U.S. patent application number 13/084931 was filed with the patent office on 2011-10-20 for hot press forming method.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Hiroyuki Eguchi, Toshifumi Matsuda, Kohei Teshima.
Application Number | 20110252856 13/084931 |
Document ID | / |
Family ID | 44787105 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110252856 |
Kind Code |
A1 |
Matsuda; Toshifumi ; et
al. |
October 20, 2011 |
HOT PRESS FORMING METHOD
Abstract
A steel plate is heated so that an average temperature becomes
Ac3 point or higher, a press forming is performed on the steel
plate by use of a mold, and the steel plate is cooled during press
forming. A formed product is removed from the mold, and the formed
product is cooled further. In the hot press forming method,
although a quantity of change in shape of the formed product
decreases as a removal temperature of the formed product decreases.
When the removal temperature of the formed product is lower than
322.degree. C., no more improvement on an accuracy of the formed
product can be realized, and a time during which the mold is
occupied exclusively to cool the formed product becomes long. When
the removal temperature of the formed product is 510.degree. C. or
lower, the rate of change of the quantity of change in shape with
change in temperature is increased drastically, whereby an increase
in accuracy of the formed product can be realized. Consequently,
the removal temperature of the formed product is set to a
temperature range from 322.degree. C. or higher to 510.degree. C.
or lower.
Inventors: |
Matsuda; Toshifumi;
(Tochigi, JP) ; Teshima; Kohei; (Tochigi, JP)
; Eguchi; Hiroyuki; (Tochigi, JP) |
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
44787105 |
Appl. No.: |
13/084931 |
Filed: |
April 12, 2011 |
Current U.S.
Class: |
72/364 |
Current CPC
Class: |
B21D 22/022 20130101;
C21D 11/00 20130101; C21D 1/673 20130101; C21D 2211/008 20130101;
C21D 11/005 20130101; C21D 6/00 20130101 |
Class at
Publication: |
72/364 |
International
Class: |
B21D 31/00 20060101
B21D031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 14, 2010 |
JP |
2010-092863 |
Claims
1. A hot press forming method comprising: heating a steel plate so
that an average temperature of the steel plate becomes Ac3 point or
higher; executing a press forming on the steel plate whose average
temperature is so set by use of a mold; cooling the steel plate
during press forming; removing a formed product from the mold,
while the average temperature of the formed product stays within a
temperature range from 322.degree. C. or higher to 510.degree. C.
or lower; and cooling the formed product further.
2. The method according to claim 1, further comprising: setting the
average temperature of the steel plate to 366.degree. C. or higher,
when the steel plate is removed from the mold.
3. The method according to claim 1, further comprising: setting the
average temperature of the steel plate to 451.degree. C. or lower,
when the steel plate is removed from the mold.
4. The method according to claim 1, further comprising: setting the
average temperature of the steel plate to 451.degree. C. or lower,
when the steel plate is removed from the mold.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a hot press forming method
for press forming a heated steel plate and more particularly to an
improvement in technique for removing a formed steel plate from a
mold.
[0003] 2. Related Art
[0004] In a press forming method, for example, a mold 100 shown in
FIGS. 4A to 4C is used. In the mold 100, as is shown in FIG. 4A,
when an upper mold 110 is lowered towards a lower die 120 after a
workpiece W is placed on an upper surface of the lower die 120, a
pad 112 of the upper mold 110 is brought into abutment with the
workpiece W and then presses the workpiece W to thereby effect a
form shaping on the steel plate.
[0005] Next, when the upper mold 110 is lowered further following
the form shaping, the abutment state of the pad 112 with the
workpiece W is maintained. Then, a trimming blade 115 of a trimming
portion 111 of the upper mold 110 is brought into abutment with the
workpiece W, and a cutting operation of the workpiece W by the
trimming blade 15 is effected. A piercing blade 114 of the upper
mold 110 is brought into abutment with the workpiece W, and a
cutting operation of the workpiece W by the piercing blade 114 is
effected. Reference numeral 113 denotes an elastic member which
supports the pad 112, reference numeral 121 a trimming blade
provided on the lower die 120, and reference numeral 122 a hole
portion into which a portion of the workpiece W which is cut by the
piercing blade 114 falls.
[0006] In the hot press forming method, the steel plate which is
heated at a high temperature of 800.degree. C. or higher is used as
the workpiece, and the press forming is performed by the mold 100,
and the press formed plate steel is cooled. A hot press forming
method like the one described above is promising as a method for
obtaining a highly strong press formed product. Specifically,
making the steel plate highly strong is realized by changing the
structure of the steel plate from an austenite structure (resulting
when the steel plate is heated) to a martensite structure
(executing so-called quenching) at the same time as press forming
by making use of the press mold 100 as a heat removal material (a
refrigerant) for removing heat that the steel plate possesses.
[0007] In the method above, it is generally necessary that the
cooling speed of the steel plate is equal to or faster than the
quenching limit speed (27.degree. C./s) and that the temperature of
the formed product is reduced to 100 to 200.degree. C. just after
it is removed from the press mold to complete the quenching.
[0008] However, since the mold needs to be occupied exclusively for
cooling the formed product in the way described above until the
cooling is completed, compared with a cold press forming method,
the hot press forming method requires a longer cycle time, and
hence, the productivity thereof becomes worse than that of the cold
press forming method. Then, to shorten the cycle time,
JP-A-2005-288528 proposes that the formed product be removed from
the press mold prior to cooling for quenching in a separate step.
However, as this occurs, since the formed product is removed from
the press mold prior to cooling, there exists a serious risk of
occurrence of a deformation in the shape of the formed product in
association with cooling, making it difficult to ensure the
accuracy of the formed product with respect to shape.
[0009] With a view to solving the problem, deep studies have been
made on hot press forming methods, and as a result thereof, it has
been found that the accuracy of a formed product depends on the
temperature thereof when the formed product is removed from the
mold after press forming and that there exists a critical point in
the dependency on temperature of the accuracy of the formed
product. Thus, the invention has been completed based on these
findings.
SUMMARY OF THE INVENTION
[0010] One or more embodiments of the invention provide a hot press
forming method which can not only shorten a cycle time but also
ensure the accuracy of a formed product with respect to shape.
[0011] In accordance with one or more embodiments, a hot press
forming method may include the steps of: heating a steel plate so
that an average temperature of the steel plate becomes Ac3 point or
higher; executing a press forming on the steel plate whose average
temperature is so set by use of a mold; cooling the steel plate
during press forming; removing a formed product from the mold,
while the average temperature of the formed product stays within a
temperature range from 322.degree. C. or higher to 510.degree. C.
or lower; and cooling the formed product further.
[0012] The "average temperature" denotes an average value of
temperatures of a plurality of locations which are set over a whole
area of the formed product.
[0013] In the hot press forming method, although the accuracy of
the formed product increases as the average temperature of the
formed product when it is removed from the mold (hereinafter,
referred to as a removal temperature) decreases, the accuracy of
the formed product becomes substantially constant when the removal
temperature of the formed product becomes lower than 322.degree.
C., and no more improvement in accuracy cannot be realized.
Moreover, in this case, the time during which the mold is occupied
exclusively to cool the formed product becomes long. On the other
hand, when the removal temperature is 510.degree. C. or lower, the
degree at which the accuracy of the formed product increases with
change in temperature increases drastically.
[0014] Based on the understanding above, shortening the cycle time
is compatible with ensuring the accuracy of the formed product by
setting the removal temperature of the formed product to the
temperature range from 322.degree. C. or higher to 510.degree. C.
or lower.
[0015] The hot press forming method of the invention can adopt
various configurations. The degree at which the accuracy of the
formed product increases with change in temperature becomes the
largest when the removal temperature of the formed product stays in
a temperature range from 366.degree. C. or higher to 451.degree. C.
or lower. Consequently, a lower limit value for the removal
temperature of the formed product is more preferably set to
366.degree. C., and an upper limit of the removal temperature of
the formed product is more preferably set to 451.degree. C. or
lower.
[0016] According to the hot press forming method of the
embodiments, shortening the cycle time can be made compatible with
ensuring the accuracy of the formed product by setting the removal
temperature of the formed product to the temperature range from
322.degree. C. or higher to 510.degree. C. or lower.
[0017] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a graph showing a relationship between a
temperature of a workpiece (a steel plate) and time in each step of
a hot press forming method according to an exemplary
embodiment.
[0019] FIG. 2 is a graph showing a relationship between quantity of
change in shape of a formed product obtained by the hot press
forming method according to the exemplary embodiment and removal
temperature.
[0020] FIGS. 3A and 3B are diagrams explaining dimensions of formed
products obtained by the hot press forming method according to the
exemplary embodiment.
[0021] FIGS. 4A to 4C are side sectional diagrams representing
steps of the press forming method.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0022] Hereinafter, the invention will be described in detail by
reference to an exemplary embodiment. In the exemplary embodiment,
a draw forming is performed on heated steel plates which are
introduced into a mold. Average temperatures of formed products
when they are removed from the mold (hereinafter, referred to as
removal temperatures) are changed formed product by formed product,
and accuracies of the formed products are evaluated.
[0023] FIG. 1 is a graph showing a relationship between a
temperature of a workpiece (a steel plate) and time in each step of
a hot press forming method according to the exemplary embodiment.
Temperatures in the exemplary embodiment are average values
(average temperatures) of temperatures measured at a plurality of
points on steel plates by a thermo-viewer. Since temperatures of
steel plates cannot be measured during press forming, in FIG. 1,
temperatures of steel plates during press forming are obtained from
temperatures of formed products measured when press forming is
initiated and when press forming is completed on the assumption
that the temperatures of the steel plates change linearly during
press forming. In FIG. 1, a quenching limit line (a quenching limit
speed of 27.degree. C./s) of the steel plate is drawn as well.
[0024] In the exemplary embodiment, steel plates (whose carbon
content is 0.2 wt % or more) are used as workpieces. Steel plates
are firstly heated to 950.degree. C. in a heating oven (a heating
step). A material of the steel plate suitable for the invention is
an Fe-based material containing, for example, C or C and other
elements and including Fe and impurities that cannot be avoided as
the remaining portion thereof. This material preferably contains 15
wt % or more C to ensure a required hardness in quenching, and as
this occurs, Cr, Mo, Ti and B may be added as required for
quenching properties. In addition to the elements above, Si, Mn, P,
S and the like may also be added as required.
[0025] After the completion of heating, the steel plates are
removed from the heating oven, and the steel plate which are still
being heated are installed into a mold (an installation step).
Following this, the steel plates are press formed by the mold, and
the steel plates are cooled (quenching) during press forming (a
forming step). This forming step includes a step of cooling the
steel plates with the mold held at its bottom dead center (an
in-mold restrained cooling step by removing heat by the mold). In
this case, quenching is designed to start during press forming.
[0026] In the press forming described above, two types of formed
products having sectional shapes shown in FIGS. 3A, 3B are obtained
by use of two types of molds. FIG. 3A shows a sectional shape of a
formed product whose formed height is low, and FIG. 3B shows a
sectional shape of a formed product whose formed height is high.
Following this, the formed products are removed from the molds to
be transferred (a transfer step), and the formed products are
cooled (quenching) (a cooling step).
[0027] In the exemplary embodiment, a cooling speed during press
forming by the mold is set to fall within a range from 50 to
100.degree. C./s, and a removal temperature of formed products is
set to fall within a range from 250.degree. C. to 650.degree. C.
The graph shown in FIG. 1 shows the results of specific examples of
steel plates which are set to fall within the temperature range. In
the specific examples, a time from the end of heating of the steel
plate to the removal of the steel plate from the heating oven is
set to 1 second, a time from the removal of the steel plate from
the heating oven to the start of press forming (an installation
time) is set to 2.6 seconds, a press forming time is set to 5.4
seconds, a time from the completion of press forming of the steel
plate to the start of cooling thereof (a transfer time) is set to
1.7 seconds, and a cooling time is set to 2 seconds. The
temperature of the steel plates when press forming is initiated is
850.degree. C. and the temperature of the formed product when press
forming is completed is 390.degree. C.
[0028] A quantity of change t in shape of a flange portion of the
formed product (a different t in height of an edge portion of the
flange portion between a shape actually obtained and a normal shape
(FIGS. 3A, 3B)) is measured on each formed product after the
cooling step. In FIGS. 3A, 3B, a shape indicated by a solid line
represents the normal shape (a desired shape), and shape indicated
by a broken line (only a left-hand side portion is shown)
represents a shape actually obtained. Numeral values in FIG. 3
denote sizes (unit in mm) and angles of portions of sectional
shapes of the formed products. The results of measurements on those
portions of the formed products with a high formed height and the
formed products with a low formed height are shown in Table 2 and
FIG. 2.
TABLE-US-00001 TABLE 1 Formed Height Low Formed Height High
Temperature Accuracy Temperature Accuracy Remarks 261 0 280 0 276
-0.1 296 -0.1 285 0.3 305 0 285 0 306 0.1 286 0.1 322 0.3 Critical
Point [1] 331 0.1 339 0 344 0 362 -0.2 353 -0.2 366 0 Critical
Point [2] 382 0.2 389 0 385 -0.3 393 0.3 390 0 404 0.4 423 -0.1 412
0.5 431 0.1 434 0.7 440 0.6 438 0.1 451 0 443 0.4 459 0.2 447 0.8
471 0.8 450 1.5 502 1.5 451 1.2 Critical Point [3] 504 0.5 471 1
518 0.9 510 2.15 Critical Point [4] 567 1.45 510 1.1 583 2.3 533
2.6 588 2 541 1.9 600 1.6 541 1.9 606 1.1 545 1.7 622 1.95 658 2.9
630 2.2 658 2.6
[0029] FIG. 2 is a graph showing data shown in Table 1. In FIG. 2,
a curve l is a curve showing data on the formed products with a
high formed height and the curve l is also an approximate curve of
maximum values of quantities of change t at individual
temperatures. A curve m is a curve showing data on the formed
products with a low formed height and the curve m is also an
approximate curve of minimum values of quantities of change t at
individual temperatures. The curves l, m are obtained so that all
the data on the formed products with a high formed height and the
formed products with a low formed height are contained between the
curves. A broken line N is an image line emphasizing critical
points on the curve l.
[0030] As is seen from FIG. 2, it is confirmed that although the
quantity of change t in shape of the formed product decreases as
the average temperature of the formed product when it is removed
from the mold (hereinafter, referred to as a removal temperature)
decreases, the quantity of change t in shape of the formed product
becomes substantially constant when the removal temperature of the
formed product becomes lower than 322.degree. C., and no more
improvement in accuracy of the formed product cannot be realized.
Moreover, in this case, the time during which the mold is occupied
exclusively to cool the formed product becomes long. Consequently,
it is confirmed that the removal temperature of the formed product
needs to be set to 322.degree. C. or higher (Critical Point [1]).
On the other hand, it is confirmed that when the removal
temperature is 510.degree. C. or lower, the rate of change of the
quantity of change t in shape of the formed product with change in
temperature increases drastically. Consequently, it is confirmed
that the removal temperature of the formed product needs to be set
to 510.degree. C. or lower (Critical Point [4]).
[0031] Thus, it is confirmed based on the facts above that
shortening the cycle time is compatible with ensuring the accuracy
of the formed product by setting the removal temperature of the
formed product to the temperature range from 322.degree. C. or
higher to 510.degree. C. or lower. Since a force which deforms an
internal shape of the formed product becomes larger than a
resisting force to deformation of the material outside the above
temperature range, it is assumed that a change in accuracy of the
formed product is induced. In contrast to this, since the
relationship between the two forces is reversed, causing the
resisting force to deformation of the material to be larger than
the force which deforms the internal shape of the formed product
within the temperature range, it is considered that the accuracy of
the formed product can be ensured.
[0032] As is seen from FIG. 2, it is confirmed that the degree at
which the accuracy of the formed product increases with change in
temperature becomes the largest when the removal temperature of the
formed product stays in a temperature range from 366.degree. C. or
higher to 451.degree. C. or lower. Consequently, it is confirmed
that in order to obtain the advantage described above with good
efficiency, a lower limit value for the removal temperature of the
formed product is more preferably set to 366.degree. C. (Critical
Point [2]), while an upper limit of the removal temperature of the
formed product is more preferably set to 451.degree. C. (Critical
Point [3]) or lower.
[0033] While description has been made in connection with the
specific exemplary embodiment and the examples thereof, it will be
obvious to those skilled in the art that various changes and
modifications may be made therein without departing from the
present invention. It is aimed, therefore, to cover in the appended
claims all such changes and modifications falling within the true
spirit and scope of the present invention.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0034] 100 mold
* * * * *