U.S. patent application number 15/305552 was filed with the patent office on 2017-02-16 for method for manufacturing hot press forming part and hot press forming part.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Toru Minote, Tatsuya Nakagaito, Yoshikiyo Tamai, Yuichi Tokita.
Application Number | 20170043386 15/305552 |
Document ID | / |
Family ID | 54332187 |
Filed Date | 2017-02-16 |
United States Patent
Application |
20170043386 |
Kind Code |
A1 |
Nakagaito; Tatsuya ; et
al. |
February 16, 2017 |
METHOD FOR MANUFACTURING HOT PRESS FORMING PART AND HOT PRESS
FORMING PART
Abstract
In manufacturing a hot press forming part by hot pressing, using
a tool of press forming, a coated steel sheet, the tool of press
forming having a die, a blank holder, and a punch, edges of the
coated steel sheet heated to a temperature range of Ac.sub.3
transformation temperature to 1000.degree. C. are cooled with the
edges squeezed between the die and the blank holder to a
temperature of 550.degree. C. or lower and 400.degree. C. or higher
at a cooling rate of 100.degree. C./s or higher, press forming is
performed so that the press forming is started when the edges reach
a temperature of 550.degree. C. or lower and 400.degree. C. or
higher, and after the press forming, a formed body is quenched with
the formed body held at the press bottom dead center while being
squeezed by the tool of press forming.
Inventors: |
Nakagaito; Tatsuya;
(Chiyoda-ku, Tokyo, JP) ; Tokita; Yuichi;
(Chiyoda-ku, Tokyo, JP) ; Minote; Toru;
(Chiyoda-ku, Tokyo, JP) ; Tamai; Yoshikiyo;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
54332187 |
Appl. No.: |
15/305552 |
Filed: |
February 26, 2015 |
PCT Filed: |
February 26, 2015 |
PCT NO: |
PCT/JP2015/056439 |
371 Date: |
October 20, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B21D 37/16 20130101;
B21D 24/04 20130101; B21D 22/20 20130101; B21D 22/022 20130101;
B21D 22/208 20130101 |
International
Class: |
B21D 22/02 20060101
B21D022/02; B21D 37/16 20060101 B21D037/16 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2014 |
JP |
2014-088848 |
Sep 3, 2014 |
JP |
2014-179073 |
Claims
1. A method for manufacturing a hot press forming part by hot
pressing, using a tool of press forming, a coated steel sheet that
is obtained by forming a Zn--Ni coating layer on a surface of a
steel sheet, the tool of press forming having a die, a blank
holder, and a punch, the method comprising: cooling edges of the
coated steel sheet heated to a temperature range of Ac.sub.3
transformation temperature to 1000.degree. C., with the edges
squeezed between the die and the blank holder, to a temperature of
550.degree. C. or lower and 400.degree. C. or higher at a cooling
rate of 100.degree. C./s or higher; performing press forming so
that the press forming is started when the edges reach a
temperature of 550.degree. C. or lower and 400.degree. C. or
higher; and after the press forming, quenching a formed body with
the formed body held at the press bottom dead center while being
squeezed by the tool of press forming.
2. The method for manufacturing a hot press forming part according
to claim 1, wherein the cooling and the press forming are performed
while slidably moving the die with the coated steel sheet so that
slidable movement is temporarily stopped before the coated steel
sheet is brought into contact with the punch, or so that slidable
movement before the coated steel sheet coming into contact with the
punch is slower than slidable movement during press forming after
the coated steel sheet coming into contact with the punch.
3. The method for manufacturing a hot press forming part according
to claim 1, wherein in the press forming, the blank holder is
detached from the coated steel sheet to perform crush forming
without using the blank holder.
4. The method for manufacturing a hot press forming part according
to claim 1, wherein in the press forming, draw forming is performed
with the coated steel sheet squeezed between the die and the blank
holder.
5. The method for manufacturing a hot press forming part according
to claim 1, wherein the Zn--Ni coating layer contains Ni in an
amount of 9 mass % or more and 25 mass % or less.
6. A hot press forming part manufactured by the method as recited
in claim 1.
7. The method for manufacturing a hot press forming part according
to claim 2, wherein in the press forming, the blank holder is
detached from the coated steel sheet to perform crush forming
without using the blank holder.
8. The method for manufacturing a hot press forming part according
to claim 2, wherein in the press forming, draw forming is performed
with the coated steel sheet squeezed between the die and the blank
holder.
9. The method for manufacturing a hot press forming part according
to claim 2, wherein the Zn--Ni coating layer contains Ni in an
amount of 9 mass % or more and 25 mass % or less.
10. The method for manufacturing a hot press forming part according
to claim 3, wherein the Zn--Ni coating layer contains Ni in an
amount of 9 mass % or more and 25 mass % or less.
11. The method for manufacturing a hot press forming part according
to claim 4, wherein the Zn--Ni coating layer contains Ni in an
amount of 9 mass % or more and 25 mass % or less.
12. The method for manufacturing a hot press forming part according
to claim 7, wherein the Zn--Ni coating layer contains Ni in an
amount of 9 mass % or more and 25 mass % or less.
13. The method for manufacturing a hot press forming part according
to claim 8, wherein the Zn--Ni coating layer contains Ni in an
amount of 9 mass % or more and 25 mass % or less.
14. A hot press forming part manufactured by the method as recited
in claim 2.
15. A hot press forming part manufactured by the method as recited
in claim 3.
16. A hot press forming part manufactured by the method as recited
in claim 4.
17. A hot press forming part manufactured by the method as recited
in claim 5.
18. A hot press forming part manufactured by the method as recited
in claim 7.
19. A hot press forming part manufactured by the method as recited
in claim 8.
20. A hot press forming part manufactured by the method as recited
in claim 9.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a hot press forming part and a
method of manufacturing the same. The disclosure particularly
relates to a method for manufacturing a hot press forming part from
a coated steel sheet wherein, while press forming the coated steel
sheet heated beforehand into a predetermined shape, the steel sheet
is simultaneously quenched to attain a predetermined strength (such
that the tensile strength is at least 1180 MPa grade), and a hot
press forming part manufactured by the same.
BACKGROUND
[0002] In recent years, strengthening and sheet metal thinning of
automotive parts have been required. As the thin steel sheets
(hereinafter referred to as "steel sheets") used have higher
strength, press formability is deteriorated, and forming the steel
sheets into the desired part shape is more difficult.
[0003] The following technique is known to solve this problem:
while hot press forming a blank sheet heated to high temperature
into a desired shape using a tool of press forming, the steel sheet
is quenched in the die by heat extraction, thus enhancing the
hardness of the hot press formed part.
[0004] For example, GB1490535A (PTL 1) proposes a technique in
which, when manufacturing a part of a predetermined shape by hot
pressing a blank sheet (steel sheet) heated to an austenite single
phase region of about 900.degree. C., the blank sheet is quenched
in a tool of press forming simultaneously with the hot press
forming, thus enhancing the strength of the part.
[0005] However, the technique proposed in PTL 1 has a problem in
that, when heating the steel sheet to high temperature of about
900.degree. C. before the pressing, oxide scale (iron oxide) forms
on the surface of the steel sheet, and the oxide scale peels during
the hot press forming and damages the die or the surface of the hot
press formed part. Besides, the oxide scale remaining on the
surface of the part causes poor appearance and lower coating
adhesion. Accordingly, the oxide scale on the surface of the part
is typically removed by a process such as pickling or shot
blasting. Such a process, however, causes lower productivity.
Furthermore, while suspension parts of vehicles, automotive body
parts, and the like are also required to have excellent corrosion
resistance, the corrosion resistance of the hot press formed part
by the technique proposed in PTL 1 is insufficient because a rust
preventive film such as a coating layer is not provided on the
blank sheet.
[0006] For these reasons, there is demand for hot press forming
techniques that can suppress the generation of oxide scale during
heating before hot press forming and also improve the corrosion
resistance of the hot press formed part. To meet this demand,
coated steel sheets having films such as coating layers on their
surfaces, hot press forming methods using coated steel sheets, etc.
are proposed.
[0007] For example, JP2001353548A (PTL 2) proposes a technique in
which a steel sheet coated with Zn or a zinc-based alloy is heated
to 700.degree. C. to 1200.degree. C. and then hot press formed to
obtain a hot press formed part having a Zn--Fe-based compound or a
Zn--Fe--Al-based compound on its surface. PTL 2 describes that the
use of the steel sheet coated with Zn or a Zn-based alloy
suppresses the oxidation of the surface of the steel sheet during
heating before hot press forming, and also enables a hot press
formed part having excellent corrosion resistance to be
obtained.
[0008] With the technique proposed in PTL 2, the generation of
oxide scale on the surface of the hot press formed part is
suppressed to some extent. However. Zn in the coating layer may
cause liquid metal embrittlement cracking, resulting in cracks of
about 100 .mu.m in depth in the surface layer part of the hot press
formed part. Such cracks pose various problems such as a decrease
in fatigue characteristics of the hot press formed part.
[0009] In view of this problem, JP201391099A (PTL 3) proposes a
method in which a coated steel sheet obtained by providing a
Zn--Fe-based coating layer on a surface of a steel sheet is heated
to a temperature not less than Ac.sub.1 transformation temperature
of the steel sheet and not more than 950.degree. C. and then cooled
to a temperature not more than the freezing point of the coating
layer, before starting the forming. PTL 3 describes that liquid
metal embrittlement cracking can be suppressed by starting the
forming after the coated steel sheet is cooled to the temperature
not more than the freezing point of the coating layer.
CITATION LIST
Patent Literature
[0010] PTL 1: GB1490535A [0011] PTL 2: JP2001353548A [0012] PTL 3:
JP201391099A
SUMMARY
Technical Problem
[0013] It is believed that the technique proposed in PTL 3 can
suppress liquid metal embrittlement cracking, i.e., cracks in the
surface of the hot press formed part, which are about 100 .mu.m in
depth from the interface between the coating layer and the steel
toward the inside of the steel and in which Zn is detected from its
interface (such cracks referred to hereinafter as "macro-cracks").
For suppressing such macro-cracks, we studied the use of Zn--Ni
alloy coating obtained by blending Zn with about 9% to 25% of Ni as
a coating layer with high fusing point. The .gamma.-phase in the
phase equilibrium diagram of Zn--Ni alloy has a fusing point of
860.degree. C. or higher, which is very high as compared to that a
normal Zn or Zn alloy coated layer, making it possible to suppress
macro-cracks under normal press conditions.
[0014] However, in addition to the macro-cracks, minute cracking
which is about 30 .mu.m or less in depth from the interface between
the coating layer and the steel toward the inside of the steel and
in which Zn is not detected from its interface may also occur in
the surface of the hot press formed part. Such minute cracking is
called "micro-cracks". Micro-cracks pass through the interface
between the coating layer and the steel and reach the inside of the
steel (steel sheet), adversely affecting the characteristics
(fatigue characteristics, etc.) of the hot press formed part.
[0015] Macro-cracks also occur in, for example, a shoulder area of
die R on the punch-contacting side which is subjected to only
tensile strain while press forming of a hat-shaped section part
(also referred to hereinafter as a "hat-shaped part"). On the other
hand, micro-cracks do not occur in such area, but on the
die-contacting side of side wall portions, which are subjected to
compression (due to bending) followed by tensile strain (due to
bend restoration). It is thus estimated that macro-cracks and
micro-cracks are produced by different mechanism.
[0016] PTL 3 may suppress the occurrence of macro-cracks in a
coated steel sheet having a Zn--Fe-based coating layer formed
thereon, but is not necessarily effective for suppressing the
occurrence of micro-cracks, because it does not consider potential
micro-cracks occurring in a coated steel sheet having a Zn--Ni
coating layer formed thereon.
[0017] Additionally, PTL 3 teaches that the coated steel sheet is
press formed as a whole while being cooled to a temperature at or
below the freezing point of the coating layer, without specifying
the lowest temperature at which the press forming is started,
leading to the problem of lower forming temperature resulting in
higher strength of the steel sheet during press forming, and
deteriorating the shape fixability (which is a characteristic that
maintains the shape at press bottom dead center even after die
release, because of little springback and the like).
[0018] It could thus be helpful to provide a method for, when
manufacturing a hot press formed part by hot pressing a coated
steel sheet having a Zn--Ni-based coating layer formed thereon,
manufacturing a hot press forming part while preventing a reduction
in the shape fixability during hot press forming and suppressing
the occurrence of micro-cracks, and a hot press forming part
manufactured by the same.
Solution to Problem
[0019] We studied means for suppressing micro-cracks (minute
cracking) caused when hot press forming a Zn-based coated steel
sheet.
[0020] Although the micro-crack occurrence mechanism is still
unclear, press forming the Zn-based coated steel sheet at high
temperature may induce minute cracking in the surface of the coated
steel sheet, and such minute cracking also occurs in Zn--Ni
coating. The minute cracking has a depth of about 30 .mu.m from the
interface between the coating layer and the steel (steel sheet),
and passes through the interface between the coating layer and the
steel (steel sheet) and reaches the inside of the steel sheet. As a
result of making various research on this problem, we discovered
that micro-cracks are suppressed by performing hot press forming at
low temperature. Further, the effect of significantly reducing the
amount of coating attached to the tool of press forming, which
would be quite large to cause problems with conventional coated
steel sheets for hot press forming, was obtained by setting a low
temperature for press forming as mentioned above.
[0021] However, when the press forming temperature is lower, the
strength of the steel sheet is higher, and accordingly the shape
fixability is lower. Thus, the advantages of the hot press forming
cannot be exploited.
[0022] We then conceived cooling, before performing hot press
forming, only those portions of the steel sheet that are processed
such that micro-cracks would occur during the press process. Then
we made further research on the influence of strain during forming
on the occurrence of micro-cracks, and discovered that micro-cracks
are not caused by compressive deformation or bending deformation
alone, but are caused at those portions when being subjected to
bending-bend restoration deformation resulting from the portions
being bent and stretched afterwards.
[0023] A portion of the formed part, called a side wall portion,
principally undergoes such bending-bend restoration deformation.
FIG. 17 shows the forming conditions. Most automotive press-formed
parts are of so-called hat-like shape such as shown by "Final
shape" in FIG. 17, and are manufactured by a process such as draw
forming ((a) in FIG. 17) in which press forming is performed by
squeezing a steel sheet between a blank holder and a die to
suppress wrinkle formation, or crush forming ((b) in FIG. 17)
without using a blank holder. As illustrated in FIG. 17, in either
case, a steel sheet is bent against a die, and then restored to its
original shape as the punch rises to have side wall portions.
[0024] In draw forming, side wall portions are formed by those
portions squeezed between the die and the blank holder prior to
press forming. We further investigated the way of effectively
cooling only such portions. As a result, it was discovered that it
is possible to suppress defects in shape accuracy and to suppress
the occurrence of micro-cracks in the side wall portions by, prior
to press forming, squeezing the steel sheet between the die and the
blank holder and, through heat extraction at the tool of press
forming, cooling the steel sheet continuously (for 0.5 seconds to 3
seconds) until the temperature of the portions at which the steel
sheet is squeezed between the die and the blank holder reaches
550.degree. C. or lower and 400.degree. C. or higher.
[0025] The exact mechanism behind this phenomenon in which cooling
in the die and blank the holder prevented lowering shape accuracy
is unclear, yet the reason may be as stated below.
[0026] For hat-shaped parts, typical defects in shape accuracy
include angle change such that the angle formed by two faces across
the bending ridgeline becomes large relative to the die angle, and
wall camber such that the planes of the side wall portions have
curvature. Both of these defects occur due to the difference of any
stress distribution in the sheet thickness direction, and the
higher the flow stress of the steel sheet during forming, the shape
accuracy decreases. In other words, in hot pressing, as the forming
temperature becomes lower, the flow stress increases during forming
of the steel sheet, and the shape accuracy decreases. In this
respect, by performing cooling in the tool of press forming as
described above, the aforementioned angle change becomes small,
because the portion of the steel sheet in contact with the punch
shoulder portion at the time of press forming is not cooled during
the cooling process in the die and the blank holder, and this
portion is processed under high-temperature conditions. It is also
believed that the side wall portions are reduced in shape accuracy
since the temperature of the steel sheet during processing is
decreased by cooling in the die and the blank holder. However,
almost no deterioration of shape accuracy was observed over a
holding time (within three seconds) when the temperature of the
steel sheet is 400.degree. C. or higher. The reason may be that at
a steel plate temperature of 400.degree. C. or higher (over a
holding time of 3 seconds or less), the metallographic structure
during the press forming was still austenite, and the stress that
had been introduced during the press forming was eased by
martensitic transformation after the forming process, causing no
deterioration of shape accuracy. By contrast, it is considered that
if the holding time exceeds 3 seconds, the metallographic structure
will have already been transformed to martensite at the time of
press forming, and wall camber will be caused by the stress
introduced during the press forming.
[0027] The disclosure is based on the aforementioned discoveries.
We thus provide the following.
[0028] [1] A method for manufacturing a hot press forming part by
hot pressing, using a tool of press forming, a coated steel sheet
that is obtained by forming a Zn--Ni coating layer on a surface of
a steel sheet, the tool of press forming having a die, a blank
holder, and a punch, the method comprising:
[0029] cooling edges of the coated steel sheet heated to a
temperature range of Ac.sub.3 transformation temperature to
1000.degree. C., with the edges squeezed between the die and the
blank holder, to a temperature of 550.degree. C. or lower and
400.degree. C. or higher at a cooling rate of 100.degree. C./s or
higher;
[0030] performing press forming so that the press forming is
started when the edges reach a temperature of 550.degree. C. or
lower and 400.degree. C. or higher; and
[0031] after the press forming, quenching a formed body with the
formed body held at the press bottom dead center while being
squeezed by the tool of press forming.
[0032] [2] The method for manufacturing a hot press forming part
according to [1] above, wherein the cooling and the press forming
are performed while slidably moving the die with the coated steel
sheet so that slidable movement is temporarily stopped before the
coated steel sheet is brought into contact with the punch, or so
that slidable movement before the coated steel sheet coming into
contact with the punch is slower than slidable movement during
press forming after the coated steel sheet coming into contact with
the punch.
[0033] [3] The method for manufacturing a hot press forming part
according to [1] or [2], wherein in the press forming, the blank
holder is detached from the coated steel sheet to perform crush
forming without using the blank holder.
[0034] [4] The method for manufacturing a hot press forming part
according to [1] or [2], wherein in the press forming, draw forming
is performed with the coated steel sheet squeezed between the die
and the blank holder.
[0035] [5] The method for manufacturing a hot press forming part
according to any one of [1] to [4], wherein the Zn--Ni coating
layer contains Ni in an amount of 9 mass % or more and 25 mass % or
less.
[0036] A hot press forming part manufactured by the method as
recited in any one of [1] to [5].
Advantageous Effect
[0037] According to this disclosure, micro-cracks do not occur, and
it is possible to manufacture a hot press forming part with
sufficient strength and hardness as well as satisfactory shape
fixability, without causing a significant increase in load of press
forming.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] In the accompanying drawings:
[0039] FIG. 1 illustrates a method for manufacturing a hot press
forming part according to one of the disclosed embodiments;
[0040] FIG. 2 is a first schematic diagram illustrating the
relationship between metallographic structures, temperature, and
cooling time;
[0041] FIG. 3 is a second schematic diagram illustrating the
relationship between metallographic structure, temperature, and
cooling time;
[0042] FIG. 4 illustrates a general press forming method;
[0043] FIG. 5 illustrates a cooling-time control method according
to one of the disclosed embodiments;
[0044] FIG. 6 illustrates a test piece used in experiments
according to one of the disclosed embodiments;
[0045] FIG. 7 is a graph illustrating experimental results
according to one of the disclosed embodiments, showing the change
in temperature of the test piece;
[0046] FIG. 8 is a partial enlarged view of FIG. 7, with emphasis
on the horizontal axis;
[0047] FIG. 9 shows SEM (scanning electron microscope) images of
side wall portions demonstrating experimental results according to
one of the disclosed embodiments;
[0048] FIG. 10 is a graph illustrating experimental results
according to one of the disclosed embodiments, showing the
relationship between press forming start temperature and press
load;
[0049] FIG. 11 is a graph illustrating experimental results
according to one of the disclosed embodiments, showing the
relationship between press forming start temperature and amount of
mouth opening deformation;
[0050] FIG. 12 illustrates different modes of operation for cooling
in the tool of press forming according to one of the disclosed
embodiments;
[0051] FIG. 13 illustrates a forming method according to one of the
disclosed embodiments;
[0052] FIG. 14 illustrates a press forming part to be press formed
in examples;
[0053] FIG. 15 illustrates a micro-crack examined in examples;
[0054] FIG. 16 illustrates the amount of mouth opening deformation
examined in examples; and
[0055] FIG. 17 illustrates the stress state during the process of
press forming a forming part to have a hat-shaped cross
section.
DETAILED DESCRIPTION
[0056] The method for manufacturing a hot press forming part
according to one of the disclosed embodiments manufactures a hot
press forming part by hot pressing, using a tool of press forming,
a coated steel sheet that is obtained by forming a Zn--Ni coating
layer on a surface of a steel sheet, the tool of press forming
having a die, a blank holder, and a punch, the method comprising,
as illustrated in FIG. 1: (S1) cooling edges of coated steel sheet
1 heated to a temperature range of Ac.sub.3 transformation
temperature to 1000.degree. C., with the edges squeezed between die
3 and blank holder 5, to a temperature of 550.degree. C. or lower
and 400.degree. C. or higher at a cooling rate of 100.degree. C./s
or higher; (S2) performing press forming so that the press forming
is started using the die 3, blank holder 5, and punch 7 when the
edges of the coated steel sheet 1 reach a temperature of
550.degree. C. or lower and 400.degree. C. or higher; and (S3)
after the press forming, quenching formed body 1' with the formed
body held at the press bottom dead center while being squeezed by
the die 3, blank holder 5, and punch 7.
[0057] The following describes the details of the blank material of
the hot press formed part, the cooling (S1), the press forming
(S2), and the quenching (S3).
[0058] <Blank Material of Hot Press Formed Part>
[0059] A coated steel sheet obtained by providing a Zn--Ni coating
layer on a surface of the steel sheet is used as the blank material
of a hot press formed part. The provision of the Zn--Ni coating
layer on the surface of the steel sheet ensures the corrosion
resistance of the hot press formed part.
[0060] The method of forming the Zn--Ni coating layer on the
surface of the steel sheet is not particularly limited, and may be
any of the methods such as hot-dip galvanizing and
electro-galvanizing. The coating weight per side is preferably 10
g/m.sup.2 or more and 90 g/m.sup.2 or less.
[0061] The Ni content in the coating layer is preferably 9 mass %
or more and 25 mass % or less. In the case of forming the Zn--Ni
coating layer on the surface of the steel sheet by
electro-galvanizing, a .gamma. phase having any of the crystal
structures of Ni.sub.2Zn.sub.11, NiZn.sub.3, and Ni.sub.5Zn.sub.21
is formed when the Ni content in the coating layer is 9 mass % or
more and 25 mass % or less. The .gamma. phase has a high fusing
point, and thus is advantageous in preventing the coating layer
from evaporating when heating the coated steel sheet before hot
press forming. The .gamma. phase is also advantageous in
suppressing liquid metal embrittlement cracking during
high-temperature hot press forming.
[0062] The coated steel sheet 1 is heated to a temperature range of
Ac.sub.3 transformation temperature to 1000.degree. C. If the
heating temperature of the coated steel sheet 1 is below Ac.sub.3
transformation temperature, a sufficient amount of austenite cannot
be obtained during heating, leading to the presence of ferrite
during press forming. As a result, sufficient strength and good
shape fixability are difficult to achieve through hot press
forming. When the heating temperature of the coated steel sheet 1
exceeds 1000.degree. C., on the other hand, the coating layer
evaporates or excessive oxide generation occurs in the surface
layer part, as a result of which the resistance to oxidation
declines or the corrosion resistance of the hot press formed part
declines. Therefore, the heating temperature is from Ac.sub.3
transformation temperature to 1000.degree. C. Preferably, the
heating temperature is from Ac.sub.3 transformation
temperature+30.degree. C. to 950.degree. C. The method of heating
the coated steel sheet 1 is not particularly limited, and may be
any of the methods such as heating in an electric furnace, an
induction heating furnace, and a direct current furnace. Although
not particularly limited, the thickness of the steel sheet is
preferably 0.8 mm to 4.0 mm from the perspective of guaranteeing
the rigidity of the press formed part and ensuring the cooling rate
during cooling in the tool of press forming. More preferably, the
thickness is 1.0 mm 3.0 mm.
[0063] <Cooling (S1) and Press Forming (S2)>
[0064] In the cooling (S1), edges of the coated steel sheet thus
heated are cooled, with the edges being squeezed between the die
and the blank holder, to a temperature of 550.degree. C. or lower
and 400.degree. C. or higher at a cooling rate of 100.degree. C./s
or higher.
[0065] Additionally, in the press forming (S2), press forming is
performed so that the press forming is started when the edges of
the coated steel sheet reach a temperature of 550.degree. C. or
lower and 400.degree. C. or higher.
[0066] It is noted here that in the cooling (S1), the temperature
at start of cooling, with the edges of the heated coated steel
sheet 1 squeezed between the die and the blank holder, is
preferably 800.degree. C. or lower from the perspective of
preventing the risk of the Zn--Ni coating layer being adhered to
the tool of press forming, and preferably 670.degree. C. or higher
from the perspective of guaranteeing the strength after hot press
forming.
[0067] As used herein, the edges refer to those portions of the
coated steel sheet that form, after subjection to the press
forming, flange portions with at least the lower portions (on the
flange side) of the side wall portions of a formed body. For
example, when a hat-shaped section part as illustrated in FIG. 14
is formed, such edges correspond to those portions that form, on
the opposite sides of the coated steel sheet, flange portions with
at least lower portions (on the flange side) of the side wall
portions of a formed body; or when a cup-shaped part is formed,
such edges correspond to those portions that form, on the entire
circumference of the coated steel sheet, flange portions with at
least lower portions (on the flange side) of the side wall portions
of a formed body.
[0068] In addition, cooling in the tool of press forming using the
die and the blank holder is adopted because, for example, when a
hat-shaped section part is formed, the edges of the steel sheet
that are squeezed between the die and the blank holder will be
rapidly cooled, while other portions of the steel sheet that are in
contact with the shoulder areas of the punch during the press
forming will hardly be cooled, and thus can be press formed while
being kept at high temperature.
[0069] Moreover, the cooling rate for cooling in the tool of press
forming is set to 100.degree. C./s or higher because, when a
forming part is press formed into a hat-shaped part, for example,
this cooling rate enables the side wall portions (portions squeezed
by the tool of press forming) of the press formed body to have a
martensite single phase structure, and thus allows for
strengthening, without increasing cost, of the side wall
portions.
[0070] In the following, this will be described in detail.
[0071] FIG. 2 is a schematic diagram illustrating the relationship
between metallographic structure, temperature, and cooling time.
Graph (a) of FIG. 2 shows a case where the press forming start
temperature is high and, after the start of press forming, the
coated steel sheet is rapidly cooled by heat extraction to the tool
of press forming, so as to have a martensite single phase
structure.
[0072] On the other hand, if the press forming start temperature is
low as shown in graph (b) of FIG. 2, ferrite and bainite are formed
before the start of press forming, leading to a decrease in the
strength of the press formed part after subjection to the press
forming.
[0073] Thus, simply lowering of the press forming start temperature
results in graph (b) of FIG. 2, whereas the present disclosure
adopts the cooling step that enables rapid cooling of only edges of
the coated steel sheet, with the edges squeezed between the die and
the blank holder before the start of press forming, so that the
side wall portions of the press formed body may have a martensite
single phase structure, as shown by a curve indicated by a broken
line in FIG. 3.
[0074] Normally, the upper limit of the cooling rate for cooling in
the tool of press forming is about 500.degree. C./s.
[0075] In the cooling step, the edges are cooled to 550.degree. C.
or lower because, above 550.degree. C., cooling becomes
insufficient, causing micro-cracks after subjection to the hot
press forming. In addition, the lower limit of the cooling
temperature is 400.degree. C. because, if the edges are cooled
below 400.degree. C., the coated steel sheet 1 will be excessively
cooled before subjection to the press forming, leading to
deterioration in shape fixability.
[0076] We conducted experiments to examine the relationship between
cooling temperature, occurrence of micro-cracks, and shape
fixability in the cooling step, and the results thereof will be
described below.
[0077] As the blank material, a Zn--Ni coated steel sheet having a
sheet thickness of 1.6 mm that was prepared by applying Zn-12% Ni
coating to both surfaces thereof with a coating weight per side of
60 g/m.sup.2 was used. The heating temperature was 900.degree. C.,
the temperature at start of cooling in the tool of press forming
was about 700.degree. C., the blank holding force (BHF) was 98 kN,
and the bottom dead center holding time was 15 s.
[0078] In the cooling step, cooling in the tool of press forming
was controlled by the time for which the blank material was held by
the die 3 and the blank holder 5 before the start of press forming.
Specifically, in conventional press forming as illustrated in FIG.
4, after a blank material is placed on punch 7 and blank holder 5
for press forming and before the press forming is started, the die
is slidably moved at a constant high speed (12 spm [Shots Per
Minute]). In contrast, in an experiment according to the disclosure
as illustrated in FIG. 5, as a cooling step, coated steel sheet 1
was squeezed between die 3 and blank holder 5 and, as-is, slidably
moved at a low speed (lower than 0.24 spm to 12 spm) before coming
into contact with the punch, while in the subsequent press forming
step after the coated steel sheet 1 coming into contact with the
punch, the die was slidably moved at a high speed (12 spm) as is
the case with the conventional press forming. Cooling time was
controlled by controlling the slidable movement speed. In the
cooling step, when the slidable movement speed is from 0.24 spm to
below 12 spm, the cooling time is from 0.16 s to less than 5.8
s.
[0079] For the change in temperature of a steel sheet, as indicated
by steel sheet 9 in FIG. 6, metal-sheathed thermocouple 16 of 0.5
.phi. was inserted through an edge of the steel sheet to be
squeezed between the die and the blank holder, and measurement was
performed twice to determine the temperature of this portion.
[0080] FIG. 7 is a graph showing the results, where the vertical
axis is temperature (.degree. C.) and the horizontal axis is time
(s). FIG. 8 is a graph representing a partial enlarged view of FIG.
7, with emphasis on the horizontal axis, and focusing on an area
enclosed by a broken line in FIG. 7.
[0081] The change in temperature of the edge of the steel sheet
caused by cooling in the tool of press forming is, as illustrated
in FIG. 8, about 190.degree. C./s, showing that cooling in the tool
of press forming enables rapid cooling of the edge of the steel
sheet. Also, a radiation thermometer was used to measure the
surface temperature of the steel sheet at those portions to be
brought into contact with the shoulder areas of the punch during
press forming. These portions showed almost no drop in temperature
until they were brought into contact with the punch.
[0082] For evaluation, observations were made to: (i) determine the
presence of micro-cracks by observing the cross sections of the
side wall portions of the press forming part; (ii) determine the
hardness of the press forming part; (iii) determine the load of
press forming; and (iv) determine the shape fixability by measuring
the amount of mouth opening deformation of the hat-like opening of
the press forming part (the difference between the width dimension
of the opening after die release following the press forming and
the width of the press forming part conformed to the tool of press
forming).
[0083] FIG. 9 shows SEM images of cross sections of steel sheet
surface layers of side wall portions on the die side. It can be
seen that no micro-cracks are observed where the cooling time in
the tool of press forming is 0.60 s or more (where the press
forming start temperature is 550.degree. C. or lower). Under all
conditions, Hv.gtoreq.380, proving that quench hardenability does
not deteriorate.
[0084] FIG. 10 is a graph illustrating the results of load of press
forming, where the vertical axis is press load (kN) and the
horizontal axis is press forming start temperature (C). As used
herein, the press forming start temperature refers to the
temperature of the edges of the steel sheet that are squeezed
between the die and the blank holder. As can be seen from the graph
of FIG. 10, press load increases with decreasing press forming
start temperature due to cooling in the tool of press forming prior
to press forming. However, at around 550.degree. C. at which
micro-cracks do not occur, load of press forming was as low as that
of mild steel (270 D, cold draw forming), which poses no
problem.
[0085] FIG. 11 is a graph illustrating the results of shape
fixability, where the vertical axis is the amount of mouth opening
deformation (mm) of the press forming part, and the horizontal axis
is press forming start temperature (.degree. C.). As shown in the
graph of FIG. 11, the amount of mouth opening deformation increases
due to a decrease in the forming start temperature caused by the
process of cooling in the tool of press forming prior to the press
forming process, which shows a tendency such that shape fixability
deteriorates accordingly. However, up to the point where the press
forming start temperature is 400.degree. C. or higher, almost no
deterioration of shape fixability is observed.
[0086] As described above, in the cooling step, by cooling the
edges of the heated coated steel sheet, with the edges squeezed
between the die and the blank holder, to a temperature of
550.degree. C. or lower to 400.degree. C. or higher at a cooling
rate of 100.degree. C./s or higher before the start of press
forming, it becomes possible for the press forming part to have a
sufficient strength, and it becomes possible to prevent the
occurrence of micro-cracks, prevent an increase in the load of
press forming, and achieve satisfactory shape fixability.
[0087] While the method for cooling the coated steel sheet 1 in the
tool of press forming prior to press forming is not particularly
limited, as mentioned above, cooling with the blank holder 5 is
preferable because it facilitates controlling surface temperature.
FIG. 12 illustrates an exemplary cooling method with blank holder
5.
[0088] In FIG. 12(a), the holding position of the blank holder 5 is
set above the upper surface of the punch 7, the coated steel sheet
1 is squeezed between the die 3 and the blank holder 5, and then
cooling is performed during the slidable movement of the die 3
until the coated steel sheet is brought into contact with the punch
7. At this time, the cooling time of the coated steel sheet 1 can
be controlled by the slidable movement speed. After starting press
forming, it is preferable for the slidable movement speed to be
fast in order to prevent reduction in productivity and press
formability associated with the temperature drop of the coated
steel sheet 1, and it is desirable to change the slidable movement
speed before press forming and during press forming depending on
needs. However, with some press machines, it may be difficult to
freely change the slidable movement speed as described above, and
even if the slidable movement speed during press forming is the
same as or slower than the movement speed before press forming, the
effect of the disclosure will not be impaired as long as the
cooling effect by the tool of press forming is obtained during the
slidable movement.
[0089] Further, the press forming start temperature at which the
press forming is started is normally controlled by the cooling
time. For example, the relation between the time of cooling in the
tool of press forming and the decrease in blank temperature is
measured beforehand, and based on this relation, the press forming
start temperature is controlled. It is also possible to dispose
temperature measuring elements such as a thermocouple on the
surface of the tool of press forming to directly measure the
temperature of the coated steel sheet 1 and control the press
forming start temperature.
[0090] Further, in order to suppress the rise in temperature of the
tool of press forming during continuous press forming and reduce
the variation in the cooling rate, it is also possible to perform
cooling of the tool of press forming by disposing water cooling
piping in the die 3 or the blank holder 5, or to use material with
high thermal conductivity for the surface of the die 3 or the blank
holder 5.
[0091] As shown in FIG. 12(b), it is also possible to squeeze the
coated steel sheet 1 between the die 3 and the blank holder 5, and
then stop the slidable movement for a certain period of time to
cool the coated steel sheet 1, and then perform press forming.
[0092] Further, as shown in FIG. 12(c), pressing may be performed
by setting the holding position of the blank holder 5 above the
upper surface of the punch 7, squeezing the coated steel sheet 1
between the die 3 and the blank holder 5 and stopping for a certain
period of time, and then performing slidable movement. In such
case, the stop time and the slidable movement time until the coated
steel sheet 1 and the punch 7 are brought into contact added
together is the cooling time of the coated steel sheet 1 before
press forming. FIG. 12(d) is an example of utilizing a pad 10. For
the unformed part, it is preferable that cooling is started
quickly. It is also possible to utilize the pad 10 and start
cooling with the pad 10 abutted against the unformed part before
press forming.
[0093] While FIG. 12(d) is an example of utilizing the pad 10 for
FIG. 12(a), the pad 10 can also be utilized in a similar way for
the examples of FIG. 12(b) and FIG. 12(c).
[0094] Although the press forming machine to be used is not
particularly limited, when the slidable movement speed is changed
in FIG. 12(a), or when control is performed in which the slidable
movement is temporarily stopped as in FIG. 12(B) and FIG. 12(c), a
servo-press machine needs to be used.
[0095] Further, the press forming method is not particularly
limited either. Possible methods include draw forming where forming
is performed with the coated steel sheet 1 squeezed between the die
3 and the blank holder 5 as shown in FIG. 13(a), or crush forming
where the coated steel sheet 1 is squeezed between the die 3 and
the blank holder 5 to be cooled and then forming is performed with
the blank holder 5 once detached from the coated steel sheet 1 as
shown in FIG. 13(b). From the perspective of suppressing
micro-cracks, crush forming in which the degree of processing the
sidewall portion is small is preferable.
[0096] <Quenching (S3)>
[0097] In the quenching (S3), a formed body 1' after the press
forming is quenched with the formed body 1' held at the press
bottom dead center while being squeezed by the tool of press
forming. In order to quench the formed body after press forming,
the slidable movement is stopped at the press bottom dead center
after press forming. Although the stop time i.e. the holding time
at the press bottom dead center differs depending on the amount of
heat extraction by the tool of press forming, it is preferably 3
seconds or more. Further, although the upper limit is not
particularly limited, it is preferably 20 seconds or less from the
perspective of productivity.
[0098] In order to hold the formed body in the tool of press
forming for a certain period of time to obtain a quenched
structure, for example, a hot rolled steel sheet or cold rolled
steel sheet having a chemical composition that includes (consists
of), in mass %, C: 0.15% or more and 0.50% or less, Si: 0.05% or
more and 2.00% or less, Mn: 0.50% or more and 3.00% or less, P:
0.10% or less, S: 0.050% or less, Al: 0.10% or less and N: 0.010%
or less, with the balance including Fe and inevitable impurities is
preferably used as the steel sheet. The reasons of limitation of
each component are given below. Here, "%" indicating the content of
each component is "mass %", unless otherwise stated.
[0099] C: 0.15% or More and 0.50% or Less
[0100] C is an element that improves the strength of steel. To
enhance the strength of the hot pressed part, the C content is
preferably 0.15% or more. When the C content exceeds 0.50%, on the
other hand, the weldability of the hot press formed part and the
blanking workability of the raw material (steel sheet) decrease
significantly. Accordingly, the C content is preferably 0.15% or
more and 0.50% or less, and more preferably 0.20% or more and 0.40%
or less.
[0101] Si: 0.05% or More and 2.00% or Less
[0102] Si is an element that improves the strength of steel, as
with C. To enhance the strength of the hot pressed part, the Si
content is preferably 0.05% or more. When the Si content exceeds
2.00%, on the other hand, a surface defect called red scale
increases significantly during hot rolling when manufacturing the
steel sheet. Accordingly, the Si content is preferably 0.05% or
more and 2.00% or less, and more preferably 0.10% or more and 1.50%
or less.
[0103] Mn: 0.50% or More and 3.00% or Less
[0104] Mn is an element that enhances the quench hardenability of
steel, and is effective in suppressing the ferrite transformation
of the steel sheet and improving quench hardenability in the
cooling process after the hot press forming. Mn also has a function
of decreasing the Ac.sub.3 transformation temperature, and so is an
element effective in lowering the heating temperature of the coated
steel sheet 1 before the hot pressing. To achieve these effects,
the Mn content is preferably 0.50% or more. When the Mn content
exceeds 3.00%, on the other hand, Mn segregates and the uniformity
of the characteristics of the steel sheet and hot press formed part
declines. Accordingly, the Mn content is preferably 0.50% or more
and 3.00% or less, and more preferably 0.75% or more and 2.50% or
less.
[0105] P: 0.10% or Less
[0106] When the P content exceeds 0.10%, P segregates to grain
boundaries, and the low temperature toughness of the steel sheet
and hot press formed part decreases. Accordingly, the P content is
preferably 0.10% or less, and more preferably 0.01% or less.
Excessively reducing P, however, leads to longer refining time and
higher cost, and accordingly, P content is preferably 0.003% or
more.
[0107] S: 0.050% or Less
[0108] S is an element that forms a coarse sulfide by combining
with Mn and causes a decrease in ductility of steel. The S content
is preferably reduced as much as possible, though up to 0.050% is
allowable. Accordingly, the S content is preferably 0.050% or less,
and more preferably 0.010% or less. Excessively reducing S,
however, leads to longer refining time and higher cost, and
accordingly, S content is preferably 0.001% or more.
[0109] Al: 0.10% or Less
[0110] When the Al content exceeds 0.10%, oxide inclusions in steel
increase, and the ductility of steel declines. Accordingly, the Al
content is preferably 0.10% or less, and more preferably 0.07% or
less. Meanwhile, Al functions as a deoxidizer, and so the Al
content is preferably 0.01% or more to improve the cleanliness of
steel.
[0111] N: 0.010% or Less
[0112] When the N content exceeds 0.010%, nitrides such as AlN form
in the steel sheet, which causes lower formability during hot
pressing. Accordingly, the N content is preferably 0.010% or less,
and more preferably 0.005% or less. Excessively reducing N,
however, leads to longer refining time and higher cost, and
accordingly, N content is preferably 0.001% or more.
[0113] These are the preferable basic components of the steel
sheet. The steel sheet may further include the following elements
when necessary.
[0114] At least one type selected from the group consisting of Cr:
0.01% or more and 0.50% or less, V: 0.01% or more and 0.50% or
less, Mo: 0.01% or more and 0.50% or less, and Ni: 0.01% or more
and 0.50% or less Cr, V, Mo, and Ni are each an element effective
in enhancing the quench hardenability of steel. This effect is
achieved when the content is 0.01% or more for each of the
elements. When the content exceeds 0.50% for each of Cr, V, Mo, and
Ni, however, the effect saturates and the cost increases.
Accordingly, in the case where at least one type of Cr, V, Mo, and
Ni is included, the content is preferably 0.01% or more and 0.50%
or less, and more preferably 0.10% or more and 0.40% or less.
[0115] Ti: 0.01% or More and 0.20% or Less
[0116] Ti is effective for strengthening steel. The strengthening
effect of Ti is achieved when the content is 0.01% or more. Ti
within the specified range can be used to strengthen steel without
any problem. When the Ti content exceeds 0.20%, however, the effect
saturates and the cost increases. Accordingly, in the case where Ti
is included, the content is preferably 0.01% or more and 0.20% or
less, and more preferably 0.01% or more and 0.05% or less.
[0117] Nb: 0.01% or More and 0.10% or Less
[0118] Nb is also effective for strengthening steel. The
strengthening effect of Nb is achieved when the content is 0.01% or
more. Nb within the specified range can be used to strengthen steel
without any problem. When the Nb content exceeds 0.10%, however,
the effect saturates and the cost increases. Accordingly, in the
case where Nb is included, the content is preferably 0.01% or more
and 0.10% or less, and more preferably 0.01% or more and 0.05% or
less.
[0119] B: 0.0002% or More and 0.0050% or Less
[0120] B is an element that enhances the quench hardenability of
steel, and is effective in suppressing the generation of ferrite
from austenite grain boundaries and obtaining a quenched structure
when cooling the steel sheet after the hot press forming. This
effect is achieved when the B content is 0.0002% or more. When the
B content exceeds 0.0050%, however, the effect saturates and the
cost increases. Accordingly, in the case where B is included, the
content is preferably 0.0002% or more and 0.0050% or less, and more
preferably 0.0005% or more and 0.0030% or less.
[0121] Sb: 0.003% or More and 0.030% or Less
[0122] Sb has an effect of suppressing a decarburized layer
generated in the surface layer part of the steel sheet from when a
steel sheet is heated before the hot press forming to when the
steel sheet is cooled by the process of hot press forming. To
achieve this effect, the Sb content is preferably 0.003% or more.
When the Sb content exceeds 0.030%, however, the rolling load
increases during steel sheet manufacture, which may cause lower
productivity. Accordingly, in the case where Sb is included, the
content is preferably 0.003% or more and 0.030% or less, and more
preferably 0.005% or more and 0.010% or less.
[0123] The components (balance) other than the above components are
Fe and inevitable impurities.
[0124] The manufacturing condition of the coated steel sheet 1 used
as the raw material of the hot press formed part is not
particularly limited. The manufacturing condition of the steel
sheet is not particularly limited. For example, a hot rolled steel
sheet (pickled steel sheet) having a predetermined chemical
composition or a cold rolled steel sheet obtained by subjecting the
hot rolled sheet to cold rolling may be used as the steel
sheet.
[0125] The condition when forming the Zn--Ni coating layer on the
surface of the steel sheet to obtain the coated steel sheet 1 is
not particularly limited. In the case where a hot rolled steel
sheet (pickled steel sheet) is used as the steel sheet, the coated
steel sheet 1 may be obtained by subjecting the hot rolled steel
sheet (pickled steel sheet) to Zn--Ni coating treatment.
[0126] In the case where a cold rolled steel sheet is used as the
steel sheet, the coated steel sheet 1 may be obtained by subjecting
the cold rolled steel sheet to Zn--Ni coating treatment after cold
rolling.
[0127] In the case of forming a Zn--Ni coating layer on the surface
of the steel sheet, for example, the Zn--Ni coating layer may be
formed by cleaning and pickling the steel sheet, and then
subjecting the steel sheet to electroplating treatment with a
current density of 10 A/dm.sup.2 or more and 150 A/dm.sup.2 or less
in a plating bath having a pH of 1.0 or more and 3.0 or less and a
bath temperature of 30.degree. C. or more and 70.degree. C. or less
and containing: 100 g/L or more and 400 g/L or less nickel sulfate
hexahydrate; and 10 g/L or more and 400 g/L or less zinc sulfate
heptahydrate. In the case where a cold rolled steel sheet is used
as the steel sheet, the cold rolled steel sheet may be subjected to
annealing treatment before the cleaning and pickling. The Ni
content in the coating layer may be set to a desired Ni content
(for example, 9 mass % or more and 25 mass % or less) by
appropriately adjusting the concentration of the zinc sulfate
heptahydrate or the current density within the above-mentioned
range. The coating weight of the Zn--Ni coating layer may be set to
a desired coating weight (for example, 10 g/m.sup.2 or more and 90
g/m.sup.2 or less per side) by adjusting the current passage
time.
Examples
[0128] Experiments were performed to confirm the effect of the
method for manufacturing a hot press forming part described herein.
These experiments will be described below.
[0129] Steels having the compositions shown in Table 1 were each
smelted into a casting slab, and the casting slab was heated to
1200.degree. C., hot rolled at a finisher delivery temperature of
870.degree. C., and coiled at 600.degree. C. to obtain a hot rolled
steel sheet.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Ac.sub.3
transformation Steel C Si Mn P S Al N Cr V Mo Ni Ti Nb B Sb
temperature (.degree. C.) A 0.21 0.23 1.51 0.02 0.003 0.03 0.004 --
-- -- -- -- -- -- -- 808 B 0.32 0.64 1.15 0.01 0.005 0.02 0.003 --
-- -- -- -- -- -- -- 804 C 0.18 1.30 2.30 0.03 0.007 0.04 0.004 --
-- -- -- -- -- -- -- 850 D 0.25 0.12 0.75 0.02 0.003 0.03 0.003 --
-- -- -- -- -- -- -- 817 E 0.23 0.35 0.82 0.02 0.004 0.03 0.005
0.40 -- -- -- -- -- -- -- 830 F 0.26 0.15 1.75 0.01 0.002 0.04
0.002 -- -- -- 0.15 0.04 0.02 -- -- 784 G 0.20 0.20 1.43 0.02 0.003
0.02 0.003 -- 0.20 -- -- 0.02 -- 0.0025 -- 807 H 0.25 0.50 1.30
0.01 0.002 0.03 0.004 -- -- 0.30 -- -- -- -- 0.007 811
[0130] The hot rolled steel sheet was then pickled and cold rolled
with a rolling reduction of 50%, to obtain a cold rolled steel
sheet with a thickness of 1.6 mm. The Ac.sub.3 transformation
temperature in Table 1 was calculated according to the following
Formula (1) (see William C. Leslie, The Physical Metallurgy of
Steels, translation supervised by Nariyasu Kouda, translated by
Hiroshi Kumai and Tatsuhiko Noda. Maruzen Co., Ltd., 1985, p.
273).
Ac.sub.3(.degree. C.)=910-203
[C]+44.7.times.[Si]-30.times.[Mn]+700.times.[P]/400.times.[Al]
(1)
where [C], [Si], [Mn], [P], and [Al] are the contents (mass %) in
steel of the respective elements (C, Si, Mn, P, and Al).
[0131] Using the cold rolled steel sheet obtained as described
above as a steel sheet, each of a pure Zn coating layer, a Zn--Fe
coating layer, and a Zn--Ni coating layer was formed on the surface
of the steel sheet to obtain a coated steel sheet 1. Each coating
layer was formed under the following condition.
[0132] <Pure Zn Coating Layer>
[0133] The cold rolled steel sheet was passed through a continuous
galvanizing line, heated to a temperature range of 800.degree. C.
or more and 900.degree. C. or less at a heating rate of 10.degree.
C./s, and held in the temperature range for 10 s or more and 120 s
or less. After this, the cold rolled steel sheet was cooled to a
temperature range of 460.degree. C. or more and 500.degree. C. or
less at a cooling rate of 15.degree. C./s, and immersed into a
galvanizing bath of 450.degree. C. to form a Zn coating layer. The
coating weight of the Zn coating layer was adjusted to a
predetermined coating weight by the gas wiping method.
[0134] <Zn-Fe Coating Layer>
[0135] The cold rolled steel sheet was passed through a continuous
galvanizing line, heated to a temperature range of 800.degree. C.
or more and 900.degree. C. or less at a heating rate of 10.degree.
C./s, and held in the temperature range for 10 s or more and 120 s
or less. After this, the cold rolled steel sheet was cooled to a
temperature range of 460.degree. C. or more and 500.degree. C. or
less at a cooling rate of 15.degree. C./s, and immersed into a
galvanizing bath of 450.degree. C. to form a Zn coating layer. The
coating weight of the Zn coating layer was adjusted to a
predetermined coating weight by the gas wiping method. Immediately
after adjusting the Zn coating layer to the predetermined coating
weight by the gas wiping method, the cold rolled steel sheet was
heated to 500.degree. C. to 550.degree. C. in an alloying furnace
and held for 5 s to 60 s, to form a Zn--Fe coating layer. The Fe
content in the coating layer was set to a predetermined content by
changing the heating temperature in the alloying furnace or the
holding time at the heating temperature within the above-mentioned
range.
[0136] <Zn-Ni Coating Layer>
[0137] The cold rolled steel sheet was passed through a continuous
annealing line, heated to a temperature range of 800.degree. C. or
more and 900.degree. C. or less at a heating rate of 10.degree.
C./s, and held in the temperature range for 10 s or more and 120 s
or less. After this, the cold rolled steel sheet was cooled to a
temperature range of 500.degree. C. or less at a cooling rate of
15.degree. C./s. The cold rolled steel sheet was then cleaned and
pickled, and subjected to electroplating treatment of applying
current for 10 s to 100 s with a current density of 30 A/dm.sup.2
to 100 A/dm.sup.2 in a plating bath having a pH of 1.3 and a bath
temperature of 50.degree. C. and containing: 200 g/L nickel sulfate
hexahydrate; and 10 g/L to 300 g/L zinc sulfate heptahydrate, thus
forming a Zn--Ni coating layer. The Ni content in the coating layer
was set to a predetermined content by appropriately adjusting the
concentration of the zinc sulfate heptahydrate or the current
density within the above-mentioned range. The coating weight of the
Zn--Ni coating layer was set to a predetermined coating weight by
appropriately adjusting the current passage time in the
above-mentioned range.
[0138] A blank sheet of 200 mm r 400 mm was punched from each
coated steel sheet 1 obtained as described above, and heated in an
electric furnace having an air atmosphere. The blank sheet was set
on a tool of press forming (material: SKD61), and then cooling and
press forming was performed using the tool of press forming. After
this, the blank sheet was quenched in the tool of press forming and
released from the tool of press forming, thus manufacturing a press
formed part with a hat-shaped cross section illustrated in FIG. 14.
Regarding the shape of the tool of press forming, a tool of press
forming with a shoulder area of punch R: 6 mm and a shoulder area
of die R: 6 mm was used, and the press forming was performed with a
punch-die clearance of 1.6 mm. Cooling in the tool of press forming
prior to press forming was performed by squeezing the blank sheet
between the die 3 and the blank holder 5. Press forming was
performed by draw forming where forming is performed while applying
a blank holder force of 98 kN, and crush forming where, after the
cooling prior to press forming, the blank holder 5 is removed and
forming is performed without the blank holder. As indicated in
FIGS. 7 and 8, the relation between the time of cooling in the tool
of press forming and the decrease in blank temperature was measured
beforehand, and based on this relation, the press forming start
temperature was obtained using time of cooling in the tool of press
forming until press forming.
[0139] The type of coating layer, heating condition, cooling
condition and press forming condition are shown in Table 2.
TABLE-US-00002 TABLE 2 Coating layer Heating condition Cooling
Condition Press forming Coating Heating Holding Cooling Cooling
Cooling condition weight temp. time start temp. time rate Press
forming Steel Type (g/m.sup.2) (.degree. C.) (s) Cooling method
(.degree. C.) (s) (.degree. C./s) method A Zn-12Ni % 65 900 5
Cooling in tool 720 0.8 190 Crush of press forming Zn-10Ni % 40 870
90 Cooling in tool 690 1.5 147 Draw of press forming Zn-22Ni % 45
880 30 Cooling in tool 750 1.9 158 Draw of press forming Zn-15Ni %
50 910 10 Cooling in tool 670 0.7 229 Crush of press forming Zn-9Ni
% 60 900 15 Cooling in tool 720 2.5 116 Draw of press forming
Zn-12Ni % 30 850 20 Cooling in tool 780 1.2 233 Crush of press
forming Zn-12Ni % 40 880 5 Cooling in tool 660 0.5 240 Crush of
press forming Zn-12Ni % 65 900 60 Cooling in tool 700 0 -- Crush of
press forming Zn-10Ni % 40 920 90 Cooling in tool 680 0.3 233 Draw
of press forming Zn-22Ni % 45 880 30 Cooling in tool 720 3.5 106
Draw of press forming Zn-15Ni % 50 900 120 Cooling in tool 740 5.0
102 Crush of press forming Zn-12Ni % 65 900 5 Gas cooling 680 1.2
25 Draw Zn-12Ni % 65 880 15 Gas cooling 660 2.8 21 Crush Zn-10Ni %
50 910 5 Gas cooling 700 10.0 17 Crush Zn 45 890 10 Cooling in tool
710 1.0 170 Crush of press forming Zn-10Fe % 50 900 5 Cooling in
tool 730 1.2 167 Draw of press forming B Zn-13Ni % 35 880 60
Cooling in tool 730 1.2 208 Crush of press forming C Zn-11Ni % 50
900 90 Cooling in tool 710 1.8 161 Draw of press forming D Zn-10Ni
% 60 870 30 Cooling in tool 690 1.0 190 Crush of press forming E
Zn-12Ni % 40 890 60 Cooling in tool 170 1.5 140 Crush of press
forming F Zn-10Ni % 60 870 30 Cooling in tool 690 1.8 117 Crush of
press forming G Zn-13Ni % 50 910 30 Cooling in tool 740 1.0 210
Draw of press forming H Zn-15Ni % 30 870 10 Cooling in tool 700 1.1
182 Crush of press forming Press forming condition Evaluation Press
Amount forming Press bottom Surface of steel sheet of mouth start
dead center Presence or Average opening Tensile temp. holding time
absence of crack depth deformation Hardness strength Steel
(.degree. C.) (s) crack (.mu.m) (mm) (Hv) (MPa) Remarks A 530 12
Absence -- 0 465 1520 Example 1 470 15 Absence -- 0 459 1515
Example 2 450 10 Absence -- 0 471 1550 Example 3 510 8 Absence -- 0
460 1520 Example 4 430 10 Absence -- 0 463 1530 Example 5 500 15
Absence -- 0 482 1570 Example 6 540 12 Absence -- 0 458 1505
Example 7 700 10 Presence 7 0 463 1500 Comparative Example 1 610 15
Presence 12 0 452 1480 Comparative Example 2 350 12 Absence -- 8
481 1560 Comparative Example 3 230 8 Absence -- 10 485 1570
Comparative Example 4 650 15 Presence 14 0 452 1490 Comparative
Example 5 600 10 Presence 8 2 378 1160 Comparative Example 6 530 10
Absence -- 3 341 1080 Comparative Example 7 540 15 Presence 35 0
465 1505 Comparative Example 8 530 15 Presence 5 0 473 1550
Comparative Example 9 B 480 10 Absence -- 0 557 1720 Example 8 C
420 3 Absence -- 0 425 1330 Example 9 D 500 15 Absence -- 0 493
1580 Example 10 E 500 10 Absence -- 0 475 1540 Example 11 F 480 12
Absence -- 0 497 1590 Example 12 G 530 15 Absence -- 0 460 1500
Example 13 H 500 7 Absence -- 0 492 1580 Example 14
[0140] A sample was collected from the side wall portion of each
obtained press formed part with a hat-shaped cross section, and the
section of its surface was observed using a scanning electron
microscope (SEM) with 1000 magnification for 10 fields per sample,
to examine the presence or absence of micro-cracks (minute cracking
in the surface of the sample, which passes through the interface
between the coating layer and the steel sheet and reaches the
inside of the steel sheet) and the average depth of micro-cracks.
The average depth of micro-cracks was calculated by averaging the
micro-crack depths of any 20 micro-cracks. The micro-crack depth
mentioned here means the length (length h in FIG. 15) of a
micro-crack 11 measured from the interface between a coating layer
13 and a steel sheet 15 toward the center in the thickness
direction, as illustrated in FIG. 15. In the case where the number
of micro-cracks observed was less than 20, the average depth of the
depths of all observed micro-cracks was used.
[0141] Regarding the shape accuracy of the obtained press formed
part, the difference between the width W of the press forming part
after die release and the width W.sub.0 of the press forming part
conformed to the tool of press forming of the hat-shaped section
part shown in FIG. 16 (W-W.sub.0) was evaluated as the amount of
mouth opening deformation.
[0142] Furthermore, a sample for hardness measurement was collected
from the side wall portion of each obtained press formed part. The
hardness of the cross section of the sample was measured using a
micro-Vickers hardness meter. A test was conducted with a test load
of 9.8 N at 5 points in the center position in the thickness
direction, and the average value thereof was used as the hardness
of the samples. Here, the target hardness is 380 Hv or more.
[0143] In addition, a JIS No. 13 B tensile test piece was collected
from the side wall portion of each obtained press formed part. A
tensile test was conducted using the collected test piece according
to JIS G 0567 (1998), to measure the tensile strength at room
temperature (22.+-.5.degree. C.). The tensile test was conducted at
a crosshead speed of 10 mm/min.
[0144] These results are also shown in Table 2.
[0145] In examples 1 to 12, the type of coating layer (Zn--Ni
coating layer), cooling method (cooling in tool of press forming),
cooling rate (appropriate range: 100.degree. C./s or more), and
press forming start temperature (appropriate range: 400.degree. C.
to 550.degree. C.) are all within the range of the disclosure.
[0146] In each of the samples after pressing of examples 1 to 12,
micro-cracks were not generated and the amount of mouth opening
deformation was 0 mm. From these results, it can be seen that, with
the press forming method described herein, it is possible to
suppress the occurrence of micro-cracks while ensuring good shape
fixability. Further, in all of examples 1 to 12, the hardness was
380 Hv or more, and tensile strength was 1180 MPa or more.
[0147] In comparative example 1, the type of coating layer is a
Zn--Ni coating layer. However, forming was performed without
performing cooling in the tool of press forming. Further, in
comparative examples 2 to 4, the type of coating layer is a Zn--Ni
coating layer. However, the press forming start temperatures for
each of comparative examples 2 to 4 were out of the appropriate
range. The press forming start temperature for comparative example
2 was 610.degree. C. which is higher than the appropriate range,
and the press forming start temperatures for comparative examples 3
and 4 were 350.degree. C. and 230.degree. C. which are lower than
the appropriate range.
[0148] As for the samples after pressing of comparative examples 1
and 2, the amount of mouth opening deformation is 0 mm. However,
micro-cracks are generated. From these results, it can be seen
that, when the press forming start temperature of the steel sheet
is higher than 550.degree. C., micro-cracks are generated.
[0149] In comparative examples 3 and 4, micro-cracks are not
generated. However, the amount of mouth opening deformation is 8 mm
to 10 mm. From these results, it can be seen that when the cooling
time is too long and the forming start temperature of the steel
sheet becomes lower than 400.degree. C., the strength of the steel
sheet increases, and thus the shape fixability decreases.
[0150] In comparative examples 5 to 7, the type of coating layer is
a Zn--Ni coating layer. However, the cooling method is gas cooling
and the cooling rate is not 100.degree. C./s or more. Therefore, in
comparative examples 5 and 6, the press forming start temperatures
of the steel sheet are out of the appropriate range (over
550.degree. C.), and micro-cracks are generated. Further, in
comparative example 7, the press forming start temperature of the
steel sheet was 530.degree. C. which is within the appropriate
range. However, the amount of mouth opening deformation was 3 mm,
and shape fixability was decreased. This is because, since the
cooling method was gas cooling, the cooling rate was slow and the
metallographic structure during the press forming was ferrite or
bainite instead of an austenite single phase, and therefore the
martensitic transformation after forming was decreased, and the
stress introduced during the forming was hardly eased. As a result,
it is believed that, angle change such that the angle formed by two
faces across the bending ridgeline becomes large relative to the
die angle occurred.
[0151] Further, in comparative examples 6 and 7, quenching was
performed after mild cooling to a certain degree by gas cooling and
pressing. Therefore, the hardness of the samples after pressing was
decreased.
[0152] In comparative examples 8 and 9, the cooling method (cooling
in tool of press forming), cooling rate (167.degree. C./s,
170.degree. C./s), and press forming start temperature (530.degree.
C. to 540.degree. C.) are appropriate. However, the type of coating
layer is different. Specifically, comparative example 8 is a
coating layer of only Zn, and comparative example 9 is a coating
layer of Zn--Fe, and therefore micro-cracks are generated in the
samples after pressing.
REFERENCE SIGNS LIST
[0153] 1 Coated steel sheet [0154] 1' Formed body [0155] 3 Die
[0156] 5 Blank holder [0157] 7 Punch [0158] 9 Steel sheet [0159] 10
Pad [0160] 11 Micro-crack [0161] 13 Coating layer [0162] 15 Steel
sheet [0163] 16 Thermocouple
* * * * *