were
heat treated at an atmospheric temperature of 930C for a
total holding time of 6 seconds. Then area fractions of
components forming an alloy plating layer formed by the
heat treatment were measured, and specifically, area
fractions of a diffusion layer, Fe2Al5 and a tau phase
forming the alloy plating layer are shown in the following
Table 2. As shown in the following Table 2, it was seen
that Examples 1 to 4 prepared under the manufacturing
process conditions of the present invention all had the tau
phase area ratio of 10% or greater in the alloy plating
layer. However, Comparative Examples 1 to 3 having the
manufacturing process condition outside the scope of the
present invention all had the tau phase area fraction of
less than 10%.
【Table 2】
Plated
Steel
Sheet
High Temperature
Forming Heat
Treatment
Alloy Plating Layer
Forming Area Ratio (%)
Note
Page 26
No. Atmospheric
Temperature
(C)
Total
Holding
Time (s)
Diffusion
Layer
Fe2Al5
Tau
phase
1 930 6 16 61 23
Invention
Example 1
2 930 6 20 59 21
Invention
Example 2
3 930 6 18 67 15
Invention
Example 3
4 930 6 27 63 10
Invention
Example 4
5 930 6 33 59 8
Comparative
Example 1
6 930 6 74 26 0
Comparative
Example 2
8 930 6 28 63 9
Comparative
Example 3
Meanwhile, a longitudinal section of the plated steel
sheet heat treated as above was observed and analyzed, and
the results are shown in the following Table 3. Herein,
hardness is a Vicker’s (Hv,1 g) hardness value measured
using a micro hardness tester under a 1 g load condition,
and a difference in hardnesses was calculated and listed
after each measured hardness of the diffusion layer and
hardness of the alloy layer. The tau phase area ratio in
the alloy layer was calculated as an area fraction using an
image analyzer, and an area ratio of the tau phase having
an average diameter of 5 μm or less, and an aspect ratio of
the tau phase were also measured. The results are shown in
the following Table 3.
In addition, delamination resistance evaluation on
the alloy layer in the following Table 3 was carried out
such that an internal angle was formed to be 30 degrees
Page 27
using a 3 point bending tester, tape was attached to the
inside and then removed, and a state of delamination
smearing on the tape was evaluated. Specifically, the
following evaluation criteria were used.
[Model Example of Delamination Resistance Evaluation]
○: No delamination occurred and delaminated pieces
were not present on the tape.
X: Delamination occurred and delaminated pieces were
present on the tape.
【Table 3】
Plated
Steel
Sheet
No
Difference
in
hardnesses
(Hv)
between
Diffusion
Layer and
Alloy
Layer
Tau
phase
in
Alloy
Layer
(%)
Area
Ratio
of Tau
phase
with 5
μm or
Less
(%)
Aspect
Ratio
of Tau
phase
Alloy Layer
Delamination
Resistance
Note
1 384 23 82 3.3 ○
Invention
Example 1
2 395 21 73 2.4 ○
Invention
Example 2
3 384 15 62 2.8 ○
Invention
Example 3
4 350 10 61 2.1 ○
Invention
Example 4
5 583 8 55 2.8 X
Comparative
Example 1
6 552 0 0 0 X
Comparative
Example 2
8 485 9 16 7.4 X
Comparative
Example 3
As shown in Table 3, it was seen that Invention
Examples 1 to 4 all had the tau phase area fraction of 10%
or greater in the alloy layer, and in addition to this, the
Page 28
area surface of the tau phase having an average diameter of
5 μm or less and the aspect ratio of the tau phase
satisfied the scope of the present invention. Accordingly,
it was seen that a difference in hardnesses between the
alloy layer and the diffusion layer became 400 (Hv) or less.
Meanwhile, FIG. 1 is a structural picture of a
longitudinal section showing the plating layer of the
plated steel sheet 3 of Invention Example 3, and FIG. 2 is
a diagram of the structural picture of FIG. 1. As shown in
FIGs. 1 and 2, the tau phases (5) forming the alloy layer
are irregularly and discontinuously formed inside the
plating layer in the present invention. Having such tau
phase distribution enables effective accomplishment of the
above-described difference in hardnesses of 400 (Hv) or
less between the alloy layer and the diffusion layer.
In FIG. 2, reference numeral 1 represents a substrate
steel sheet, 2 represents a diffusion layer, 3 represents
an alloy layer, 4 represents Fe2Al5 and 5 represents a tau
phase particle, and an alloy plating layer is formed
combining the diffusion layer (2) and the alloy layer (3).
Meanwhile, it was seen that Comparative Examples 1 to
3 having the manufacturing process outside the scope of the
present invention all had a difference in hardnesses of
greater than 400 (Hv) between the alloy layer and the
diffusion layer, and accordingly, it was identified through
Table 3 that delamination resistance was also degraded.
Hereinbefore, the present invention has been
described with reference to the examples, however, it is to
be understood that those skilled in the art is capable of
diversely modify and change the present invention within
the scope ideas and areas of the present invention
described in the following claims.

【WE CLAIM:】
【Claim 1】
A hot press formed (HPF) article having excellent
delamination resistance, a hot dip aluminum plating layer
being formed on a surface of a substrate steel sheet,
wherein the substrate steel sheet includes, in % by weight,
C: 0.18% to 0.25%, Si: 0.1% to 1.0%, Mn: 0.9% to 1.5%, P:
0.03% or less, S: 0.01% or less, Al: 0.01% to 0.05%, Cr:
0.05% to 0.5%, Ti: 0.01% to 0.05%, B: 0.001% to 0.005%, N:
0.009% or less, and a remainder of Fe and other impurities;
the plating layer is formed to have a soft diffusion
layer and a hard alloy layer; and
a tau phase is irregularly and discontinuously
dispersed and distributed inside the alloy layer in an
amount of 10% or greater based on the total area fraction
so that a difference in hardnesses between the alloy layer
and the diffusion layer is 400 (Hv) or less.
【Claim 2】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the substrate
steel sheet is a cold rolled steel sheet or a hot rolled
steel sheet.
【Claim 3】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the tau phase
is present in a range of 10% by weight to 20% by weight
inside the alloy layer.
【Claim 4】
Page 30
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the tau phase
has an aspect ratio in a range of 1 to 4.
【Claim 5】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the tau phase
having a size of 5 μm or less occupies an area of 50% or
greater with respect to the total tau phase fraction.
【Claim 6】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the substrate
steel sheet further includes Mo+W in an amount of 0.001% to
0.5%.
【Claim 7】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the substrate
steel sheet further includes a sum of one or more types of
Nb, Zr or V in a range of 0.001% to 0.4%.
【Claim 8】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the substrate
steel sheet further includes Cu+Ni in a range of 0.005% to
2.0%.
【Claim 9】
The hot press formed (HPF) article having excellent
delamination resistance of Claim 1, wherein the substrate
steel sheet further includes one or more types of Sb, Sn or
Bi in an amount of 0.03% or less.
【Claim 10】
Page 31
A method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance, the
method comprising:
preparing a steel sheet including, in % by weight, C:
0.18% to 0.25%, Si: 0.1% to 1.0%, Mn: 0.9% to 1.5%, P:
0.03% or less, S: 0.01% or less, Al: 0.01% to 0.05%, Cr:
0.05% to 0.5%, Ti: 0.01% to 0.05%, B: 0.001% to 0.005, N:
0.009% or less, and a remainder of Fe and other impurities;
hot dip aluminum plating the steel sheet by, after
heating the steel sheet to a temperature of 550C to 850C,
immersing the steel sheet in a hot dip aluminum plating
bath maintained at a temperature of 640C to 680C and
composed of, in % by weight, Si: 7% to 13%, Fe: less than
3%, and the remainder of Al and other unavoidable
impurities;
skin pass milling (SPM) the hot dip aluminum plating
steel sheet with an elongation of 0.5% to 3% after cooling
the hot dip galvanized steel sheet;
alloying a hot dip aluminum plating layer on a
surface of the hot dip aluminum plated steel sheet by
heating the hot dip aluminum plated steel sheet to a
temperature of 850C to 950C and maintaining the
temperature for a certain period of time; and
manufacturing a hot press formed (HPF) product by
rapidly cooling the alloyed hot dip aluminum plated steel
sheet to a temperature of 300C or lower while hot press
forming the alloyed hot dip aluminum plated steel sheet.
【Claim 11】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
10, wherein the alloyed hot dip aluminum plating layer is
formed to have a soft diffusion layer and a hard alloy
Page 32
layer; and a tau phases is irregularly and discontinuously
dispersed and distributed inside the alloy layer in an
amount of 10% or greater based on the total area fraction.
【Claim 12】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
10, wherein the steel sheet is a cold rolled steel sheet or
a hot rolled steel sheet.
【Claim 13】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
11, wherein the tau phase is present in a range of 10% by
area to 20% by area inside the alloy layer.
【Claim 14】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
11, wherein the tau phase has an aspect ratio in a range of
1 to 4.
【Claim 15】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
11, wherein the tau phase having a size of 5 μm or less
occupies an area of 50% or greater with respect to the
total tau phase fraction.
【Claim 16】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
10, wherein the substrate steel sheet further includes Mo+W
in an amount of 0.001% to 0.5%.
【Claim 17】
Page 33
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
10, wherein the substrate steel sheet further includes a
sum of one or more types of Nb, Zr or V in a range of
0.001% to 0.4%.
【Claim 18】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
10, wherein the substrate steel sheet further includes
Cu+Ni in a range of 0.005% to 2.0%.
【Claim 19】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
10, wherein the substrate steel sheet further includes one
or more types of Sb, Sn or Bi in an amount of 0.03% or less.
【Claim 20】
A method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance, the
method comprising:
skin pass milling (SPM) a hot dip aluminum plated
steel sheet on which a hot dip aluminum plating layer is
formed on a surface of a substrate steel sheet including,
in % by weight, C: 0.18% to 0.25%, Si: 0.1% to 1.0%, Mn:
0.9% to 1.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.01%
to 0.05%, Cr: 0.05% to 0.5%, Ti: 0.01% to 0.05%, B: 0.001%
to 0.005%, N: 0.009% or less, and a remainder of Fe and
other impurities;
alloying a hot dip aluminum plating layer on a
surface of the hot dip aluminum plated steel sheet by
heating the hot dip aluminum plated steel sheet to a
temperature of 850C to 950C and maintaining the
temperature for a certain period of time; and
Page 34
manufacturing a hot press formed (HPF) product by
rapidly cooling the alloyed hot dip aluminum plated steel
sheet to a temperature of 300C or lower while hot press
forming the alloyed hot dip aluminum plated steel sheet.
【Claim 21】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
20, wherein the alloyed hot dip aluminum plating layer is
formed to have a soft diffusion layer and a hard alloy
layer; and a tau phases is irregularly and discontinuously
dispersed and distributed inside the alloy layer in an
amount of 10% or greater based on the total area fraction.
【Claim 22】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
20, wherein the substrate steel sheet is a cold rolled
steel sheet or a hot rolled steel sheet.
【Claim 23】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
21, wherein the tau phase is present in a range of 10% by
area to 20% by area inside the alloy layer.
【Claim 24】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
21, wherein the tau phase has an aspect ratio in a range of
1 to 4.
【Claim 25】
The method for manufacturing a hot press formed (HPF)
article having excellent delamination resistance of Claim
21, wherein the tau phase having a size of 5 μm or less
Page 35
occupies an area of 50% or greater with respect to the
total tau phase particle fraction.