【DESCRIPTION】
【Invention Title】
HOT PRESS FORMED PARTS HAVING EXCELLENT BENDING PROPERTIES
AND METHOD FOR MANUFACTURING SAME
5
【Technical Field】
The present invention relates to a hot press forming
(HPF) part used as a structural part of a vehicle or the
like, requiring impact resistance characteristics, and more
10 particularly, to a ultra-strong HPF part having a tensile
strength of 1300 MPa or greater and a method for
manufacturing the same by heating a steel material to a
temperature at which an austenite single phase may be
formed, and quenching and hot forming thereof using a mold.
15 【Background Art】
Recently, in the global automobile industry, the
development of steels having excellent impact resistance
and the application thereof have been in strong demand for
weight reductions of a vehicle bodies in addition to
20 passenger protection. In order to secure the above
properties, research into ultra-strong steels having a
tensile strength of 1300 MPa or greater has been actively
undertaken, but it has been difficult to form automobile
parts having complicated shapes therefrom, due to a lack of
25 formability through having ultra-high strength and also to
secure shape precision due to spring-back.
In order to solve the above problems, the Hot Press
Forming (HPF) method has been suggested (for example, a
technique suggested in US 6296805 and the like). The
30 technique presented in the above patent is a method for
Page 3
manufacturing an HPF part having ultra-high strength by
heating a hot dip aluminum plated steel sheet, having
thermal resistance at a high temperature, to a high
temperature, forming a part by hot press forming, and then
5 quenching the part to secure martensite throughout the
entire part. However, the HPF part manufactured by the
above technique is composed of martensite as a main phase
in the entire thickness of the HPF part in a percentage of
100% to secure ultra-high strength. Sometimes, in the case
10 that a cooling speed of a part in a mold is slow or the
formed part is a thin plate (1 mm or less), ferrite and/or
bainite may be formed at the martensite grain boundaries,
but these microstructures cause a reduction of strength and
bending properties of the HPF part. Accordingly, these
15 microstructures are considered to be unintended
microstructures.
Further, bending properties are considered as a
representative index for evaluating impact resistance
characteristics of the HPF part. For example, like a car
20 B-pillar, fracture toughness, a measurement of steel
properties determined by measuring a degree to which a car
part can endure deformation without fracturing to a certain
degree (angle) after the HPF part is bent by car side
impact (bending properties), is required. However, it is
25 known that the bending properties of HPF parts containing
martensite and/or grain boundary ferrite throughout the
entire thickness of the part may be degraded due to the
presence of martensite, which degrades the ultra-high
strength of the part itself and the bending properties
30 structurally. When forming an HPF part to have a bainite
microstructure in order to improve the bending properties,
there is a problem that it is difficult to secure the
Page 4
ultra-high strength.
Thus, the present inventors have developed a method for
securing impact resistance characteristic of the HPF part
by improving bending properties thereof, as well as
5 securing ultra-high strength therein.
【Disclosure】
【Technical Problem】
An aspect of the present invention is directed to
providing an ultra-strong HPF part having excellent bending
10 properties and a tensile strength of 1300 MPa or greater,
used in the structural parts of vehicles and the like
requiring an impact resistance characteristics.
Further, the present invention is directed to providing
a method for manufacturing the HPF part.
15 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, will be
clearly understood by those skilled in the art from the
20 descriptions below.
【Technical Solution】
The present invention relates to an HPF part wherein a
hot dip aluminum plating layer is formed on the surface of
a base steel sheet, an HPF part having excellent bending
25 properties,
wherein the base steel sheet comprises C : 0.18% to
0.25%, Si : 0.1% to 0.5%, 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%
30 or less, a balance of Fe and other impurities by wt%;
Page 5
a ferrite phase is continuously or discontinuously
formed on the surface of the base steel sheet to a
thickness of 50 μm or thinner, and a percentage of a
ferrite phase in the surface is 5% or less; and
5 carbide particles having a size of 1 μm or smaller are
dispersively distributed in the surface of the base steel
sheet so as to occupy 90% or greater in overall carbide
particle distribution.
In the present invention, an amount of the carbide
10 particles in a size range of 1 μm to 10 μm may be 5 or less
per 10 mm2 in the surface.
Further, the base steel sheet may be any one of a cold
rolled steel sheet and a hot rolled steel sheet. The base
steel sheet may preferably further comprise Mo + W within a
15 range of 0.001% to 0.5%.
Further, the base steel sheet may preferably further
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
20 comprise Cu + Ni: within a range of 0.005% to 2.0%.
Moreover, the base steel sheet may preferably further
comprise at least one of Sb, Sn and Bi in an amount of
0.03% or less.
Further, the present invention relates to a method for
25 manufacturing an HPF part having excellent bending
properties, which comprises:
a process of manufacturing a hot rolled steel sheet
having a composition as described above;
a process of coiling the hot rolled steel sheet at a
30 temperature within a range of 450°C to 750°C for a time
satisfying Relational Expression 1;
a process of cold rolling, annealing and then
Page 6
conducting hot dip aluminum plating on the annealed steel
sheet;
a process of heating the hot dip aluminum plated steel
sheet to a temperature of 850°C to 1000°C and then
5 maintaining the hot dip aluminum plated steel sheet at the
temperature for a certain period of time; and
a process of hot forming the heated steel material and
cooling thereof at a temperature within a range of 200°C or
lower at a cooling speed of 20°C/s to 1000°C/s at the same
10 time to manufacture an HPF part to manufacture an HPF part.
[Relational Expression 1]
190,000 ≤ [coiling temperature (CT) × Time (min)]/2 ≤
350,000
in the Relational Expression 1, Time refers to a time
15 taken to reach 200°C from coiling temperature.
Further, in the present invention, the steel material
may preferably be maintained for a time of 1 second to 1000
seconds after the heating process.
Further, the annealing temperature may be preferably be
20 managed at a temperature within a range of 700°C to 900°C.
Further, a cold reduction ratio during the cold rolling
may preferably be in a range of 30% to 80%.
Further, in the HPF part, a ferrite phase may
preferably be continuously or discontinuously formed on the
25 surface of the base steel sheet to a thickness of 50 μm or
thinner, and a percentage of a ferrite phase in the surface
may preferably be 5% or less; and carbide particles having
a size of 1 μm or smaller may preferably be dispersively
distributed in the surface of the base steel sheet so as to
30 occupy 90% or greater in a whole carbide particle
distribution.
Page 7
Further, an amount of the carbide particles in a size
range of 1 μm to 10 μm may preferably be 5 or less per 10
mm2 in the surface.
Further, the present invention relates to a method for
5 manufacturing a cold rolled steel sheet comprising:
a process of manufacturing a hot rolled steel sheet
having a composition as described above;
a process of coiling the manufactured hot rolled steel
sheet at a temperature within a range of 450°C to 750°C for
10 a time satisfying Relational Expression 1; and
a process of cold rolling the coiled hot rolled steel
sheet.
[Relational Expression 1]
190,000 ≤ [coiling temperature (CT) × Time (min)]/2 ≤
15 350,000
where Time refers to a time taken to reach 200°C from
coiling temperature
Further, a cold reduction ratio during the cold rolling
may preferably be in a range of 30% to 80%.
20 Further, the present invention relates to a method for
manufacturing an HPF part having excellent bending
properties comprising:
a process of annealing the manufactured cold rolled
steel sheet, and then conducting hot dip aluminum plating;
25 a process of heating the hot dip aluminum plated steel
sheet to a temperature of 850°C to 1000°C and then
maintaining the hot dip aluminum plated steel sheet at the
temperature for a certain period of time; and
a process of hot forming the heated steel material and
30 cooling thereof at a temperature within a range of 200°C or
lower at a cooling speed of 20°C/s to 1000°C/s at the same
time to manufacture an HPF part.
Page 8
Further, in the present invention, the steel material
may preferably be maintained for a time of 1 second to 1000
seconds after the heating process.
Further, the annealing temperature may preferably be
5 managed at a temperature within a range of 700°C to 900°C.
Further, in the HPF part, a ferrite phase may
preferably be continuously or discontinuously formed on the
surface of the base steel sheet to a thickness of 50 μm or
thinner, and a percentage of a ferrite phase in the surface
10 may preferably be 5% or less; and carbide particles having
a size of 1 μm or smaller may preferably be dispersively
distributed in the surface of the base steel sheet so as to
occupy 90% or greater in a whole carbide particle
distribution.
15 Further, an amount of the carbide particles in a size
range of 1 μm to 10 μm may preferably be 5 or less per 10
mm2 in the surface.
【Best Mode】
The present invention has an effect of providing an
20 ultra-strong HPF part having excellent bending properties
and tensile strength of 1300 MPa or greater, thereby
effectively used to structural parts of vehicles and the
like requiring an impact resistance characteristic.
【Description of Drawings】
25 The above and other aspects, features and other
advantages of the present disclosure will be more clearly
understood from the following detailed description taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is a graph showing correlation between coiling
30 temperature × Time and bending angle according to an
Page 9
embodiment of the present invention;
FIG. 2 is a structure image showing a microstructure of
the surface of a base steel sheet directly under a plating
layer after preheating treatment according to an embodiment
5 of the present invention in comparison with Comparative
Example;
FIG. 3 is an image showing Mn distribution (EPMA) of
the surface of a base steel sheet directly under a plating
layer after preheating treatment according to an embodiment
10 of the present invention in comparison with Comparative
Example; and
FIG. 4 is a structure image of the hot rolled surface
layer of a steel sheet of Comparative Example.
【Best Mode】
15 Hereinafter, the present invention will be described.
The present inventors repeated studies and experiments
about a method for providing bending properties to an HPF
part in manufacturing the HPF part having ultra-high
strength of tensile strength of 1300 MPa or greater. As a
20 result, the inventors found that ultra-high strength can be
secured and also bending properties can be remarkably
improved by controlling a microstructure of the surface of
the HPF part and also controlling size and a percentage of
carbide particles in the surface at the same time.
25 In detail, the present inventors found that there are
problems in that a microstructure of the surface of a
general HPF part is composed of martensite without ferrite,
and bending properties of the formed part may be remarkably
reduced because the carbide particles remaining inside the
30 martensite are not precisely controlled. In particular, in
the case of a hot dip aluminum plated steel sheet, cracks
Page 10
are easily formed in the plating layer by an intermetallic
compound phase having undesired softness, which is formed
during heat treatment for hot forming. Accordingly, crack
propagation from this to a martensite base is easy and the
5 coarse carbide particles remained in the base functions as
a starting point of crack growth. Thus, it was found that
bending properties is remarkably reduced.
Considering this, the present inventors found that
forming ferrite continuously or discontinuously on the
10 surface of the base steel sheet directly under the plating
layer of the HPF part is very effective to inhibit the
propagation of the fine cracks formed on the plating layer
to the base. Moreover, the present inventors suggest the
present invention after confirmed that size and a
15 percentage of the carbide particles in the surface is very
important to inhibit growth speed of the small amount of
the fine cracks already propagated to the base.
First of all, the HPF part of the present invention
having excellent bending properties will be described.
20 In the present invention, in an HPF part wherein a hot
dip aluminum plating layer is formed on the surface of a
base steel sheet, the base steel sheet comprises C : 0.18%
to 0.25%, Si : 0.1% to 0.5%, 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%
25 to 0.5%, 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%.
Hereinafter, each ingredient of the base steel sheet
and reasons for limiting thereof will be described in
detail.
30
C: 0.18% to 0.25%
C is an essential element increasing the strength of
Page 11
martensite. If a C content is less than 0.18%, it may be
difficult to obtain sufficient strength to secure impact
resistance. Further, if a C content is greater than 0.25%,
impact toughness of a slab may be deteriorated, and
5 weldability of the HPF part may be deteriorated. In this
regard, in the present invention, it is preferable to limit
the C content to 0.18 wt% to 0.25 wt% (hereinafter, just
called %).
10 Si: 0.1% to 0.5%
Si is added as a deoxidizing agent in steel making. If
Si content is less than 0.1%, enough deoxidizing may be
difficult, and if the Si content is greater than 0.5%, it
may be difficult to secure good quality of the hot dip
15 aluminum plated surface by Si oxides formed on the surface
of the steel sheet. Therefore, in the present invention,
it is preferable to limit the Si content to a range of 0.1%
to 0.5%.
20 Mn: 0.9% to 1.5%
Mn is added to secure hardenability of steel such as Cr,
B and the like. If 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
25 sufficient strength. Further, if the Mn content is greater
than 1.5%, a cost of manufacturing a steel sheet may
increase, and also bending properties of the HPF part may
be remarkably deteriorated as the Mn is segregated inside
the steel material. In this regard, in the present
30 invention, it is preferably to limit the Mn content within
a range of 0.9% to 1.5%.
Page 12
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 P in as small an amount as possible.
5 If a P content is greater than 0.03%, bending properties,
impact resistance properties, 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%.
10 S: 0.01% or less (not including 0%)
S is an element existing in a steel as an impurity and
hindering a bending properties and weldability of the
formed part. Thus, it is preferable to contain the S in as
small an amount as possible. If S content is greater than
15 0.01%, the bending properties and weldability of the formed
part may be deteriorated. Thus, it is preferable to limit
the upper limit of the content to 0.01%.
Al: 0.01% to 0.05%
20 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 qualities of the plated material may be
25 deteriorated. 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 steel such as Mn,
30 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%, sufficient hardenability can
Page 13
be secured. However, the effect may be saturated and also
a cost of manufacturing the steel may increase. In this
regard, in the present invention, it is preferable to limit
the Cr content to a range of 0.05% to 0.5%.
5
Ti: 0.01% to 0.05%
Ti is added to form TiN by binding to nitrogen remained
in steel as an impurity, thereby leaving solid B essential
to secure hardenability. If Ti content is less than 0.01%,
10 it may be difficult to expect a sufficient effect, and if
the content is greater than 0.05%, the characteristic may
be saturated and also a cost of manufacturing steel may
increase. In this regard, in the present invention, it is
preferable to limit the Ti content to a range of 0.01% to
15 0.05%.
B: 0.001% to 0.005%
B is added to secure hardenability of the HPF part like
Mn and Cr. To achieve this purpose, B should be added in
20 an amount of 0.001% or greater, and if the content is
greater than 0.005%, the effect may be saturated, and also
hot rolling properties may be remarkably reduced. Thus, in
the present invention, it is preferable to limit the B
content to a range of 0.001% to 0.005%.
25
N: 0.009% or less
N exists in steel as an impurity, and it is preferably
to add the N in as small an amount as possible. If an N
content is greater than 0.009%, it may cause bad surface
30 qualities of steel. Thus, it is preferable to limit the
upper limit of the content to 0.009%. Then, more preferably,
the base steel sheet making the HPF part of the present
Page 14
invention may further contain the following ingredients.
Mo + W: 0.001% to 0.5%
Mo and W are elements reinforcing hardenability and
5 precipitation, and are very effective in further securing
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
precipitation, and if the content is greater than 0.5%, the
10 effect may be saturated and manufacturing costs may also
increase. Thus, in the present invention, it is preferable
to limit the Mo + W content to a range of 0.001% to 0.5%.
Sum of at least one of Nb, Zr or V: 0.001% to 0.4%
15 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
Nb, Zr and V is less than 0.001%, it may be difficult to
expect the above effect, and if the content is greater than
20 0.4%, manufacturing costs may increase excessively. Thus,
in the present invention, it is preferable to limit the
contents of the elements to a range of 0.001% to 0.4%.
Cu + Ni: 0.005% to 2.0%
25 The Cu is an element improving strength by forming fine
Cu precipitates, and the Ni is an element effective to
increase strength and improve heat treatment
characteristics. If the sum of the above ingredients is
less than 0.005%, it may difficult to obtain enough desired
30 strength, and if the content is greater than 2.0%,
workability may become worse, and manufacturing costs may
increase. In this regard, in the present invention, it is
Page 15
preferable to control the Cu + Ni content to a range of
0.005% to 2.0%.
At least one of Sb, Sn or Bi: 0.03% or less
5 The Sb, Sn and Bi are grain segregation elements and
during the HPF process, the elements are concentrated on
the interface between the plating layer and the base iron
and can improve adhesion of the plating layer. The
elements can play a role in preventing the detachment of
10 the plating layer during hot forming by improving adhesion
of the plating layer. Because the Sb, Sn and Bi have
similar characteristics, it is possible to use the three
elements as a mixture, and in such a case, the amount of at
least one may preferably be 0.03% or less. If the sum of
15 the above ingredients is greater than 0.03%, there may be a
problem that brittleness of the base iron may be
deteriorated during the hot forming.
Hereinafter, a microstructure of the formed part of the
20 present invention hot formed using the hot dip aluminum
plating steel material will be described.
First of all, the present invention, is characterized
in that a ferrite phase is continuously or discontinuously
formed to a thickness of 50 μm or less under a plating
25 layer of the HPF part having the hot dip aluminum plating
layer, on the surface directly below the base steel sheet
interface. Herein, the surface means a region in depth of
50 μm from the base steel sheet interface to the inside
thereof.
30 In the present invention, the ferrite is a very
important phase in the surface of the base steel sheet, and
the ferrite should be continuously or discontinuously
Page 16
formed to a thickness of 50 μm or less. If the ferrite is
not continuously or is discontinuously formed, cracks
formed on the alloyed aluminum plating layer may penetrate
into the base steel sheet composed of martensite thereby
5 deteriorating bending properties of the formed part. Thus,
the surface ferrite should be continuously or
discontinuously formed within the observed thickness of 50
μm or less.
And in the present invention, it is required that area
10 percentage of the ferrite phase on the surface is 5% or
less of the entire structure of the surface. If the area
percentage is greater than 5%, an excessive increase of a
hot rolled coiling temperature may occur, leading to the
formation of surface ferrite. Consequently, if the surface
15 layer is decarbonized, Si and/or Mn oxide may be formed in
the ferrite grain boundary of the surface layer thereby
reducing bending properties of the manufactured HPF part.
Moreover, if the surface layer is excessively decarbonized,
strength of the manufactured HPF part may be reduced. Thus,
20 in the present invention, it is preferable to limit the
area percentage of the ferrite on the surface to 5% or less.
On the other hand, the present invention is
characterized by controlling size and distribution of
carbide particles existing in the surface in order to
25 manufacture an HPF part having excellent bending properties.
Specifically, the present invention is characterized by
controlling that carbide particles having a size of 1 μm or
smaller are dispersively distributed in the surface of the
base steel sheet so as to occupy 90% or greater in overall
30 carbide particle distribution. Although cracks propagated
from the plating layer to the base steel sheet by the
ferrite formed on the surface as mentioned above becomes
Page 17
slower, there is a limit to which cracks can be easily
propagated to the inside of the base steel sheet along the
boundary between martensite and the coarse carbide particle.
Considering this, the present invention allows the carbide
5 particles having a size of 1 μm or smaller to be
dispersively distributed so as to occupy 90% or greater in
overall carbide particle distribution for inhibiting crack
propagation to the inside of the base steel sheet as
mentioned above. The carbide particles, finely
10 dispersively distributed as described above, are almost
unaffected by crack propagation as mentioned above. Thus,
the carbide particles can effectively inhibit crack
propagation to the inside of the base steel sheet thereby
improving bending properties of the formed part.
15 More preferably, an amount of the carbide particles in
a size range of 1 μm to 10 μm is controlled to be 5 or less
per 10 mm2 in the surface. This control of the number of
coarse carbide particles can block a pathway of crack
propagation thereby reducing the bending properties of the
20 HPF part.
Hereinafter, a method for manufacturing an HPF part
having excellent bending properties of the present
invention will be described. First of all, in the present
25 invention, a hot rolled steel sheet having the alloy
composition as described above is manufactured. This
process of manufacturing a hot rolled steel sheet may be a
common process, but is not limited to particular
manufacturing conditions. For example, the hot rolled
30 steel sheet can be manufactured by reheating the steel slab
composed of the alloy composed as described above at 1000°C
to 1300°C, and then subjecting the steel slab to finishing
Page 18
hot rolling at a temperature within a range of an Ar3
transformation point to 1000°C.
Then, in the present invention, the hot rolled steel
sheet manufactured as described above is coiled at a
5 temperature within a range of 450°C to 750°C for a time
satisfying Relational Expression 1.
At this time, in the present invention, the coiling
temperature is a technical configuration playing an
important role in obtaining a microstructure and carbide
10 particle distribution of the surface of the manufactured
HPF part. If the coiling temperature is lower than 450°C,
a sufficient amount of ferrite cannot be formed on the base
steel sheet directly below the plating layer and the base
steel sheet interface after HPF heating. The reason for
15 this is that a small amount of elements such as C, Mn, Cr
and the like existing on the steel sheet surface are
released from the surface during coiling. In detail, the
reason is presumed to be that in the HPF heating process,
the ferrite is mostly formed when transferring the steel
20 sheet to a mold after austenite heating, and when the
ferrite forming elements such as C, Mn, Cr and the like are
present in a sufficient amount in the steel sheet surface,
the ferrite is not formed on the surface, but when the
elements are not enough, the ferrite is formed on the
25 surface layer. Thus, if a content of C, Mn and Cr of the
surface of the steel sheet is insufficient in the coiling
process due to decarbonizing and the like, in the
subsequent HPF heating process, the ferrite may be formed
on the surface. Moreover, the ferrite also can be obtained
30 by increasing the cooling speed of the surface of the steel
sheet in the HPF heating process faster than that of the
middle part.
Page 19
On the other hand, if the coiling temperature is higher
than 750°C, the elements are sufficiently released out the
surface layer thereby forming enough ferrite after the HPF
heating, but at the same time, oxides can be formed on the
5 grain boundary by binding of Si and/or Mn existing in the
steel to oxygen in the air and also many coarse carbide
particles are formed directly under the surface layer.
Therefore, the carbide particles existing even after the
HPF heating work as crack initiation points and a pathway
10 of crack propagation during a bending test, thereby
deteriorating bending properties. Thus, in order to obtain
a certain desired structure and carbide particle
distribution after HPF heating, the coiling temperature may
preferably be 450°C to 750°C.
15 In the present invention, it is preferable to manage
the coiling time to satisfy Relational Expression 1, and
the time satisfies Relational Expression 1, the effect of
limiting the coiling temperature described above may be
maximized. This coiling time may be easily controlled by
20 inserting the coiled hot rolled sheet into a slow cooling
furnace or a heating furnace. As shown in FIG. 1, it can
be found that excellent bending angle can be shown within a
range of satisfying Relational Expression 1.
[Relational Expression 1]
25 190,000 ≤ [coiling temperature (CT) × Time (min)]/2 ≤
350,000
in the Relational Expression 1, Time refers to a time
taken to reach 200°C from coiling temperature.
30 Then, in the present invention, the hot rolled steel
sheet is pickled, immediately hot dip aluminum plated
Page 20
without cold rolling, and then used as a steel material for
hot forming.
Further, the hot rolled steel material may be cold
rolled, subjected to hot dip aluminum plating, and then
5 used as a steel material for hot forming. At this time, in
the present invention, during cold rolling, a cold
reduction ratio may preferably be 30% to 80% but is not
limited thereto. If the cold reduction ratio is less than
30%, there may be a problem that the hot rolled steel
10 material should be thinner to secure a certain cold rolling
thickness and also there may be a problem in cold rolling
treading. On the contrary, if the ratio is greater than
80%, there may be problems that cracks may be easily formed
at edges of the steel material, and a cold rolling load may
15 be increased.
Then, the hot rolled steel sheet or the cold rolled
steel sheet manufactured as described above is subjected to
a certain annealing process, and then immersed in an
aluminum plating bath to manufacture a hot dip aluminum
20 plated steel sheet. At this time, the present invention is
not limited to the annealing condition mentioned above, but
it is preferable to manage the annealing temperature within
a range of 700°C to 900°C. Further, the present invention
is not limited to the above hot dip aluminum plating
25 conditions, and the aluminum plating bath may contain Al as
a major ingredient and Si in a range of 7% to 12%.
In the present invention, the hot dip aluminum plated
steel sheet is heated to an austenite single phase
30 temperature range of 850°C to 1000°C and then maintained
for 1 second to 1000 seconds. If the single phase heating
temperature is lower than 850°C, ferrite is formed on the
Page 21
steel sheet while transferring the sheet to a mold after
heating thereof in a heating furnace thereby cannot secure
certain strength of the final formed part manufactured
after the heating. However, if the temperature is higher
5 than 1000°C, manufacturing costs may rise and also
weldability may become worse. And it is more preferable to
maintain the heating rate of 1°C/s to 100°C/s during the
heating.
Further, after the heating, it is preferable to
10 maintain the temperature for 1 second to 1000 seconds. The
reason is that if the maintaining time is shorter than 1
second, it may be difficult to make enough austenite, and
if the time is longer than 1000 seconds, weldability of the
formed parte manufactured after the heating may be
15 deteriorated.
Then, in the present invention, a ultra-strong HPF part
having excellent bending properties and tensile strength of
1300 MPa or greater can be manufactured by hot forming the
heated steel sheet with a mold and cooling thereof to 200°C
20 or lower at cooling speed of 20°C/s to 1000°C/s at the same
time. If the cooling speed is slower than 20°C/s, it may
be difficult to secure a certain strength due to bainite
formation, and if the speed is faster than 1000°C/s, the
strengthening effect may be saturated and also an excessive
25 manufacturing cost may be required.
In the HPF part manufactured by the manufacturing
process as mentioned above, a ferrite phase is continuously
or discontinuously formed on the surface of the base steel
30 sheet to a thickness of 50 μm or thinner, and a percentage
of a ferrite phase in the surface is 5% or less. Further,
carbide particles having a size of 1 μm or smaller are
Page 22
dispersively distributed in the surface of the base steel
sheet so as to occupy 90% or greater in overall carbide
particle distribution. Thus, the HPF part having excellent
bending properties can be provided.
5 【Mode for Invention】
Hereinafter, the present disclosure will be described
in greater detail with reference to examples. However, the
following examples are for illustrative purposes only, and
should not be seen as limiting the scope of the present
10 disclosure. The scope of the present disclosure should be
determined by the claims and information reasonably
inferable therefrom.
(Example)
15 Table 1
Chemical ingredient (wt%)
Note
C Si Mn P S Al Ti Cr B N
0.22 0.3 1.2 0.014 0.002 0.03 0.03 0.2 0.0023 0.004
Inventive
Steel
A steel slab composed as shown in the above Table 1 was
subjected to vacuum melting, heated at a reheating
temperature of 1200°C for 1 hour and then hot rolled. At
20 this time, a hot rolled steel sheet was manufactured in the
conditions of the finishing hot rolling temperature of
900°C, and coiling temperature(CT) and time as shown in the
following Table 2. The manufactured hot rolled steel sheet
was pickled and then cold rolled at cold reduction ratio of
25 50% to manufacture a cold rolled steel sheet having final
thickness of 1.5 mm.
The cold rolled steel sheet was annealed at 780°C, and
then hot dip aluminum plated. At this time, a hot dip
Page 23
aluminum plating bath was composed of aluminum as a major
ingredient, 8.5% Si, 2% Fe and other impurities. Using the
steel sheet hot dip aluminum plated as described above,
heating for mimicking hot forming was conducted. Namely,
5 the plated steel sheet was inserted into a heating furnace,
heated to 930°C, transferred to a mold after 6 min, and
then quenched in the mold.
For the steel sheets manufactured as described above,
whether a ferrite phase exists in the surface of a base
10 steel sheet or not and percentage thereof, whether Si and
Mn oxides exist or not, carbide particle percentage on the
surface, and the like were measured. The results were
shown in the following Table 2. Further, mechanical
properties of the formed parts manufactured as described
15 above were measured, and the results are shown in the
following Table 3.
On the other hand, in this experiment, whether the
ferrite is formed on the surface of the base steel sheet
directly under the hot dip aluminum plating layer or not
20 and percentage thereof were observed at least three regions
using an optical microscope and then decided by image
analysis. Further, whether the Si and Mn oxides, which can
exist on the ferrite grain boundary, exist or not was
analyzed using SEM, and in order to analyze the carbide
25 particle distribution on the surface of the base steel
sheet, replica was extracted directly under the surface,
and size and number thereof were measured using TEM and
EPMA.
Moreover, for the steel sheets manufactured as
30 described above, mechanical properties were measured using
a JIS Z 2201 No.5 tension test specimen. As bending
properties, bending angle at maximum load was measured
Page 24
according to VDA 238-xxx test method, and when bending test
was conducted in a condition that a flexure line is
perpendicular to a rolling direction, bending angle of less
than 60° was decided as a failure, and bending angle of 60°
5 or greater was determined to be a pass.
Page 25
Table 2
Manufactu
ring
Condition
s
Hot
rolling
condition
s
Surface structure after heating Remarks
CT
(°
C)
S* Wheth
er
ferri
te
exist
s
withi
n 50
μm of
surfa
ce
Ferrite
percent
age
(%)
Wheth
er
Si/Mn
oxide
exist
s or
not
1 μm
carbide
particl
e
percent
age
(%)
1μm
to 10
μm
carbi
de
parti
cle
numbe
r
A 48
0
220,8
00
○ 0.5 X 99 1 Example1
B 60
0
230,0
00
○ 2.1 X 98 1 Example2
C 60
0
246,0
00
○ 3.2 X 97 2 Example3
D 70
0
302,0
00
○ 4.5 X 95 3 Example4
E
40
0
184,0
00
X 0 X 100 0
Comparat
ive
Example1
F
78
0
360,0
00
○ 7.2 ○ 83 10
Comparat
ive
Example2
*In Table 2, S* refers to [coiling temperature(CT) ×
Time(min)]/2, i.e., a time taken to reach 200°C from
5 coiling temperature
Page 26
Table 3
Condition
Mechanical
property
Bending
properties
Note
YS
(MPa)
TS
(MPa)
El
(%)
Angle Pass/Fail
A 1121 1536 6.5 63.5 Pass Example 1
B 1107 1518 6.6 64.6 Pass Example 2
C 1102 1508 6.4 65.2 Pass Example 3
D 1096 1487 6.9 67.8 Pass Example 4
E 1133 1538 6.2 58.1 Fail
Comparative
Example 1
F 1047 1465 6.7 55.8 Fail
Comparative
Example 2
As shown in the above Tables 1 to 3, in Examples 1 to 4
whose manufacturing process as well as steel composition is
5 within a range of the present invention, it can be found
that the ferrite phase exists on the surface of the base
steel sheet, and percentage thereof is 5% or less.
Further, the Si and/or Mn oxides did not exist on the
surface mentioned above, and coarse carbide particles of 1
10 μm to 10 μm wherein percentage of the carbide particle
having a size of 1 μm or less is 90% or greater did not
exist thereby securing a certain bending properties. This
can be confirmed through images of FIGS. 2 and 3, and in
the above Figures, Example refers to Example 1 in Tables 2
15 and 3.
On the contrary, in the case of Comparative Example 1,
it can be found that enough ferrite was not formed on the
surface of the base steel sheet due to too small (coiling
Page 27
temperature × Time)/2, thereby a certain bending angle
cannot be secured. On the other hand, in FIGS 2 and 3,
Comparative Example refers to Comparative Example 1 in
Tables 2 and 3.
5 Further, in Comparative Example 2, it can be found that
enough ferrite was formed on the surface of the base steel
sheet, but the Si and/or Mn oxides existed in the form of a
ban on the ferrite grain boundary and a lot of coarse
carbide particles were formed, thereby the bending angle
10 was further worse. FIG. 4 is a structural image in
Comparative Example 2.
While the invention has been shown and described with
reference to exemplary embodiments thereof, it will be
15 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 scope of the invention
is defined not by the detailed description of the invention
20 but by the appended claims, and all differences within the
scope will be construed as being included in the present
invention.

【WE CLAIM:】
【Claim 1】
In an HPF part wherein a hot dip aluminum plating layer
5 is formed on the surface of a base steel sheet, an HPF part
having excellent bending properties,
wherein the base steel sheet comprises C : 0.18% to
0.25%, Si : 0.1% to 0.5%, 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
10 0.5%, 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 ferrite phase is continuously or discontinuously
formed on the surface of the base steel sheet to a
thickness of 50 μm or thinner, and a percentage of a
15 ferrite phase in the surface is 5% or less; and
carbide particles having a size of 1 μm or smaller are
dispersively distributed in the surface of the base steel
sheet so as to occupy 90% or greater in overall carbide
particle distribution.
20 【Claim 2】
The HPF part having excellent bending properties of
claim 1, wherein an amount of the carbide particles in a
size range of 1 μm to 10 μm is 5 or less per 10 mm2 in the
surface.
25 【Claim 3】
The HPF part having excellent bending properties of
claim 1, wherein the base steel sheet is any one of a cold
rolled steel sheet and a hot rolled steel sheet.
【Claim 4】
Page 29
The HPF part having excellent bending properties of
claim 1, wherein the base steel sheet further comprises Mo
+ W : 0.001% to 0.5%.
【Claim 5】
5 The HPF part having excellent bending properties 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 6】
10 The HPF part having excellent bending properties of
claim 1, wherein the base steel sheet further comprises Cu
+ Ni: within a range of 0.005% to 2.0%.
【Claim 7】
The HPF part having excellent bending properties of
15 claim 1, wherein the base steel sheet further comprises at
least one of Sb, Sn and Bi : 0.03% or less.
【Claim 8】
A method for manufacturing an HPF part having
excellent bending properties comprising:
20 a process of manufacturing a hot rolled steel sheet
prepared by comprising C : 0.18% to 0.25%, Si : 0.1% to
0.5%, 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, a
25 balance of Fe and other impurities by wt%;
a process of coiling the hot rolled steel sheet at a
temperature within a range of 450°C to 750°C for a time
satisfying Relational Expression 1;
a process of cold rolling, annealing and then
30 conducting hot dip aluminum plating on the annealed steel
Page 30
sheet;
a process of heating the hot dip aluminum plated steel
sheet to a temperature of 850°C to 1000°C and then
maintaining the hot dip aluminum plated steel sheet at the
5 temperature for a certain period of time; and
a process of hot forming the heated steel material and
cooling thereof at a temperature within a range of 200°C or
lower at a cooling speed of 20°C/s to 1000°C/s at the same
time to manufacture an HPF part to manufacture an HPF part.
10 [Relational Expression 1]
190,000 ≤ [coiling temperature (CT) × Time (min)]/2 ≤
350,000
in the Relational Expression 1, Time refers to a time
taken to reach 200°C from coiling temperature.
15 【Claim 9】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein the steel
material is maintained for a time of 1 second to 1000
seconds after the heating process.
20 【Claim 10】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein the
annealing temperature is managed at a temperature within a
range of 700°C to 900°C.
25 【Claim 11】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein a cold
reduction ratio during the cold rolling is in a range of
30% to 80%.
30 【Claim 12】
Page 31
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein in the HPF
part, a ferrite phase is continuously or discontinuously
formed on the surface of the base steel sheet to a
5 thickness of 50 μm or thinner, and a percentage of a
ferrite phase in the surface is 5% or less; and carbide
particles having a size of 1 μm or smaller are dispersively
distributed in the surface of the base steel sheet so as to
occupy 90% or greater in a whole carbide particle
10 distribution.
【Claim 13】
The method for manufacturing an HPF part having
excellent bending properties of claim 12, wherein an amount
of the carbide particles in a size range of 1 μm to 10 μm
is 5 or less per 10 mm2 15 in the surface.
【Claim 14】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein the hot
rolled steel sheet further comprises Mo + W : 0.001% to
20 0.5%.
【Claim 15】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein the hot
rolled steel sheet further comprises at least one of Nb, Zr
25 and V : within a range of 0.001% to 0.4% (as the sum).
【Claim 16】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein the hot
rolled steel sheet further comprises Cu + Ni: within a
30 range of 0.005% to 2.0%.
Page 32
【Claim 17】
The method for manufacturing an HPF part having
excellent bending properties of claim 8, wherein the hot
rolled steel sheet further comprises at least one of Sb, Sn
5 and Bi : 0.03% or less.
【Claim 18】
A method for manufacturing a cold rolled steel sheet
comprising:
a process of manufacturing a hot rolled steel sheet
10 comprising C : 0.18% to 0.25%, Si : 0.1% to 0.5%, 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, a balance of Fe and
other impurities by wt%;
15 a process of coiling the manufactured hot rolled steel
sheet at a temperature within a range of 450°C to 750°C for
a time satisfying Relational Expression 1; and
a process of cold rolling the coiled hot rolled steel
sheet.
20 [Relational Expression 1]
190,000 ≤ [coiling temperature (CT) × Time (min)]/2 ≤
350,000
in the Relational Expression 1, Time refers to a time
taken to reach 200°C from coiling temperature.
25 【Claim 19】
The method for manufacturing a cold rolled steel
sheet of claim 18, wherein a cold reduction ratio during
the cold rolling is in a range of 30% to 80%.
【Claim 20】
30 A method for manufacturing an HPF part having
Page 33
excellent bending properties comprising:
a process of annealing the cold rolled steel sheet
manufactured in claim 18, and then conducting hot dip
aluminum plating;
5 a process of heating the hot dip aluminum plated steel
sheet to a temperature of 850°C to 1000°C and then
maintaining the hot dip aluminum plated steel sheet at the
temperature for a certain period of time; and
a process of hot forming the heated steel material and
10 cooling thereof at a temperature within a range of 200°C or
lower at a cooling speed of 20°C/s to 1000°C/s at the same
time to manufacture an HPF part.
【Claim 21】
The method for manufacturing an HPF part having
15 excellent bending properties of claim 20, wherein the steel
material is maintained for a time of 1 second to 1000
seconds after the heating process.
【Claim 22】
The method for manufacturing an HPF part having
20 excellent bending properties of claim 20, wherein the
annealing temperature is managed at a temperature within a
range of 700°C to 900°C.
【Claim 23】
The method for manufacturing an HPF part having
25 excellent bending properties of claim 20, wherein in the
HPF part, a ferrite phase is continuously or
discontinuously formed on the surface of the base steel
sheet to a thickness of 50 μm or thinner, and a percentage
of a ferrite phase in the surface is 5% or less; and
30 carbide particles having a size of 1 μm or smaller are
Page 34
dispersively distributed in the surface of the base steel
sheet so as to occupy 90% or greater in a whole carbide
particle distribution.
【Claim 24】
5 The method for manufacturing an HPF part having
excellent bending properties of claim 23, wherein an amount
of the carbide particles in a size range of 1 μm to 10 μm
is 5 or less per 10 mm2 in the surface.