U.S. patent application number 14/347531 was filed with the patent office on 2014-12-04 for method of forming parts from sheet steel.
This patent application is currently assigned to Imperial Innovations Limited. The applicant listed for this patent is IMPERIAL INNOVATIONS LIMITED. Invention is credited to Daniel Balint, Trevor Anthony Dean, Jianguo Lin.
Application Number | 20140352388 14/347531 |
Document ID | / |
Family ID | 44994074 |
Filed Date | 2014-12-04 |
United States Patent
Application |
20140352388 |
Kind Code |
A1 |
Balint; Daniel ; et
al. |
December 4, 2014 |
METHOD OF FORMING PARTS FROM SHEET STEEL
Abstract
A method is provided of forming a part from sheet steel. The
method comprises the steps of (a) heating the sheet to a
temperature at which austenitisation occurs; and (b) forming the
sheet between dies into the part, father cooling the formed sheet.
There is an additional step between (a) and (b) of applying cooling
means to the sheet.
Inventors: |
Balint; Daniel; (London,
GB) ; Dean; Trevor Anthony; (West Midlands, GB)
; Lin; Jianguo; (London, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMPERIAL INNOVATIONS LIMITED |
London |
|
GB |
|
|
Assignee: |
Imperial Innovations
Limited
London
GB
|
Family ID: |
44994074 |
Appl. No.: |
14/347531 |
Filed: |
September 27, 2012 |
PCT Filed: |
September 27, 2012 |
PCT NO: |
PCT/GB2012/052399 |
371 Date: |
March 26, 2014 |
Current U.S.
Class: |
72/342.5 |
Current CPC
Class: |
C21D 1/673 20130101;
C21D 9/46 20130101; C21D 1/62 20130101; B21D 22/022 20130101; C21D
9/0062 20130101; C21D 11/00 20130101 |
Class at
Publication: |
72/342.5 |
International
Class: |
B21D 22/02 20060101
B21D022/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 2011 |
GB |
1116668.3 |
Claims
1. A method of forming a part from sheet steel, the method
comprising the steps of: (a) heating the sheet to a temperature at
which austenitisation occurs; and (b) forming the sheet between
dies into the part, further cooling the formed sheet; wherein there
is an additional step between (a) and (b) of applying cooling to
the sheet.
2. A method according to claim 1, wherein the additional step takes
place before the heated sheet is placed between the dies.
3. A method according to claim 1, wherein the additional step takes
place while the heated sheet is placed between the dies.
4. A method according to claim 1, wherein the cooling comprises a
cooling fluid such as a gas, for example air; and/or wherein the
cooling fluid comprises a liquid, for example water, the method
comprising directing the cooling fluid at the heated sheet.
5. A method according to claim 1, wherein the cooling fluid is
directed as a pressurised flow of the fluid.
6. A method according to claim 1, wherein the cooling fluid is
directed as a jet and/or is directed as a mist spray.
7. A method according to claim 1, wherein the cooling fluid is
directed by controlling the duration, temperature and/or mass flow
of the cooling fluid.
8. A method according to claim 1, wherein the cooling comprises
cooling plates, for example cool copper plates.
9. A method according to claim 1, wherein the cooling can be
achieved by increasing the transfer time with natural air
cooling.
10. A method according to claim 1, wherein the additional step
comprises directing the cooling fluid at the heated sheet such that
the sheet is cooled sufficiently rapidly to avoid the steel
entering the bainite phase.
11. A method according to claim 1, wherein the additional step
comprises directing the cooling fluid at the heated sheet such that
the sheet is cooled at more than 25.degree. C./second on
average.
12. A method according to claim 1, wherein the additional step
comprises directing the cooling fluid at the heated sheet such that
the temperature of the sheet remains above the austenitisation
temperature for the steel.
13. A method according to claim 1, wherein the additional step
comprises directing the cooling fluid at the heated sheet such that
the sheet is cooled to between 500.degree. C. and 600.degree.
C.
14. A method according to claim 1 and comprising the further step
of directing cooling fluid to cool the dies.
15. A method according to claim 1 and comprising the further step
of directing cooling fluid to clean the dies.
16. A method according to claim 1, comprising the further step of
directing cooling fluid to cool and clean the dies in the same
step.
Description
FIELD
[0001] The present invention relates to the forming of parts from
metal. In embodiments, it relates to the forming of parts from
metal sheet, such as steel and steel alloys.
BACKGROUND
[0002] Processes using "hot stamping" are emerging as preferred
solutions for forming high-strength parts from steel sheet for
applications in, for example, automotive "body in white" (BiW), and
chassis and suspension (C&S) parts. The development of Boron
steel makes such process feasible for the production of automotive
safety critical panel parts, such as A-pillars, B-pillars, bumpers,
roof rails, rocker rails and floor tunnels for Body-in-White and
tubular parts and twist beams for C&S. The global demand for
such ultra-high-strength steel parts has been growing sharply in
recent years.
[0003] A typical Boron steel hot stamping process is shown in FIG.
1. Essentially it comprises the steps of:
[0004] (1) Heating the steel blank to above its austenitisation
temperature, say 925.degree. C., and soaking at that temperature to
enable all the metal to be transformed into austenite. In this
state the metal is soft and has high ductility (easy to form);
[0005] (2) Quickly transferring the austenitised material blank to
the press;
[0006] (3) Forming the blank into the shape of the component using
a cold die set, which is normally water cooled;
[0007] (4) Holding the formed part within the cold die set for a
certain period (e.g. 6-10 seconds depending on geometry, sheet
thickness, pressure, etc.) for quenching, enabling the hard phase
of the material, e.g. martensite, (for a high strength component)
to be formed; and
[0008] (5) Releasing the die when the part temperature has dropped
to a sufficiently low level, say 250.degree. C., and taking the
component out.
[0009] Such a process is sometimes referred to as a "hot stamping,
cold die forming and quenching" process.
[0010] Most of the heat in the work-piece goes to the die in the
hot stamping process. The cooling rate is largely related to the
tool surface temperature. Even if the die set is water cooled,
under mass production conditions, it is difficult to keep the tool
surface temperature sufficiently low. A high tool surface
temperature causes the following problems:
[0011] In this conventional hot stamping process for forming
complex parts from sheet steel, a sheet work-piece is transferred,
as quickly as possible, from a furnace to tools at room temperature
in which it is deformed and quenched simultaneously. The quench
rate is sufficiently rapid to produce a martensitic microstructure
in the steel, which form the basis for high strength products.
[0012] (i) The cooling rate in die quenching might become too low,
which would cause undesirable soft phases to be formed in the case
of steel (a low strength part produced); in the case of a light
alloy, e.g. aluminium, a die quenching rate that is too low could
cause undesirable grain boundary precipitation which can lead to
stress corrosion cracking and a low strength part;
[0013] (ii) The cold die holding period required may be too long
(because the heat transfer from the sheet is slower as a result of
a warmer die, hence a greater time is required to achieve the final
temperature), which reduces the productivity (increased forming
cycle time);
[0014] (iii) The requirement for adequate die cooling is important,
but providing it artificially (by ad hoc methods, i.e. cooling
ducts with forced cooling fluid, etc.) increases tooling costs
making an efficient method difficult to design and install, and can
raise the tooling and maintenance costs significantly.
[0015] (iv) Tool wear and or die surface distortion are accelerated
when the tool surface temperature is high, reducing tool life, the
costs of which are exacerbated by ad hoc cooling systems described
in (iii).
[0016] Thus, in summary, when parts are produced using this process
in rapid succession, the continual contact of work-pieces from the
furnace causes the temperature of the tools to increase. As a
result, the quenching rate reduces, which can lead to finished
products with a sub-standard microstructure. To avoid this, tool
temperature can be kept low either by reducing production rate, or
by using cooling systems, such as internal coolant-carrying
conduits or sprays of coolant onto the tools. Often, a combination
of these two methods is used to achieve a desired microstructure at
the highest production rate possible for the given cooling
strategy. A drawback is that all of these measures increase
cost.
SUMMARY
[0017] In general terms, a two-stage cooling method is proposed to
improve the productivity of high-strength sheet parts. In the
proposed two-stage cooling method, the heated sheet is rapidly
cooled between heating and forming. It is envisaged that this rapid
cooling is by some artificial means, rather than just by ambient,
still, air. For example, a high heat conductivity transfer device,
an air jet or air/liquid mist spray may be used. In this way, the
temperature of the blank can be reduced by the time it starts to be
formed in the die. Therefore, in the forming process (in which
further quenching ensues) less heat is absorbed by the tools and
the rise in their temperature is reduced. Thus, maintaining a low
base-line temperature is made easier, costs are reduced and
productivity is increased. Other beneficial effects result from
optional features.
[0018] According to a first aspect of this invention, there is
provided a method of forming a part from sheet steel, the method
comprising the steps of:
[0019] (a) heating the sheet to a temperature at which
austenitisation occurs; and
[0020] (b) forming the sheet between dies into the part;
[0021] wherein there is an additional step between (a) and (b) of
applying cooling means to the sheet to extract heat therefrom.
[0022] The additional step may include applying the cooling means
to rapidly cool the sheet.
[0023] By rapidly cooling the heated sheet before forming the sheet
between the dies, the sheet can be formed in the cold dies at a
lower starting temperature than is conventional. This has the
following effects: the sheet can cool sufficiently quickly in the
dies that the hardest phase, martensite, is formed; the sheet can
reach the temperature at which it is suitable for release from the
dies more quickly than in the conventional process, speeding up
production; the damage to tools from elevated surface temperature
is reduced, increasing tool life; and reducing the need for tool
cooling structures such as cooling ducts and thereby reducing the
cost of the dies.
[0024] The additional step may comprise extracting heat using
cooling means such as high conductivity transfer devices or by
impinging cooling means such as cooling medium on the heated
sheet
[0025] The cooling medium may be a fluid. It may be a gas, for
example air. The cooling fluid may be a liquid, for example water.
The cooling fluid of may comprise gas and liquid, for example air
and water. The cooling fluid may be directed as a pressurised flow
of the fluid. The cooling fluid may be directed as a jet. The
cooling fluid may be directed as a mist spray. The cooling fluid
may be used to cool the dies. It may be used to clean the dies. It
may be used to both cool and clean the dies. The cooling fluid may
be directed at the dies. It may be directed at the dies
subsequently to being directed at the heated sheet and/or it may be
directed simultaneously at the dies and at the heated sheet.
[0026] The cooling means may be a high heat conductivity solid,
such as a copper transfer grip or plate.
[0027] The cooling means may be applied when the blank is between
the dies.
[0028] The cooling between (a) and (b) may also be achieved by
increasing the transfer time between the two steps, for example
from the furnace to the dies.
[0029] The additional step may comprise directing the cooling fluid
at the heated sheet such that the sheet is cooled sufficiently
rapidly to avoid the steel entering the bainite phase. The
additional step may comprise directing the cooling fluid at the
heated sheet such that the sheet is cooled at more than 25.degree.
C./second on average. The additional step may comprise directing
the cooling fluid at the heated sheet. The cooling fluid may be
directed with duration, temperature and/or mass flow such that the
sheet is cooled sufficiently rapidly to avoid the steel entering
the bainite phase. The cooling fluid may be directed with duration,
temperature and/or mass flow such that the sheet is cooled at more
than 25.degree. C./second on average.
[0030] The additional step may comprise directing the cooling fluid
at the heated sheet such that the temperature of the sheet remains
above the austenitisation temperature for the steel while being
cooled in this way. The additional step may comprise directing the
cooling fluid at the heated sheet such that the sheet is cooled to
between 500.degree. C. and 600.degree. C. The cooling fluid may be
directed with duration, temperature and/or mass flow such that
temperature of the sheet maintains the austenitisation state for
the steel while being cooled in this way. The cooling fluid may be
directed with duration, temperature and/or mass flow such that that
the sheet is cooled to between 500.degree. C. and 600.degree. C.
Surprisingly, this has the effect of increasing the formability of
the alloy since the strain hardening of the steel increases while
the ductility remains substantially the same. The method may
comprise commencing step (b) while the sheet is at a temperature at
which it is in the austenite phase. The method may also comprise
carrying out step (b) until the temperature of the sheet is such
that it is in the martensite phase.
[0031] Step (a) may contain some or all of the features of that
step of the conventional process described herein.
[0032] The method may be a method of forming parts for automotive
applications. The method may be a method of forming panel parts for
automotive applications. The method may be a method of forming
load-bearing parts and parts adapted to bearing load in automotive
applications; for example, the method may be a method of forming
one or more of: pillars including A-pillars and B-pillars, bumpers,
door beams, roof rails, rocker rails and floor tunnels. The method
may be a method of forming Chassis and Suspension parts; for
example tubular parts and twist beams.
[0033] The sheet steel may be of an alloy that contains boron.
[0034] In another aspect of the invention, a method of forming a
part is provided in which the part is formed from a material other
than steel. For example, the material may be an aluminium alloy. It
may be in sheet form. It is therefore envisaged that the method of
the first aspect may be used with aluminium alloys, for example
those in sheet form. In the method of this other aspect, step (a)
may comprise heating the sheet to a temperature at which a change
in crystal structure substantially equivalent to austenitisation
occurs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 shows in schematic form an existing hot-stamping
process;
[0036] FIG. 2 shows a CCT diagram for a typical Boron steel;
[0037] FIG. 3 shows a temperature profile in cold die quenching;
and
[0038] FIG. 4 shows the stress-strain relationships for a Boron
steel tested at temperatures of 500, 600, 700 and 800.degree. C. at
a strain rate of 1.0 s.sup.-1.
SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS
[0039] As described above, an existing method is shown in FIG. 1. A
very important aspect in this existing method is that, as the hot
stamped part is held in cold dies, the cooling rate should be
sufficiently high, e.g. more than 25.degree. C./second on average,
as shown in FIG. 2, to enable the hardest phase of the material,
martensite, to be formed. In this way, high strength components can
be made. The cooling rate is not constant during the cold die
quenching, as shown in FIG. 3. Initially, the temperature
difference between the work-piece and the die is high and a high
cooling rate can be achieved. As the work-piece temperature drops
close to the tool surface temperature (which increases due to heat
transfer), the cooling rate reduces significantly. In a continuous
hot stamping process, the tool surface temperature can be as high
as 150.degree. C. This results in a low work-piece cooling rate in
the temperature range 500.degree. C. to 250.degree. C. This is the
sensitive range for metallurgical transformation and a low
quenching rate could result in the formation of lower strength
bainite instead of martensite (see FIG. 2). Thus, a low strength
part would be formed.
[0040] The present embodiment provides a method in which the amount
of heat transferred from the workpiece to the cold die is reduced
when compared with such an existing method, thereby reducing the
tool temperature in comparison with the existing method and
addressing the problems of the existing method described above.
This embodiment reduces the amount of heat absorbed by the die
while maintaining the necessary rate of quenching, and of
production.
[0041] In overview, in the present embodiment, the sheet of boron
steel is rapidly cooled as it is transported from furnace to die by
a solid medium of high heat conductivity, or by a fluid such as an
air jet or air/liquid mist spray, and thus its temperature is
reduced by the time it is placed on the die. Therefore, in the
forming process (in which further quenching ensues) less heat is
absorbed by the tools and the rise in their temperature is reduced.
Thus, maintaining a low base-line temperature of the tools is made
easier, costs are reduced and productivity is increased.
[0042] The new method involves the following steps.
[0043] First, a sheet metal blank of boron steel is heated in a
furnace to above its austenitisation temperature. In the present
embodiment, the blank is heated to 925.degree. C. The blank is then
soaked at this temperature to ensure the material is transformed
entirely into the austenite phase. In this state the metal is soft
and has high ductility (easy to form), as in the conventional
process.
[0044] The next step is to transfer the austenitised material blank
to the press in which it is to be formed into the shape of the
part. During the transfer or, in other embodiments, after the
transfer to the die but before the hot metal blank touches the die,
the blank is cooled quickly by contacting it with a substance with
high heat conductivity. This substance, that is this cooling means,
may take the form of one, more or all of: copper grips, blowing
air, directing an air-water mist or other fluid/liquid cooling
medium at the blank. In the present embodiment, an air-water mist
is applied to the blank. This is done by directing a fine spray of
pressurized water at the blank through a plurality of nozzles. In
this way, the blank is cooled to a temperature of about 600.degree.
C. The cooling rate is adjusted to be sufficiently rapid to
maintain an austenite structure per the CCT diagram in FIG. 2.
During this stage of work-piece cooling, it is envisaged that the
same cooling medium is also used to cool and clean the tools.
[0045] The remainder of the method is the same as in the
conventional method described herein. Thus, the method may be
illustrated as the conventional process shown in FIG. 1, but with
additional cooling during the transfer between the furnace and the
die.
[0046] From the typical-stress strain curves for Boron steel shown
in FIG. 4, it can be observed that when temperature decreases from
800.degree. C. to about 600.degree. C., the ductility of the alloy
does not change very much. However, the strain hardening of the
alloy leads to a near doubling of the strength. This strain
hardening feature increases the formability of the alloy
significantly, by causing the deformation to be more uniform (i.e.
an area deformed more becomes stronger, causing deformation to
occur in other areas, which then become stronger, etc.), thereby
mitigating the tendency for localised necking. This is particularly
important in hot stamping, since friction is normally high and the
strain hardening feature could reduce the friction effects. Thus,
if, as is the case in the present embodiment, a part can be formed
at a temperature starting at about 600.degree. C. in the dies,
rather than 800.degree. C. as is done conventionally; more
complex-shaped components can be formed. It should be emphasized
that this effect cannot be achieved by simply heating the sheet to
a lower initial temperature, as it must first be fully
austenitised.
[0047] The CCT diagram for Boron steel in FIG. 2 shows that the
alloy is still in the austenite state if it is cooled quickly to
about 500-600.degree. C. If the cooling is too slow, the
lower-strength bainite phase begins to form; the present method,
however, avoids this. In the present method, as the blank is
transferred to cold dies while in this temperature range and
maintained at a temperature between 450-500.degree. C. during the
entire forming process, all phase transformation takes place during
the cold die holding period and the austenite is entirely converted
to martensite to produce a high strength part.
[0048] In existing methods, the formed part is released from the
die as soon as the part temperature drops to about 250.degree. C.
At this temperature, phase transformation has been completed and no
obvious thermal distortion is observed by further cooling in the
air without the tool constraint. The cold die quenching period
(i.e. the time for which the part is held in the die) required to
cool a part from about 800.degree. C. to about 250.degree. C.
(550.degree. C. difference), is about 5 to 15 seconds in these
existing methods, depending on the thickness and shape of the
work-piece and part shape. Thus, a significant amount of heat has
to be absorbed by the die directly, which makes cooling the die
difficult.
[0049] In the present embodiment, the part is formed at about
600-500.degree. C. Thus, in the cold die quenching period, the only
need is to bring the part temperature down from, at the lower end
of this range, 500.degree. C. to about 250.degree. C. (250.degree.
C. difference). Only about half the amount of heat therefore needs
to be extracted from the die, and so the cooling requirement for
the tool is much lower. The tool design can therefore be simpler
and the tool can be cheaper. The lower temperature of the tool
surface reduces the cold die holding period significantly, and also
increases the cooling rate significantly during the temperature
range of 500.degree. C. to 250.degree. C. The holding time can be
reduced to about 2 to 8 seconds. Thus, productivity can be
increased significantly. This is vital for, for example, a
competitive automotive company. In addition, the lower tool surface
temperature reduces tool wear, thus increasing tool life
significantly, which is an additional benefit for reducing
production costs.
* * * * *