DESCRIPTION】
【Invention Title】
HOT PRESS FORMED PARTS HAVING EXCELLENT POWDERING
RESISTANCE DURING HOT PRESS FORMING, AND METHOD FOR
5 MANUFACTURING SAME
【Technical Field】
The present disclosure relates to manufacturing a Hot
Press Forming (HPF) part having an aluminum plated layer on
10 the surface thereof, and more specifically, to an HPF part
having excellent powdering resistance during press forming
by minimizing destruction and powdering of a plated layer
during hot press forming, and a method of manufacturing the
same.
15
【Background Art】
An aluminum plated steel sheet for Hot Press Forming
(HPF) is generally prepared by immersing a steel sheet
having a high degree of hardenability in a plating bath
20 containing a plating solution based on Al for plating, and
then the plated steel sheet having an Al plated layer on
the surface thereof is hot pressed. Such a steel sheet is
widely used in the manufacturing of vehicle parts having
complicated shapes and strengths of 1300 MPa or higher.
Page 3
However, in the HPF heating process, the plated layer
is composed of an alloyed layer comprising an intermetallic
compound consisting of FeAl or Fe2Al5 and the like as an
upper layer, and a diffusion layer consisting of Fe 80% to
5 95 wt% (hereinafter all steel ingredients are in wt%) as a
lower layer. However, because the alloyed layer formed
uppermost in the plated layer has brittleness as compared
to the diffusion layer, the alloyed layer may be detached
from the plated layer during hot press forming and may be
10 attached to a pressing side. Therefore, this disadvantage
makes continuous hot press forming difficult.
Namely, if the Al plated material is hot press formed
by heating thereof in a heating furnace at a temperature of
900°C to 930°C, the plated layer may be detached from a
15 region of high surface friction, and at this time, an
overall alloyed layer or a portion thereof may be detached
from the region of high surface friction, and therefore,
there may be a problem in that the detached plated layer
may be attached to the surface of a hot press forming mold.
20 Thus, the development of an HPF part which can overcome
the above-mentioned problems and have excellent press
formability is required.
【Disclosure】
25 【Technical Problem】
Page 4
An aspect of the present disclosure is directed to
providing an HPF part, which can minimize the problem that
a plated layer is detached from a plating object and
attached to the surface of a mold during hot press forming
5 by optimizing thickness of an alloyed layer, and a
percentage of a tau phase and contents of Si and Cr in a
plated layer.
Further, an aspect of the present disclosure is
directed to provide a method of manufacturing the HPF part.
10 However, problems sought to be resolved by the present
disclosure are not limited to the above-described problems.
Other problems, which are sought to be resolved by the
present disclosure but are not described herein, can be
clearly understood by those skilled in the art from the
15 descriptions below.
【Technical Solution】
The present disclosure relates to, in an HPF part
wherein a hot-dip plated layer comprising Al is formed on
the surface of a base steel sheet, an HPF part having
20 excellent powdering resistance during hot press forming,
wherein the base steel sheet comprises 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%
25 or less, and a balance of Fe and other impurities by wt%;
Page 5
the hot-dip plated layer consists of a soft diffusion
layer and a hard alloyed layer;
in the alloyed layer, a tau phase exists at area%
within a range of 10% to 30%; and
5 the tau phase is prepared by comprising Si of 10% or
greater and Cr of 0.2% or greater by wt% thereof, so as to
allow the alloyed layer to have a thickness of 35 μm or
less.
The base steel sheet may be a cold rolled steel sheet
10 or a hot rolled steel sheet.
The tau phase may preferably be prepared by comprising
Si: 10% to 12%, Mn+Cr: 1.3% to 2.0%, and a balance of Fe
and Al by wt% thereof.
In the hot-dip plated layer, a thickness ratio of the
15 alloyed layer/diffusion layer may preferably satisfy 1.5 to
3.0.
The tau phase may preferably be formed on the boundary
between the alloyed layer and the diffusion layer and
inside the alloyed layer, and the tau phase formed inside
20 the alloyed layer may preferably take the form of a band
interconnected in 50% or longer of a zone perpendicular to
the thickness of the plated layer.
The base steel sheet may preferably further comprise Mo
+ W: 0.001% to 0.5%.
25 Further, the base steel sheet may preferably further
Page 6
comprise at least one of Nb, Zr and V: within a range of
0.001% to 0.4% (as the sum).
Further, the base steel sheet may preferably further
comprise Cu + Ni: within a range of 0.005% to 2.0%.
5 Moreover, the base steel sheet may preferably further
comprise at least one of Sb, Sn, and Bi: 0.03% or less.
Further, the present disclosure relates to a method of
manufacturing an HPF part having excellent powdering
resistance during hot press forming, which comprises:
10 a process of preparing a steel sheet having a
composition as described above;
a process of heating the steel sheet at a temperature
of 550°C to 850°C followed by maintaining a temperature at
640°C to 680°C, and then immersing the steel sheet in a hot
15 dip aluminum plating bath prepared by comprising Si: 9% to
11%, Fe: 3% or less, and a balance of Al and other
inevitable impurities by wt% for hot dip aluminum plating;
a process of heating the hot dip aluminum plated steel
sheet at a temperature of 880°C to 930°C followed by being
20 maintained for a certain time to alloy a hot dip aluminum
plated layer of the surface thereof; and
a process of hot forming the alloyed hot dip aluminum
plated steel sheet and quenching at a temperature range of
300°C or lower at the same time to manufacture an HPF part.
25 In the present disclosure, the plated steel sheet may
Page 7
preferably be cooled at an average cooling speed of 15°C/s
or faster after the hot dip plating until the plated layer
is solidified.
Further, in the present disclosure, the alloyed hot dip
5 aluminum plated layer may preferably consist of a soft
diffusion layer and a hard alloyed layer; in the alloyed
layer, a tau phase may preferably exist at area% within a
range of 10% to 30%; and the tau phase may preferably be
prepared by comprising Si of 10% or greater and Cr of 0.2%
10 or greater by wt% thereof, so as to allow the alloyed layer
to have a thickness of 35 μm or less.
The steel sheet may be a cold rolled steel sheet or a
hot rolled steel sheet.
The tau phase may preferably be prepared by comprising
15 Si: 10% to 12%, Mn+Cr: 1.3% to 2.0%, and a balance of Fe
and Al by wt% thereof.
In the hot dip aluminum plated layer, a thickness ratio
of the alloyed layer/diffusion layer may preferably satisfy
1.5 to 3.0.
20 The tau phase may preferably be formed on the boundary
between the alloyed layer and the diffusion layer and
inside the alloyed layer, and the tau phase formed inside
the alloyed layer may preferably take the form of a band
interconnected in 50% or longer of a zone perpendicular to
25 the thickness of the plated layer.
Page 8
Further, the method may further comprise a process of
cooling the alloyed hot dip aluminum plated steel sheet at
a temperature range of 700°C to 780°C before hot forming
5 the alloyed hot dip aluminum plated steel sheet.
At this time, a cooling speed thereof may preferably be
controlled within a range of 20°C/s to 100°C/s.
Further, the present disclosure relates to a method of
manufacturing an HPF part having excellent powdering
10 resistance during hot press forming, which comprises:
a process of heating the hot dip aluminum plated steel
sheet at a temperature of 880°C to 930°C followed by being
maintained for a certain time to alloy a hot dip aluminum
plated layer of the surface thereof; and
15 a process of hot forming the alloyed hot dip aluminum
plated steel sheet and quenching at a temperature range of
300°C or lower at the same time to manufacture an HPF part.
Further, in the present disclosure, the alloyed hot dip
aluminum plated layer may preferably consist of a soft
20 diffusion layer and a hard alloyed layer; in the alloyed
layer, a tau phase may preferably exist at area% within a
range of 10% to 30%; and the tau phase may preferably be
prepared by comprising Si of 10% or greater and Cr of 0.2%
or greater by wt% thereof, so as to allow the alloyed layer
25 to have a thickness of 35 μm or less.
Page 9
The steel sheet may be a cold rolled steel sheet or a
hot rolled steel sheet.
The tau phase may preferably be prepared by comprising
Si: 10% to 12%, Mn+Cr: 1.3% to 2.0%, and a balance of Fe
5 and Al by wt% thereof.
In the hot dip aluminum plated layer, a thickness ratio
of the alloyed layer/diffusion layer may preferably satisfy
1.5 to 3.0.
【Advantageous Effects】
10 The present disclosure has an effect of effectively
providing an HPF part, which can minimize the problem that
a plated layer is detached and attached on the surface of a
mold during hot press forming by optimizing thickness of an
alloyed layer making a hot dip aluminum plated layer,
15 percentage, and composition of a tau phase in a plated
layer and the like.
【Description of Drawings】
FIG. 1 is an image of a structure showing a cross
section of a plated layer after hot pressing in an Example
20 of the present disclosure.
FIG. 2 is an image of a structure showing a cross
section of a plated layer after hot pressing in a
Comparative Example.
Page 10
【Best Mode for Invention】
Hereinafter, the present disclosure will be described.
In general, there is a problem in that when hot press
forming a hot dip aluminum plated steel sheet, the plated
5 layer is detached after hot pressing, and the plated layer
is attached to the surface of a press mold thereby
deteriorating press formability. The present inventors
have repeated studies and experiments to solve these
problems, and as a result, have looked for solutions to
10 reduce thickness of the alloyed layer having brittleness in
the hot dip aluminum plated layer to be as thin as possible.
Moreover, the inventors found that thickness of the alloyed
layer is closely related to area percentage and composition
of a tau phase in the alloyed layer.
15 In describing in detail, in the alloyed layer, a tau
phase that is a FeAl phase having a base, which consists of
a Fe2Al5 phase and has brittleness, and having softness is
distributed. And in the lower part of the alloyed layer, a
soft layer is formed on an interface with the base steel
20 sheet.
The present inventors suggest the present disclosure
after discovering that the composition (contents of Si and
Cr) of the tau phase making the alloyed layer is important.
In particular, when the tau phase contains Si of 10% or
25 greater and Cr of 0.2% or greater by wt% thereof, the tau
Page 11
phase can be distributed so that a percentage of the tau
phase in the whole alloyed layer is 10% or greater, and
thickness of the alloyed layer is within 35 μm, thereby
minimizing detachment of the plated layer during hot press
5 forming. In other words, the present inventors suggests
the present disclosure after discovering that after the HPF
process, the percentage of the tau phase in the alloyed
layer and the contents of Si and Cr in the tau phase affect
press formability of the plated layer.
10 Hereinafter, the HPF part of the present disclosure
having excellent powdering resistance during hot press
forming will be described.
The HPF part of the present disclosure refers to a
formed part manufactured by hot press forming a hot dip
15 aluminum plated steel sheet wherein a hot dip aluminum
plated layer is formed on the surface of a base steel sheet.
In the present disclosure, the base steel sheet may be a
common cold rolled steel sheet, but may also be a hot
rolled steel sheet.
20 The base steel sheet making the HPF part is prepared by
comprising 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 balance of Fe and other
25 impurities by wt%. Each ingredient of the base steel sheet
Page 12
and reasons for limiting thereof will be described in
detail as follows.
C: 0.18% to 0.25%
5 C is an essential element increasing strength of
martensite. If a C content is less than 0.18%, it may be
difficult to obtain enough strength to secure impact
resistance. Further, if the C content is greater than
0.25%, impact toughness of a slab may be deteriorated, and
10 weldability of the HPF part may be deteriorated.
Considering that, in the present disclosure, it is
preferable to limit the C content to 0.18 wt% to 0.25 wt%
(hereinafter, just referred to as “%”).
15 Si: 0.1% to 1.0%
Si is effective to uniformity of a steel material after
the HPF, and may contribute to formation of the tau phase
of the plated layer by diffusion to the plated layer during
the HPF process. If the Si content is less than 0.1%, it
20 may be difficult to obtain enough effect to uniformity of a
material and diffusion to the plated layer, and if the Si
content is greater than 1.0%, it may be difficult to secure
good quality of the hot dip aluminum plated surface by Si
oxides formed on the surface of the steel sheet during
25 annealing. Therefore, the Si is added in an amount of 1.0%
Page 13
or less.
Mn: 0.9% to 1.5%
Mn is added to secure hardenability of a steel such as
Cr, B, and the 5 like. If a Mn content is less than 0.9%, it
may be difficult to secure enough hardenability thereby
forming bainite. Therefore, it may be difficult to secure
enough strength. Further, it the Mn content is greater
than 1.5%, a cost of manufacturing a steel sheet may
10 increase, and also a bending property of the HPF part may
be remarkably deteriorated as the Mn is segregated inside
the steel material. Considering that, in the present
disclosure, it is preferable to limit the Mn content within
a range of 0.9% to 1.5%.
15
P: 0.03% or less (not including 0%)
P is a grain boundary segregation element hindering
many characteristics of the HPF part. Thus, it is
preferable to contain the P as small as possible. If the P
20 content is greater than 0.03%, a bending property, an
impact property, weldability, and the like of the formed
part may be deteriorated. Thus, it is preferable to limit
the upper limit of the content to 0.03%.
25 S: 0.01% or less (not including 0%)
Page 14
S is an element existing in a steel as an impurity and
hindering a bending property and weldability of the formed
part. Thus, it is preferable to contain the S as small as
possible. If the S content is greater than 0.01%, the
5 bending property and weldability of the formed part may
become worse. Thus, it is preferable to limit the upper
limit of the content to 0.01%.
Al: 0.01% to 0.05%
10 Al is added for the purpose of deoxidation for steel
making like the Si. In order to achieve the purpose, the
Al is added in an amount of 0.01% or greater. If the
content is greater than 0.05%, the effect may be saturated,
and the surface quality of the plated material may become
15 worse. Thus, it is preferable to limit the upper limit of
the content to 0.05%.
Cr: 0.05% to 0.5%
Cr is added to secure hardenability of a steel such as
20 Mn, B, and the like. If a Cr content is less than 0.05%,
it may be difficult to secure enough hardenability, and if
the content is greater than 0.5%, the hardenability can be
sufficiently secured. But the characteristic may be
saturated, and also a cost of manufacturing the steel may
25 increase. Considering that, in the present disclosure, it
Page 15
is preferable to limit the Cr content to a range of 0.05%
to 0.5%.
Ti: 0.01% to 0.05%
5 Ti is added to form TiN by being bound to a nitrogen
remained in a steel as an impurity, thereby leaving solid B
essential to secure hardenability. If a Ti content is less
than 0.01%, it may be difficult to expect the sufficient
effect, and if the content is greater than 0.05%, the
10 characteristic may be saturated and a cost of manufacturing
a steel may increase. Considering that, in the present
disclosure, it is preferable to limit the Ti content to a
range of 0.01% to 0.05%.
15 B: 0.001% to 0.005%
B is added to secure hardenability of the HPF part like
Mn and Cr. To achieve the purpose, the B should be added
in an amount of 0.001% or greater, and if the content is
greater than 0.005%, the effect may be saturated, and also
20 a hot rolling property may be remarkably reduced. Thus, in
the present disclosure, it is preferable to limit the B
content to a range of 0.001% to 0.005%.
N: 0.009% or less
25 N exists in a steel as an impurity, and it is
Page 16
preferable to add as little N as possible. If a N content
is greater than 0.009%, it may cause bad surface of a steel.
Thus, it is preferable to limit the upper limit of the
content to 0.009%.
5
Then, more preferably, the base steel sheet of the HPF
part of the present disclosure may further contain the
following ingredients.
Mo + W: 0.001% to 0.5%
10 Mo and W are elements reinforcing hardenability and
precipitation, and are very effective to further secure
high strength. If the sum of the amounts of the Mo and the
W is less than 0.001%, it may be difficult to obtain a
sufficient effect of reinforcing hardenability and
15 precipitation, and if the content is greater than 0.5%, the
effect may be saturated and also a manufacturing cost may
increase. Thus, in the present disclosure, it is
preferable to limit the Mo + W content to a range of 0.001%
to 0.5%.
20
A sum of at least one of Nb, Zr, or V: 0.001% to 0.4%
Nb, Zr, and V are elements increasing strength of a
steel sheet, and improving grain refinement and heat
treatment characteristics. If a content of at least one of
25 the Nb, Zr, and V is less than 0.001%, it may be difficult
Page 17
to expect the above effect, and if the content is greater
than 0.4%, a manufacturing cost may excessively increase.
Thus, in the present disclosure, it is preferable to limit
the contents of the elements to a range of 0.001% to 0.4%.
5
Cu + Ni: 0.005% to 2.0%
Cu is an element improving strength by forming fine Cu
precipitates, and Ni is an element effective to increase
strength and improve heat treatment characteristics. If
10 the sum of the above ingredients is less than 0.005%, it
may difficult to obtain enough desired strength, and if the
content is greater than 2.0%, workability may become worse,
and a manufacturing cost may increase. Considering that,
in the present disclosure, it is preferable to control the
15 Cu + Ni content to a range of 0.005% to 2.0%.
At least one of Sb, Sn, or Bi: 0.03% or less
Sb, Sn, and Bi are grain segregation elements, and
during the HPF process, the elements are concentrated on
20 the interface between the plated layer and the base iron
and can improve adhesion of the plated layer. The elements
can play a role in preventing the detachment of the plated
layer during hot forming by improving adhesion of the
plated layer. Because Sb, Sn, and Bi have similar
25 characteristics, it is possible to use the three elements
Page 18
as a mixture, and at this time, the sum of at least one may
preferably be 0.03% or less. If the sum of the above
ingredients is greater than 0.03%, there may be a problem
that brittleness of the base iron may be deteriorated
5 during the hot forming.
The HPF part of the present disclosure has a hot dip
aluminum plated layer formed on the surface of the base
steel sheet having the above mentioned composition, and as
10 known, the plated layer consists of a soft diffusion layer
and a hard alloyed layer. And the alloyed layer is
composed of an Fe2Al5 base phase having brittleness and a
tau phase (FeAl) having softness. At this time, in the
present disclosure, the tau phase is formed on the boundary
15 between the alloyed layer and the diffusion layer and
inside the alloyed layer, and the tau phase formed inside
the alloyed layer may preferably take the form of a band
interconnected in 50% or longer of a zone perpendicular to
the thickness of the plated layer.
20 In the present disclosure, in the alloyed layer, the
tau phase (FeAl) may preferably exist in a range of 10% to
30% by area%. If the area percentage of the tau phase is
less than 10%, the plated layer may often detach during
press processing because the plated layer is mechanically
25 brittle, and if the percentage is greater than 30%,
Page 19
weldability may become worse.
Further, in the present disclosure, the tau phase may
preferably be prepared by comprising Si of 10% or greater
and Cr of 0.2% or greater (remaining ingredients are Al and
5 Fe) by wt% thereof. By controlling the ingredients of the
tau phase as described above, it is possible to control
thickness of the alloyed layer having brittleness to be 35
μm or less and also the area percentage of the tau phase
can be controlled. Thus, the HPF part having excellent
10 powdering resistance during hot press forming can be
provided.
More preferably, the tau phase may be prepared by
comprising Si: 10% to 12%, Mn+Cr: 1.3% to 2.0%, and a
balance of Fe and Al by wt% thereof.
15 Further, in the present disclosure, in the hot dip
aluminum plated layer, a thickness ratio of the alloyed
layer/diffusion layer may preferably satisfy 1.5 to 3.0.
Due to the steel composition of the base steel sheet
and constitution of the plated layer as mentioned above,
20 the HPF part of the present disclosure can prevent defects
such as detachment of the plated layer during hot forming
thereby improving powdering resistance.
Next, a method of manufacturing the HPF part having
excellent powdering resistance during hot press forming
25 will be described.
Page 20
First of all, in the present disclosure, a steel sheet
having the steel composition as mentioned above is prepared.
In the present disclosure, as the steel sheet, a cold
rolled steel sheet as well as a hot rolled steel sheet can
5 be used.
Specifically, as the steel sheet, a scale removed hot
rolled steel sheet, or a cold rolled steel sheet obtained
after cold rolling the hot rolled sheet can be used. And
at this time, as the cold rolled steel sheet, a case of
10 cold rolling the hot rolled steel sheet and then annealing
thereof at an atmosphere of reducing gas of 750°C to 850°C
is also included.
Then, in the present disclosure, hot dip aluminum
15 plating is conducted by heating the steel sheet at a
temperature of 550°C to 850°C followed by being maintained
at 640°C to 680°C, and then immersing the steel sheet in a
hot dip aluminum plating bath prepared by comprising Si: 9%
to 11%, Fe: 3% or less, and a balance of Al and other
20 inevitable impurities by wt%.
Namely, in the present disclosure, for hot dip aluminum
plating, the steel sheet is inserted into a heating furnace
and then heated. At this time, it is preferable to limit a
range of a heating temperature to 550°C to 850°. If the
25 heating temperature of the steel sheet is less than 550°C,
Page 21
a temperature difference between the steel sheet and the
plating bath is excessive, thereby cooling a temperature of
the plating bath during hot dip plating. Thus, plating
quality may be deteriorated. If the temperature is higher
5 than 850°C, equipment may be deteriorated by the high
temperature.
Then, the hot dip aluminum plating is conducted by
maintaing the steel sheet at 640°C to 680°C, and then
immersing the heated steel sheet in a hot dip aluminum
10 plating bath prepared by comprising Si: 9% to 11%, Fe: 3%
or less, and a balance of Al and other inevitable
impurities by wt%. If the temperature of the plating bath
is lower than 640°C, uniformity of forming plated layer
thickness may be deteriorated, and if the temperature is
15 higher than 680°C, a port of the plating bath may be
deteriorated by corrosion due to the high temperature.
On the other hand, in the present disclosure, it is
required to prepare the hot dip aluminum plating bath
composition by comprising Si: 9% to 11%, Fe: 3% or less,
20 and a balance of Al and other inevitable impurities by wt%.
If the Si content is less than 9%, formation of the
plated layer may be non-uniform, and also formation of a
tau phase of the plated layer during HPF may be inadequate,
thereby damaging the plated layer during pressing. On the
25 contrary, if the Si content is greater than 11%, there is a
Page 22
problem of raising a managing temperature of the plating
bath due to raised melting temperature of the plating bath.
Further, the Fe in the plating bath is molted from the
steel sheet to the plating bath during the plating process.
5 However, if the Fe content in the plating bath is 3% or
greater, formation of an FeAl compound mass called dross in
the plating bath may be easy, thereby hindering plating
quality. Thus, it is required to manage the content to be
less than 3%.
10 On the other hand, after hot dipping, a solidified
structure is determined during the solidifying process, and
the solidified structure has a major influence to the
alloying process and formation of the tau phase during the
HPF process. Thus, it is required to control the
15 solidifying speed. After solidifying, the Al plated layer
has a structure wherein an Al phase has hardness in a range
of Hv 70 to 100 and a ternary FeAlSi alloyed phase has a
hardness of Hv 800 to 1000. If this structure is nonuniform,
it is not good to inhibit brittleness of the
20 plated layer because the formation of the tau phase is not
enough or does not have continuity during the HPF process.
As a result of being confirmed by the present inventors,
if the cooling speed until the plated layer is solidified
after the hot dip plating is within 15°C/s on average, the
25 plated layer structure may be non-uniform, but if the
Page 23
average speed is faster than 15°C/s on average, the region
where the FeAlSi alloy phase other than the Al phase does
not exist in the middle of the plated layer may be
uniformly controlled within 50 μm on average. If the Al
5 phase region where the FeAlSi alloy phase is not
precipitated widens in the middle of the plated layer even
locally, strength of the plated layer may become nonuniform.
In this case, when unwiding or cutting a coil
made of a plated material before hot pressing, the
10 operation may be difficult because of a problem that the
plated layer is attached to a touch-type roll. Accordingly,
it is required that the length of the region where the
FeAlSi phase is precipitated in the middle of the plated
layer is within 50 μm on average and not over 100 μmat
15 maximum. More preferably, the length may be within 30 μm
on average and not over 50 μmat maximum.
In order to secure this cooling speed, it is preferable
to quenching the layer using steam from directly after the
hot dip plating to solidification. At this time, if the
20 plated surface is directly cooled by a metallic piece or
liquid droplet other than steam, it may cause nonuniformity
of the plated structure.
At this time, in the present disclosure, it is
preferable to control thickness of the plated layer formed
25 by the hot dip aluminum plating within 25μm to 35 μm. As a
Page 24
result of the hot dip melting, if the thickness of the
plated layer is thinner than 25 μm, it may be not enough to
protect the part by the plated layer, and if the thickness
is 35 μm or thicker, mechanical properties of the plated
5 layer after heating may become brittle, thereby generating
powdering in the plated layer.
Also, in the present disclosure, the hot dip aluminum
plated steel sheet may be heated at a temperature of 880°C
to 930°C followed by being maintained for a certain time to
10 alloy the hot dip aluminum plated layer of the surface
thereof. In the present disclosure, it is required to heat
the hot dip plated steel sheet at a temperature of at least
880°C. If the temperature of the plated steel sheet is
lower than 880°C, uniformity of austenite of the steel
15 structure may be deteriorated. On the contrary, if the
temperature of the steel sheet is higher than 930°C, the
plated layer may be thermally deteriorated.
Due to this heat treatment, the hot dip aluminum plated
layer is alloyed. Namely, the hot dip aluminum plated
20 layer consisting of a diffusion layer and a hard alloyed
layer can be obtained, and the alloyed layer includes a
Fe2Al5 base phase having brittleness and a tau phase (FeAl)
having softness.
In the present disclosure, preferably, the tau phase
25 (FeAl) may exist in the alloyed layer at an area% within a
Page 25
range of 10% to 30%. Further, it is preferable to prepare
the tau phase to contain Si of 10% or greater and Cr of
0.2% or greater by wt% thereof (remaining ingredients are
Al and Fe), and it is more preferable to prepare the tau
5 phase to contain Si: 10% to 12%, Mn+Cr: 1.3% to 2.0% and a
balance of Fe and Al by wt% thereof.
Moreover, in the present disclosure, more preferably, a
thickness ratio of the alloyed layer/diffusion layer in the
alloyed hot dip aluminum plated layer may satisfy 1.5 to
10 3.0.
On the other hand, in the present disclosure, the
maintaining time may preferably be managed not to be longer
than 10 min.
15 Then, in the present disclosure, the HPF part may be
manufactured by hot forming the alloyed hot dip aluminum
plated steel sheet and being quenched at a temperature
range of 300°C or lower at the same time. Namely, the
alloyed steel sheet is formed by a hot press forming mold
20 whose inside is cooled by water, and the HPF processing is
finished by ejecting the process part from the mold after a
temperature of the steel sheet becomes 300°C or lower. If
the formed part from the mold at a temperature of the steel
sheet of 300°C or higher is ejected after hot pressing,
25 there may be a problem of deformation by thermal stress.
Page 26
Further, according to one embodiments in the present
disclosure, the method may further comprise a process of
cooling the heated steel sheet before hot forming the steel
sheet by a mold. It was confirmed that this cooling
5 process has an effect of inhibiting cracking of the plated
layer when formed by a mold by preventing accumulation of
stress on the plated layer. However, this step is just a
maximized effect of the present disclosure, and therefore,
it is not necessarily required to be conducted.
10 When cooling, the cooling speed may be preferably
20°C/s to 100°C/s. If the cooling speed is slower than
20°C/s, the cooling effect can’t be expected, but if the
speed is faster than 100°C/s, a martensite transformation
effect by hot press may be reduced by excessive cooling.
15 During the cooling, a temperature of completing the
cooling may preferably be 700°C to 780°C. If the
temperature of completing the cooling is lower than 700°C,
the martensite transformation effect by hot press may be
reduced, but if the temperature is higher than 780°C, the
20 effect of inhibiting cracking of the plated layer by the
cooling may be reduced.
【Mode for Invention】
Hereinafter, the present disclosure will be described
in greater detail with reference to examples. However, the
25 following examples are for illustrative purposes only, and
Page 27
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.
5 (Example)
First of all, a cold rolled steel sheet in a thickness
of 1.4 mm having a composition of 0.227C-0.26Si-1.18Mn-
0.014P-0.0024S-0.035Al-0.183Cr-0.034 Ti-0.0023B-0.0040N by
wt% was prepared, and oils and contaminants of the surface
10 of the cold rolled steel sheet were removed by washing.
After heating the cold rolled steel sheet at 760°C, the
steel sheet was immersed in a plating bath maintained at
660°C to form a hot dip aluminum plated layer on the steel
sheet. At this time, in the plating bath, other than Al, a
15 Si content was changed to 8% to 11%, and an Fe content was
evaluated within a range of 1.7% to 2.5%. Then, as
described above, the plated steel sheet where the hot dip
aluminum plated layer is formed was cooled, and at this
time, the cooling speed of Example 1, Example 2, Example 3,
20 Comparative Example 1, and Comparative Example 2 was
controlled to 15°C/s, 35°C/s, 45°C/s, 14°C/s and 12°C/s,
respectively, in the following Table 1.
Then, as shown in Table 1, the cooled plated steel
sheet was inserted in a heating furnace of 900°C to 930°C
25 for 5 min to 6 min followed by heating thereof, and HPF was
Page 28
continuously conducted. At this time, the continuous work
was conducted until a width of defects made by debris
detached from the plated layer on the surface of the formed
part became 0.5 mm, and a number thereof became 5.
5 The following Table 1 shows a plating bath composition
and plated layer thickness, and percentage, composition,
and thickness of the tau phase after heating used in
manufacturing plated steel sheets used to a press
formability test, and also shows a possible continuous work
10 number in summary. However, an absolute value of the
continuous work number may vary depending on a shape and
material of the mold; but in these Examples, it can be
found that an increase and decrease of the continuous work
number is significantly changed according to a structure
15 and composition of the alloyed layer.
Page 29
Table 1
Secti
on
Plating Bath
composition
(wt%)
Heating
Temp.
(°C)
Heating
Time
(min)
% of
Tau
phase
Tau phase composition
(wt%)
Alloy
Layer
Thickness
(㎛)
Alloy layer/
diffusion layer
ratio
Continuous
Working
Number
Al Si Fe Al Si Cr Mn
Exam.
1
88.5 9.7 1.8 900 5 21 22.6 10.29 0.28 1.3 30.5 2.8 255
Exam.
2
88.5 9.7 1.8 930 6 27 23.5 10.50 0.23 1.4 30.2 1.75 290
Exam.
3
87.0 11 2.0 900 6 19 21.1 11.5 0.25 1.2 29.4 2.2 260
Comp.
Exam.
1
89.1 8.8 2.0 900 6 7.2 28.5 8.34 0.15 1.1 36 9.5 80
Comp.
Exam.
2
89.5 8.0 2.5 900 6 5.9 29.5 8.11 0.18 1.0 37 8.2 85
Page 30
*In Table 1, a % of tau phase means percentage of a tau
phase in an alloyed layer, and a tau phase composition
means wt% thereof (a balance of Fe).
As shown in the above Table 1, in the cases of Examples
5 1 to 3 containing Si of 10% or greater and Cr of 0.2% or
greater in the tau phase composition making the alloyed
layer, it can be found that all of them have alloyed layer
thickness of 35 μm or thinner and also an excellent
continuous working number of 255 or more.
10 On the other hand, FIG. 1 is an image showing a cross
section of the plated layer of Example 1 of the present
disclosure. As shown in FIG. 1, it can be found that after
HPF processing, the plated layer is composed of an alloyed
layer and a diffusion layer, and the tau phase is shown as
15 a dark colored region in the alloyed layer.
On the contrary, in the cases of Comparative Examples 1
and 2 containing Si of 10% or less in the tau phase
composition making the alloyed layer, it can be found that
both of them have alloyed layer thickness of 35 μm or
20 greater, and a bad continuous working number of 85 or less.
FIG. 2 is an image of a structure showing a cross section
of the plated layer of Comparative Example 1 of the present
disclosure.
While the present disclosure has been shown and
25 described with reference to exemplary embodiments thereof,
Page 31
it will be understood by those skilled in the art that
various changes in form and details may be made therein
without departing from the spirit and scope of the
invention as defined by the appended claims. Therefore, the
5 scope of the invention is defined not by the detailed
description of the invention but by the appended claims,
and all differences within the scope will be construed as
being included in the present disclosure.

【WE CLAIM】
【Claim 1】
An HPF part wherein a hot-dip plated layer comprising
Al is formed on the surface of a base steel sheet, an HPF
5 part having excellent powdering resistance during hot press
forming,
wherein the base steel sheet comprises 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%,
10 Ti: 0.01% to 0.05%, B: 0.001% to 0.005%, N: 0.009% or less,
and a balance of Fe and other impurities by wt%;
the hot-dip plated layer consists of a soft diffusion
layer and hard alloyed layer;
in the alloyed layer, a tau phase exists at area%
15 within a range of 10% to 30%; and
the tau phase is prepared by comprising Si of 10% or
greater and Cr of 0.2% or greater by wt% thereof, so as to
allow the alloyed layer to have a thickness of 35 μm or
less.
20 【Claim 2】
The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the base steel
sheet is a cold rolled steel sheet or a hot rolled steel
Page 33
sheet.
【Claim 3】
The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the tau phase
5 is prepared by comprising Si: 10% to 12%, Mn+Cr: 1.3% to
2.0% and a balance of Fe and Al by wt% thereof.
【Claim 4】
The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein a thickness
10 ratio of the alloyed layer/diffusion layer in the hot-dip
plated layer satisfies 1.5 to 3.0.
【Claim 5】
The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the tau phase
15 is formed on the boundary between the alloyed layer and the
diffusion layer and inside the alloyed layer, and the tau
phase formed inside the alloyed layer takes the form of a
band interconnected in 50% or longer of a zone
perpendicular to the thickness of the plated layer.
20
【Claim 6】
Page 34
The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the base steel
sheet further comprises Mo + W: 0.001% to 0.5%.
【Claim 7】
5 The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the base steel
sheet further comprises at least one of Nb, Zr, and V:
within a range of 0.001% to 0.4% (as the sum).
【Claim 8】
10 The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the base steel
sheet further comprises Cu + Ni: within a range of 0.005%
to 2.0%.
【Claim 9】
15 The HPF part having excellent powdering resistance
during hot press forming of claim 1, wherein the base steel
sheet further comprises at least one of Sb, Sn, and Bi:
0.03% or less.
【Claim 10】
20 A method of manufacturing an HPF part having excellent
powdering resistance during hot press forming, which
comprises:
Page 35
a process of preparing a steel sheet comprising 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:
5 0.009% or less, and a balance of Fe and other impurities by
wt%;
a process of heating the steel sheet at a temperature
of 550°C to 850°C followed by being maintained at 640°C to
680°C, and then immersing the steel sheet in a hot dip
10 aluminum plating bath prepared by comprising Si: 9% to 11%,
Fe: 3% or less, and a balance of Al and other inevitable
impurities by wt% for hot dip aluminum plating;
a process of heating the hot dip aluminum plated steel
sheet at a temperature of 880°C to 930°C followed by being
15 maintained for a certain time to alloy a hot dip aluminum
plated layer of the surface thereof; and
a process of hot forming the alloyed hot dip aluminum
plated steel sheet and being quenched at a temperature
range of 300°C or lower at the same time to manufacture an
20 HPF part.
【Claim 11】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 10, wherein the alloyed hot dip aluminum plated layer
25 consists of a soft diffusion layer and a hard alloyed
Page 36
layer; in the alloyed layer, a tau phase exists at area%
within a range of 10% to 30%; and the tau phase is prepared
by comprising Si of 10% or greater and Cr of 0.2% or
greater by wt% thereof.
5 【Claim 12】
The method of manufacturing an HPF part having
excellent powdering resistance during the hot press forming
of claim 11, wherein the tau phase is prepared by
comprising Si: 10% to 12%, Mn+Cr: 1.3% to 2.0%, and a
10 balance of Fe and Al by wt% thereof.
【Claim 13】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 11, wherein the thickness ratio of the alloyed
15 layer/diffusion layer in the alloyed hot dip aluminum
plated layer satisfies 1.5 to 3.0.
【Claim 14】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
20 claim 10, wherein the steel sheet is a cold rolled steel
sheet or a hot rolled steel sheet.
【Claim 15】
Page 37
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 10, wherein the plated steel sheet is cooled at an
average cooling speed of 15°C/s or faster after the hot dip
5 aluminum plating until the plated layer is solidified.
【Claim 16】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 10, which further comprises a process of cooling the
10 plated steel sheet at a temperature range of 700°C to 780°C
at a cooling speed of 20°C/s to 100°C/s, before hot forming
the alloyed hot dip aluminum plated steel sheet.
【Claim 17】
The method of manufacturing an HPF part having
15 excellent powdering resistance during hot press forming of
claim 10, wherein the steel sheet further comprises Mo + W:
0.001% to 0.5%.
【Claim 18】
The method of manufacturing an HPF part having
20 excellent powdering resistance during hot press forming of
claim 10, wherein the steel sheet further comprises at
least one of Nb, Zr, and V: within a range of 0.001% to
0.4% (as the sum).
Page 38
【Claim 19】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 10, wherein the steel sheet further comprises Cu +
5 Ni: within a range of 0.005% to 2.0%.
【Claim 20】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 10, wherein the steel sheet further comprises at
10 least one of Sb, Sn, and Bi: 0.03% or less.
【Claim 21】
A method of manufacturing an HPF part having excellent
powdering resistance during hot press forming, which
comprises:
15 a process of preparing a hot dip aluminum plated steel
sheet wherein a hot dip aluminum plated layer is formed on
the surface of a base steel sheet comprising 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%,
20 Ti: 0.01% to 0.05%, B: 0.001% to 0.005%, N: 0.009% or less,
a balance of Fe and other impurities by wt%;
a process of heating the hot dip aluminum plated steel
sheet at a temperature of 880°C to 930°C followed by being
Page 39
maintained for a certain time to alloy a hot dip aluminum
plated layer of the surface thereof; and
a process of hot forming the alloyed hot dip aluminum
plated steel sheet and being quenched at a temperature
5 range of 300°C or lower at the same time to manufacture an
HPF part.
【Claim 22】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
10 claim 21, wherein the alloyed hot dip aluminum plated layer
consists of a soft diffusion layer and a hard alloyed
layer; in the alloyed layer, a tau phase exists at an area%
within a range of 10% to 30%; and the tau phase is prepared
by comprising Si of 10% or greater and Cr of 0.2% or
15 greater by wt% thereof, so as to allow the alloyed layer to
have a thickness of 35 μm or less.
【Claim 23】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
20 claim 21, wherein the base steel sheet is a cold rolled
steel sheet or a hot rolled steel sheet.
【Claim 24】
The method of manufacturing an HPF part having
Page 40
excellent powdering resistance during hot press forming of
claim 22, wherein the tau phase is prepared by comprising
Si: 10% to 12%, Mn+Cr: 1.3% to 2.0% and a balance of Fe and
Al by wt% thereof.
5 【Claim 25】
The method of manufacturing an HPF part having
excellent powdering resistance during hot press forming of
claim 22, wherein a thickness ratio of the alloyed
layer/diffusion layer in the hot dip aluminum plated layer
10 satisfies 1.5 to 3.0.