【Technical Field】
[0001] This application relates to a method of manufacturing
hot press-formed member having excellent productivity,
5 weldability and formability
【Background Art】
[0002] With demand for reduced weight and improved safety in
vehicles, high-strength steel utilizing a hot press forming
10 method has been actively applied. In a hot press forming
process, heating and quenching processes of a material are
necessarily required. An aluminum plated steel material or
aluminum alloy plated steel material is used to prevent scale
from occurring at high temperatures. An aluminum plated steel
15 material or an aluminum alloy plated steel material has an issue
on melting of a plating layer during rapid heating, and is
generally heated at a low rate in an atmospheric heating
furnace.
[0003] According to a heating method in such an atmospheric
20 heating furnace, heating is performed in a furnace set to the
same atmospheric temperature or performed in a continuous
heating furnace such as a roller hearth furnace having a
plurality of heating zones in such a manner that an atmospheric
temperature is sequentially increased. However, in such
25 methods, heating is performed at a slow rate, and thus, heating
should be performed in a heating furnace for a certain period
3
of time to secure time required to reach a target temperature.
In addition, productivity may be deteriorated due to an increase
in maintaining time in the heating furnace.
[0004] Accordingly, a heating temperature may be increased to
5 reduce in-furnace maintenance time. In such a case, the heating
temperature is increased, and thus, a thickness of a diffusion
layer in an alloy layer is increased to cause poor weldability.
[0005] Therefore, a method of increasing a heating temperature
to reduce in-furnace maintenance time has been required to
10 improve productivity. In addition, a method of significantly
reducing a thickness of a diffusion layer such that a heating
temperature of blank is not maintained at a high level has been
required to secure weldability of manufactured formed products.
[0006] However, in the prior art it has not been possible to
15 simultaneously apply a reduction in in-furnace maintenance time
and a decrease in heating temperature at the same time due to
opposite effects thereof.
[0007] In addition to the above-described issue, when a
temperature of a heating furnace is continuously increased to
20 improve weldability, formability may be deteriorated. A
technology, capable of securing improved productivity,
weldability, and formability, has not been developed to date.
[0008] (Prior Art Document)
[0009] (Patent Document)
25 [0010] Korean Patent Publication No. 10-2006-0054479
4
【Disclosure】
【Technical Problem】
[0011] An aspect of the present disclosure is to provide a
method of manufacturing a hot press-formed member having
5 improved productivity, weldability, and formability.
[0012] A subject of the present disclosure is not limited to
the above description. Those of ordinary skill in the art to
which the present disclosure pertains will not have any
difficulty in understanding additional subjects of the present
10 disclosure from the general details of the present
specification.
【Technical Solution】
[0013] According to an aspect of the present disclosure, a
method of manufacturing a hot press-formed member comprises
15 heating a blank of an aluminum-based plated steel sheet in a
heating furnace, removing the heated blank from the heating
furnace and conveying the removed blank between an upper mold
portion and a lower mold portion of a mold, mounted on a press,
to be seated; and performing a forming process after the upper
20 mold portion of the mold is in contact with the seated blank.
The heating furnace is a continuous heating furnace comprising
section A, section B, and section C provided in a conveying
direction of a blank. Heating in section A satisfies conditions
specified by a figure ‘abcde’ having cumulative in-furnace
25 maintaining times and in-furnace atmospheric temperature
coordinates of approximately a(0.2 min, 750ºC), b(1.0 min,
5
750ºC), c(1.0 min, 800ºC), d(1.5 min, 900ºC), and e(0.2 min,
900ºC). Heating in section B satisfies conditions specified
by a figure ‘fghi’ having cumulative in-furnace maintaining
times and atmospheric temperature coordinates of approximately
5 f( min, 930ºC), g( min, 930ºC),
h( min, 960ºC), and i( min, 960ºC).
Heating in section C satisfies conditions specified by a figure
‘jklm’ having cumulative in-furnace maintaining times and
atmospheric temperature coordinates of approximately
10 j( min, 870ºC), k( min, 870ºC),
l( min, 940ºC), and m( min, 940ºC). A
highest atmospheric temperature of section C is lower than a
highest atmospheric temperature of section B. Relational
Expression 1 is satisfied. A time required before the forming
15 process is performed after the blank is seated is two seconds
or less.
Relation Expression 1:
T ≤ 8.2 x t + (temp-900)/30
where T denotes a sum of time required for conveying and
20 seating a blank and time required before forming is performed
after the blank is seated and a unit thereof second (s), t denotes
6
a thickness of a blank and a unit thereof is millimeters (mm),
and temp denotes a furnace extraction temperature and a unit
thereof is degrees Celsius (ºC).
【Advantageous Effects】
5 [0014] According to an aspect of the present disclosure, a
method of manufacturing a hot press-formed member having
improved productivity, weldability, and formability could be
provided.
[0015] Various advantages and effects of the present
10 disclosure are not limited to the above-described contents, and
can be more easily understood in the process of describing
specific embodiments of the present disclosure.
【BRIEF DESCRIPTION OF THE DRAWINGS】
15 [0016] FIG. 1 is a graph illustrating a heating pattern of an
aluminum plated material having a thickness of 1.2 mm.
[0017] FIG. 2 is a graph illustrating a comparison between
heating analysis experimental values and analysis values under
various furnace atmospheric temperature conditions for an
20 aluminum plated material having a thickness of 1.2 mm.
[0018] FIG. 3 is a graph illustrating preferable atmospheric
temperature conditions according to cumulative in-furnace
maintaining time of the present disclosure for heating of an
aluminum plated material having a thickness of 1.2 mm.
25 [0019] FIG. 4 is an image illustrating an observation result
of alloy layers of experimental examples in which an aluminum
7
plated material having a thickness of 1.2 mm is heated under
several heating conditions.
[0020] FIG. 5A to Fig.5C illustrate conditions for heating of
an aluminum plated material in consideration of a change in
5 thickness of a material.
[0021] FIG. 6 is a graph illustrating a comparison between
experimental values and analysis values for temperature changes
depending on times for which an aluminum plating material having
a thickness of 0.9 mm and an aluminum plating material having
10 a thickness of 1.8 mm are cooled in air after they are extracted
from a heating furnace.
【Best Mode】
[0022] Hereinafter, embodiments of the present disclosure will
be described in more detail. However, the embodiments of the
15 present disclosure can be implemented in various forms and the
scope of the present disclosure is not limited to the
embodiments described herein. In addition, the embodiments of
the present disclosure are provided in order to provide more
complete explanation of the present disclosure for a person
20 having ordinary knowledge in the field to which the present
invention pertains.
[0023] Hereinafter, a method of manufacturing a hot
press-formed member of the present disclosure will be described
in detail. Unless otherwise defined in this specification, all
25 terms and methods commonly used in the art may be applied to
8
the present disclosure.
[0024] A method of manufacturing a hot press-formed member
according to an aspect of the present disclosure may comprise
heating a blank of an aluminum-based plated steel sheet in a
5 heating furnace, removing the heated blank from the heating
furnace and conveying the removed blank between an upper mold
portion and a lower mold portion of a mold, mounted on a press,
to be seated, and performing a forming process after the upper
mold portion of the mold is in contact with the seated blank.
10 [0025] Alternatively, after performing the forming process,
the method of manufacturing a hot press-formed member may
further comprise an in-mold cooling step, in which the upper
mold portion of the mold reaches a press bottom dead center and
is then maintained to quench a formed material, and a step of
15 removing a cooled formed member.
[0026] According to an aspect of the present disclosure, the
aluminum-based plated steel sheet
may be an aluminum plated steel sheet or an aluminum alloy plated
steel sheet. In this case, although not necessarily limited,
20 as an example, a plating layer may comprise, by weight
percentage (wt%), 5 to 11% of silicon (Si), 4.5% or less of iron
(Fe), and a balance of aluminum (Al) and unavoidable impurities.
In addition, a base steel sheet may include, by wt%, 0.1 to 0.5%
of carbon (C), 0.1 to 2% of silicon (Si), 0.5 to 3% of manganese
25 (Mn), 0.01 to 0.5% of chromium (Cr), 0.001 to 1.0% of aluminum
9
(Al), 0.05% or less of phosphorus (P), 0.02% or less of sulfur
(S), 0.02% or less of nitrogen (N), 0.002 to 0.005% of boron
(B), and a balance of iron (Fe) and unavoidable impurities.
[0027] According to an aspect of the present disclosure, the
5 heating furnace may be a continuous heating furnace comprising
section A, section B, and section C provided sequentially in
a conveying direction of the blank. In this case, section A,
section B, and section C do not need to be provided adjacent
to each other in the conveying direction of the blank, and have
10 only to satisfy the above sequence in the conveying direction
of the blank. For example, each of section A, section B, and
section C may include a single heating zone, or a plurality of
heating zones may be included in each of section A, section B,
and section C. An additional section, set to a temperature
15 between pre-step and post-step temperature ranges, may be
further provided between the respective sections (for example,
between sections A and B or between sections B and C).
[0028] In a conventional atmospheric heating method, heating
is performed in a heating furnace set to the same atmospheric
20 temperature, or heating is performed in such a manner that an
atmospheric temperature is sequentially increased by a
continuous heating furnace such as a roller hearth furnace
having a plurality of heating zones.
[0029] However, since such heating is performed at a low rate,
25 heating in a heating furnace is necessarily performed for a
10
certain period of time to secure time required to reach a target
temperature. In addition, productivity may be deteriorated
due to an increase in maintaining time in the heating furnace.
[0030] In this regard, the present inventors found that when
5 an atmospheric temperature is set to be high while increasing
a temperature of section B, heating is performed more rapidly
than a conventional heating furnace setting method, and thus,
in-furnace maintaining time may be reduced to improve
productivity. In addition, the present inventors found that
10 when the temperature of section C in a subsequent process is
set to be lower than the temperature of section B, the
above-described process, a final heating temperature is set to
be low, and thus, poor weldability may be addressed.
[0031] A factor, determining the above-mentioned productivity,
15 may be significant reduction of time required to reach 900ºC,
significant reduction of time required for a temperature of a
material to reach a take-out temperature of the heating furnace
in a section in which a material is removed from the heating
furnace, or whether overall cumulative maintaining time in the
20 heating furnace until the material is removed of the heating
furnace is less than or equal to time for which a diffusion layer
has a thickness of 15 μm. By minimizing the above-mentioned
times, a cycle time in which target physical properties of a
formed member, an end product, may be secured could be reduced
25 significantly. Thus, productivity may be improved.
11
[0032] However, when section B set to the above-mentioned high
temperature is too wide, the thickness of the diffusion layer
may be increased as time required to be heated and maintained
at a high temperature is significantly reduced. Thus,
5 weldability may be deteriorated. Meanwhile, when section B set
to the high temperature is too narrow, an effect of improving
productivity by a high heating rate may not be obtained. A large
amount of energy is consumed to maintain section A, an initial
section of the heating furnace, at a high temperature. At an
10 initial stage of temperature rising, it is unnecessary to set
the temperature to an unnecessarily high atmospheric
temperature. In addition, the temperature may not be set to
a high atmospheric temperature from the beginning due to heat
radiation from an open structure of an injection portion of a
15 material and injection of a cold material.
[0033] Since a material heated to a sufficient temperature has
already been transformed into austenite, it may be only
necessary to maintain a temperature and time at which alloying
of a plating layer may be sufficiently obtained. When a high
20 atmospheric temperature is maintained even in this stage,
weldability may be deteriorated due to an excessive increase
in thickness of a diffusion layer. Therefore, the temperature
may be set to be a relatively low temperature.
[0034] In view of the foregoing, in the present disclosure,
25 as an example, a heating pattern illustrated in FIG. 1 was
12
performed on an aluminum plated material having a thickness of
1.2 mm. For example, in section A, an initial section of
temperature rising, a temperature was set to a relatively low
temperature in consideration of energy saving and inability to
5 be set to a high atmospheric temperature. In section B, the
temperature was set to a highest temperature to rapidly heat
the material, and thus, the material was set to reach a
sufficient temperature. In section C after the material
reached the sufficient temperature, the temperature was reset
10 to a lower temperature than that of section B. When the heating
furnace is set to be different for each section as illustrated
in FIG. 1, the material having a thickness of 1.2 mm is maintained
at a temperature of 900°C at a point in time at which cumulative
furnace maintaining time is 4.5 minutes. This result was
15 derived from a result of heating analysis on radiation and
convective heat transfer in a heating furnace atmosphere.
Hereinafter, heating conditions in each section will be
described in further detail.
[0035] In this specification, an atmospheric temperature in
20 each section to be described later may refer to an atmosphere
maintaining temperature in each heating zone (for example, a
temperature of a region in which an actual atmospheric
temperature is maintained in a single heating zone) in a heating
furnace having a plurality of heating zones and being able to
25 control atmospheric temperatures of the respective heating
13
zones to be distinguished from each other. For example, an
atmosphere maintaining temperature in a single heating zone may
be a temperature measured at a representative point of a region
in which the actual atmospheric temperature is maintained.
5 Although not necessarily limited, an example of the
representative point may be a point disposed in a center (1/2)
in a length direction, a 1/4 location in a width direction, and
spaced apart from a blank location by 250 mm in a height direction
with respect to a single heating zone. In this case, the
10 atmospheric temperature in each section is regarded as being
maintained at the atmosphere maintaining temperature in each
heating zone corresponding to each section.
[0036] In the above-described heating furnace having a
plurality of heating zones and being able to control atmospheric
15 temperatures of the respective heating zones to be
distinguished from each other, cumulative in-furnace
maintaining time in each section may refer to a maintaining time
from a point in time at which a blank, a material, is injected
into the heating furnace to a point in time at which the blank
20 is removed of a last heating zone, among the heating zones
corresponding to the above-mentioned sections.
[0037] In the above-described heating furnace, the heating
zones may be separated by a partition wall or the like, or may
be separated without a partition wall or the like. Therefore,
25 when the heating zones may be separated by a partition wall or
14
the like, the above-described method may be applied as it is.
[0038] When no partition wall is provided in the
above-described heating furnace, the entire heating furnace is
divided into the number of n zones (for example, five or more
5 zones) in the conveying direction of the blank. Each of the
divided zones may be regarded as a single section. In an example
embodiment, the entire heating furnace may be divided into 20
equal sections, and each of the 20 divided zones may be regarded
as a single section. In a single zone, a temperature measured
10 in a point disposed in a center (1/2) in a length direction,
a 1/4 location in a width direction, and spaced apart from a
blank location by 250 mm in a height direction may be regarded
as an atmosphere maintaining temperature in each section, as
described above.
15 [0039] As an example, according to an aspect of the present
disclosure, heating in section A may be set to an atmospheric
temperature of about 750 to 900ºC, heating in section B may be
set to an atmospheric temperature of about 930 to 960ºC, and
heating in section C may be set to about 870 ºC and to an
20 atmospheric temperature lower than an atmospheric temperature
selected in section B. When such a method is used, temperature
rise may be performed more rapidly than a case in which heating
is set to a uniform temperature of a final temperature, and thus,
the in-furnace maintaining time may be reduced. In addition,
25 formation of an excessive diffusion layer and deterioration of
15
weldability may be prevented by controlling the heating to an
appropriate range of a temperature and time at which alloying
of a plating layer may be sufficiently obtained. Accordingly,
a hot press forming method, capable of obtaining both excellent
5 productivity and weldability, may be effectively provided.
[0040] FIG. 2 is a graph illustrating feasibility of the
above-mentioned heating analysis technology and illustrating
a comparison between heating analysis experimental values and
analysis values under various furnace atmospheric temperature
10 conditions for an aluminum plated material having a thickness
of 1.2 mm. In the experimental value, one piece of temperature
data was obtained per second after a thermocouple attached to
the material was maintained in a heating furnace. The analysis
value is a result obtained by predicting such a condition using
15 the above-mentioned analysis technique. As can be seen from
FIG. 2, it can be confirmed that the analysis value expresses
the experimental value well.
[0041] The present inventors analyzed a temperature rise
pattern in various conditions to find out that a temperature
20 rise pattern of a material is dependent on a thickness of the
material, an atmospheric temperature, a maintaining time for
each temperature zone, and the like. As described above, the
present inventors found that the thickness of the material, an
atmospheric temperature, a time required to stay at each
25 atmospheric temperature are important in order to prevent a time
16
required for staying in a high atmospheric temperature zone from
excessively increasing and to avoid inability, to obtain a rapid
heating effect, caused by a significantly short time required
to stay in a high atmospheric temperature zone. Therefore, the
5 present inventors have completed the present disclosure based
on the fact that it is necessary to select an appropriate
maintaining time depending on the thickness of the material and
the atmospheric temperature. This will be described in detail
below.
10 [0042] Specifically, based on a graph in which an X axis denotes
cumulative in-furnace maintaining time and a Y axis denotes an
atmospheric temperature in the heating furnace, the heating in
section A may satisfy conditions specified by a figure ‘abcde’
having cumulative in-furnace maintaining times and in-furnace
15 atmospheric temperature coordinates of approximately a(0.2 min,
750ºC), b(1.0 min, 750ºC), c(1.0 min, 800ºC), d(1.5 min, 900ºC),
and e(0.2 min, 900ºC).
[0043] Since the heating in section A affects an initial
temperature rising rate to a temperature setting region of a
20 front portion of the heating furnace, an atmospheric
temperature in the furnace in section A may be set to, in detail,
a range of about 750ºC to about 900ºC. When the atmospheric
temperature in the heating furnace in section A is set to less
than about 750ºC, the initial temperature rising rate may be
25 significantly reduced to deteriorate productivity. Meanwhile,
17
when the atmospheric temperature in the heating furnace in
section A is set to higher than about 900ºC, an initial region
of the heating furnace is maintained at a high temperature to
increase power consumption.
5 [0044] In the heating in section A, not only the atmospheric
temperature but also the maintaining time affect a temperature
rising rate. In this case, to increase the temperature rising
rate, the maintaining time in section A may be set to be short
when the atmospheric temperature of section A is low and may
10 be set to be long when the atmospheric temperature of section
A is high. Thus, the present inventors have intensively
examined a preferable in-furnace atmospheric temperature and
a preferable maintaining time for the heating in section A to
find out that conditions of section A are preferably set as
15 illustrated in FIG. 3. For example, when the atmospheric
temperature of section A is a low temperature of about 750ºC,
the maintaining time of section A may be set to be short, in
detail, about 1 minute or less. In addition, when the
atmospheric temperature of section A is a high temperature of
20 about 900ºC, the maintaining time may be set to be, in detail,
about 1.5 minutes or less. Meanwhile, the maintaining time in
section A may be about 0.2 minute or more in consideration of
a time of passing through an entrance side of the heating
furnace.
25 [0045] In addition to the cumulative in-furnace maintaining
18
time and the atmospheric temperature, the thickness of the
material may also have an effect. However, since a
thickness-dependent effect was reflected in heating in section
B and section C to be described later and an effect in section
5 A is somewhat less, the maintaining time may be set in section
A, irrespective of the thickness of the material, from a
practical point of view (see FIG. 5A).
[0046] Then, the heating in section B may satisfy conditions
specified by a figure ‘fghi’ having cumulative in-furnace
10 maintaining times and atmospheric temperature coordinates of
approximately f( min, 930ºC), g( min,
930ºC), h( min, 960ºC), and i( min,
960ºC). In this case, a unit of the coordinates of the figure
‘fghi’ is f(1.3[min]+{(t[mm]-1.2[mm])/0.6[mm]}x0.5[min],
15 930[ºC]), g(3.8[Min]+{(t[mm]-1.2[mm])/0.6[mm]}x0.5[min],
930[ºC]), h(3.3[min]+{(t[mm]-1.2[mm])/0.6[mm]}x0.5[min],
960[ºC]), i(0.8[min]+{(t[mm]-1.2[mm])/0.6[mm]}x0.5[min],
960[ºC]).
[0047] The heating in section B is heating in a region of the
20 heating furnace in which the atmospheric temperature is highest,
and affects a temperature rising rate and a maximum temperature
of the material in a high-temperature region. When the
atmospheric temperature of section B is low, the maximum
19
temperature is decreased and the temperature rising rate is
decreased. Meanwhile, when the atmospheric temperature of
section B is high, the maximum temperature is increased and the
temperature rising rate is also increased. Therefore, the
5 atmospheric temperature of section B may be preferably set as
high as possible. However, when the atmospheric temperature
of section B is too high, the material may be heated to a
significantly high temperature to deteriorate weldability.
Therefore, it may be necessary to set a preferable range.
10 [0048] In this specification, a section from a section having
an atmospheric temperature of about 930ºC or higher to a section
having the highest atmospheric temperature (for example, a
highest atmosphere maintaining temperature) is regarded as
section B. In addition, a section subsequent from the section,
15 having an atmospheric temperature lower than the highest
atmospheric temperature, is regarded as a section distinguished
from section B. For example, when section B has a first section
B having an atmospheric temperature of about 930ºC and a second
section B having an atmospheric temperature of about 950ºC and
20 has a subsequent section having an atmospheric temperature of
about 935ºC, a section subsequent from the section having the
atmospheric temperature of about 935ºC, an atmospheric
temperature condition lower than the highest atmospheric
temperature of about 950ºC, may be regarded as section C.
25 [0049] Accordingly, in the present disclosure, the atmospheric
20
temperature of section B may be set to a range of about 930ºC
to about 960ºC. When the atmospheric temperature of section
B is higher than about 960ºC, there is a limitation of furnace
equipment, but weldability may be deteriorated because the
5 temperature is set to be significantly high in terms of alloying
of a plating layer. In addition, when the atmospheric
temperature of section B is less than about 930ºC, the
temperature rising rate may be significantly reduced to
increase a time required to reach a target temperature and to
10 deteriorate productivity due to an increase in cycle time.
[0050] In section B, not only the atmospheric temperature but
also maintaining time of section B affects a temperature rising
rate of a material and a maximum heating temperature of the
material. For example, when the maintaining time of section
15 B is too short, a sufficient temperature rising effect may not
be obtained. In addition, when the maintaining time of section
B is too long, the material may be maintained at a high
temperature for a too long period of time to cause excessive
alloying. Thus, the thickness of the diffusion layer may be
20 increased to deteriorate weldability.
[0051] Therefore, although not necessarily limited, a lower
limit of the maintaining time in section B may be about 0.5 minute
or more such that an effect of improving productivity is
obtained by a high temperature rising rate. Alternatively, an
25 upper limit of the maintaining time in section B may be about
21
4.8 minutes to prevent poor weldability caused by excessive
alloying. In this case, it should be noted that the maintaining
time of section B refers to a time for which the material is
maintained in only section B, and is conceptually distinguished
5 from the cumulative in-furnace maintaining time to be described
later.

WE CLAIM:
【Claim 1】
A method of manufacturing a hot press-formed member, the
5 method comprising:
heating a blank of an aluminum-based plated steel sheet
in a heating furnace;
removing the heated blank from the heating furnace and
conveying the removed blank between an upper mold portion and
10 a lower mold portion of a mold, mounted on a press, to be seated;
and
performing a forming process after the upper mold portion
of the mold is in contact with the seated blank,
wherein the heating furnace is a continuous heating
15 furnace comprising section A, section B, and section C provided
in a conveying direction of a blank,
heating in section A satisfies conditions specified by
a figure ‘abcde’ having cumulative in-furnace maintaining times
and in-furnace atmospheric temperature coordinates of
20 approximately a(0.2 min, 750ºC), b(1.0 min, 750ºC), c(1.0 min,
800ºC), d(1.5 min, 900ºC), and e(0.2 min, 900ºC),
heating in section B satisfies conditions specified by
a figure ‘fghi’ having cumulative in-furnace maintaining times
and atmospheric temperature coordinates of approximately
25 f( min, 930ºC), g( min, 930ºC),
72
h( min, 960ºC), and i( min, 960ºC),
and
heating in section C satisfies conditions specified by
a figure ‘jklm’ having cumulative in-furnace maintaining times
5 and atmospheric temperature coordinates of approximately
j( min, 870ºC), k( min, 870ºC),
l( min, 940ºC), and m( min, 940ºC),
a highest atmospheric temperature of section C is lower
than a highest atmospheric temperature of section B,
10 Relational Expression 1 is satisfied, and
a time required before the forming process is performed
after the blank is seated is two seconds or less,
Relation Expression 1:
T ≤ 8.2 x t + (temp-900)/30
15 where T denotes a sum of time required for conveying and
seating a blank and time required before forming is performed
after the blank is seated and a unit thereof second (s), t denotes
a thickness of a blank and a unit thereof is millimeters (mm),
and temp denotes a furnace extraction temperature and a unit
20 thereof is degrees Celsius (ºC).
【Claim 2】
The method of claim 1, wherein heating in section C is
73
performed at an atmospheric temperature of 930ºC or less.
【Claim 3】
The method of claim 1, wherein a highest atmospheric
5 temperature of section C is Tb-20ºC or less, based on a highest
atmospheric temperature (Tb) of section B.
【Claim 4】
The method of claim 1, wherein the thickness t is 1.5 mm
10 or less, and an atmospheric temperature of section B is more
than 930ºC to less than 940ºC.
【Claim 5】
The method of claim 1, wherein the thickness t is 1.5 mm
15 or less, and an atmospheric temperature of section C is 870ºC
or more to less than 880ºC.
【Claim 6】
The method of claim 1, wherein the thickness t is greater
20 than 1.5 mm, and an atmospheric temperature of section C is 870ºC
or more to less than 900ºC.
【Claim 7】
The method of claim 1, wherein the thickness t is greater
25 than 1.5 mm, and an atmospheric temperature of section B is more
than 940ºC to 960ºC or less.
74
【Claim 8】
The method of claim 1, wherein the sum of time required
for conveying and seating a blank and time required before
forming is performed after the blank is seated is greater than
5 10 seconds.
【Claim 9】
The method of claim 1, wherein a plating layer of the blank
has a thickness of 25 μm or more.
10
【Claim 10】
The method of claim 1, after the performing of the forming
process, further comprising:
an in-mold cooling step in which the upper mold portion
15 of the mold reaches a press bottom dead center and is then
maintained to quench a formed material;; and
a take-out step of removing the cooled formed member,
wherein a diffusion layer of the formed member has a
thickness of 15 μm or less, and
20 an alloy layer of the formed member has a thickness of
35 μm to 50 μm.
【Claim 11】
The method of claim 1, after the performing of the forming
25 process, the method further comprising:
an in-mold cooling step in which the upper mold portion
75
of the mold reaches a press bottom dead center and is then
maintained to quench a formed material; and
a take-out step of removing a cooled formed member,
wherein a ratio of a thickness of a diffusion layer of
5 the formed member to a thickness of an alloy layer of the formed
member (the thickness of the diffusion layer/the thickness of
the alloy layer) was 0.33 or less
【Claim 12】
10 The method of claim 1, wherein the heating is performed
to a condition that a value of Relational Expression below is
2 or more,
Relational Expression 2:
15 where Tn denotes a heating furnace atmospheric
temperature in an n-th section in a conveying direction of a
blank and a unit thereof is degrees Celsius (°C), tn denotes
a heating furnace maintaining time in the n-th section in the
conveying direction of the blank and a unit thereof is minute,
20 ttotal denotes total maintaining time in the heating furnace and
a unit thereof is minute, x denotes the number of sections
maintained at a specific atmospheric temperature in the heating
furnace, k is an integer of 3 in the case of a final section
in section B, an integer of -1 in the case in which a section
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subsequent to section B, and an integer of 1 in the other cases,
and t denotes a thickness of the blank and a unit thereof is
millimeters (mm).