U.S. patent application number 15/775834 was filed with the patent office on 2018-11-15 for device and method for providing a selective heat treatment on a metal sheet.
The applicant listed for this patent is HARDMESCH AB. Invention is credited to Nader ASNAFI, Christer SVENSSON.
Application Number | 20180327872 15/775834 |
Document ID | / |
Family ID | 54608506 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180327872 |
Kind Code |
A1 |
SVENSSON; Christer ; et
al. |
November 15, 2018 |
DEVICE AND METHOD FOR PROVIDING A SELECTIVE HEAT TREATMENT ON A
METAL SHEET
Abstract
A device for providing selective heat treatment on a metal sheet
comprising boron steel is provided. The device comprises a laser
source for emitting a laser beam onto the metal sheet to provide a
selective heat treatment thereon, wherein a defined heat treated
grid pattern is formed on the metal sheet by the selective heat
treatment. The device further comprises a control unit arranged to
control the operation of the laser source for providing the defined
heat treated pattern onto the metal sheet; and control the case
hardness depth of the defined heat treated pattern based on a
temperature parameter and a holding time parameter associated with
the operation of the laser source. A method for performing
controlling the case hardness depth of a selectively heat treated
metal sheet of boron steel is also provided.
Inventors: |
SVENSSON; Christer;
(Karlshamn, SE) ; ASNAFI; Nader; (Kristianstad,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HARDMESCH AB |
Hassleholm |
|
SE |
|
|
Family ID: |
54608506 |
Appl. No.: |
15/775834 |
Filed: |
November 13, 2015 |
PCT Filed: |
November 13, 2015 |
PCT NO: |
PCT/EP2015/076604 |
371 Date: |
May 13, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/673 20130101;
C21D 11/00 20130101; C21D 2221/00 20130101; B21D 22/022 20130101;
C21D 8/0494 20130101; C21D 1/09 20130101 |
International
Class: |
C21D 1/09 20060101
C21D001/09; C21D 11/00 20060101 C21D011/00 |
Claims
1. A device for providing selective heat treatment on a metal sheet
comprising boron steel, comprising a laser source for emitting a
laser beam onto the metal sheet to provide a selective heat
treatment thereon, wherein a defined heat treated pattern is formed
on the metal sheet by the selective heat treatment; and a control
unit arranged to: control the operation of the laser source for
providing the defined heat treated pattern onto the metal; and
control the case hardness depth of the defined heat treated pattern
based on a temperature parameter and a holding time parameter
associated with the operation of the laser source.
2. The device according to claim 1, wherein the control unit is
further arranged to control the case hardness depth based on
receipt of an input signal comprising information associated with a
desired case hardness depth of the defined heat treated
pattern.
3. The device according to claim 2, wherein the control unit, upon
receipt of said input signal, is arranged to adapt at least one of
the temperature parameter or the holding time parameter by adapting
the laser speed or sheet movement speed.
4. The device according to claim 1, wherein the control unit, upon
receipt of said input signal, is further arranged to control the
depth of the case of the metal sheet by adapting the power of the
laser source.
5. The device according to claim 1, wherein the temperature
parameter determines a temperature range for the treated metal
sheet grid pattern during the selective heat treatment.
6. The device according to claim 1, wherein the holding time
parameter determines a time period during which each metal sheet
portion of the defined heat treated pattern is above a predefined
temperature.
7. The device according to claim 1, wherein the selective heat
treatment is applied to a first surface of the metal sheet, which
first surface is selected to coincide with an inner surface after
forming or shaping the metal sheet.
8. The device according to claim 1, wherein the temperature
parameter relates to a temperature value range between a transition
temperature for the metal sheet and below a melting temperature of
the metal sheet.
9. The device according to claim 1, wherein the laser beam has a
circular spot shape resulting from a Gaussian distribution setting
on the laser source.
10. The device according to claim 9, wherein the control unit, upon
receipt of the input signal, is further arranged to control case
hardness depth by adapting the circular laser focus to a square
shaped laser beam spot shape by selecting a uniform distribution
setting for the laser source.
11. The device according to claim 1, wherein the laser source is a
carbon dioxide laser or a fiber laser.
12. A method for providing a defined heat treated pattern on a
metal sheet comprising boron steel, the method comprising emitting
a laser beam onto the metal sheet to provide the selective heat
treatment thereon; controlling the operation of the laser source
for providing the defined heat treated pattern onto the metal
sheet; and controlling the case hardness depth of the defined heat
treated pattern based on a temperature parameter and a holding time
parameter associated with the operation of the laser source.
13. The method according to claim 12, further comprising
controlling the case hardness depth based on receipt of an input
signal comprising information associated with a desired case
hardness depth of the defined heat treated pattern.
14. The method according to claim 13, further comprising, upon
receipt of said input signal, adapting at least one of the
temperature parameter or the holding time parameter by adapting the
laser speed and/or the sheet metal movement speed.
15. The method according to claim 12, further comprising, upon
receipt of said input signal, controlling the depth of the case of
the metal sheet by adapting the power of the laser source.
16. The method according to claim 12, wherein the temperature
parameter determines a temperature range for the treated metal
sheet grid pattern during the selective heat treatment.
17. The method according to claim 12, wherein the holding time
parameter determines a time period during which each metal sheet
portion of the defined heat treated pattern is above a predefined
temperature.
18. The method according to claim 12, wherein the selective heat
treatment is applied to a first surface of the metal sheet, which
first surface is selected to coincide with an inner surface after
forming or shaping the metal sheet.
19. The method according to claim 12, wherein the laser beam has a
circular spot shape resulting from a Gaussian distribution setting
on the laser source.
20. The method according to claim 19, wherein the control unit,
upon receipt of the input signal, further controls the case
hardness depth by adapting the circular laser focus to a square
shaped laser beam spot shape by selecting a uniform distribution
setting for the laser source.
21. The method according to claim 12, wherein the laser source is a
carbon dioxide laser or a fiber laser.
Description
TECHNICAL FIELD
[0001] The current disclosure relates to the field of selective
heat treatment of sheet metal materials. More specifically it
relates to the control of the case hardness depth resulting from
the selective heat treatment.
BACKGROUND
[0002] In order to increase stability and endurance of a metal
sheet it may be subjected to what is known as press hardening.
Press hardening processes allow for the production of light weight,
high strength metal sheet components. Press hardened materials are
highly deformation resistant. Press hardening techniques have
played an increasingly important role within the vehicle industry
during recent years, as the press hardened components are suitable
for absorbing great deformation energies such as in a vehicle
collision. In current press hardening processes the metal sheet is
transported through a furnace and thus heated up to its
austenization temperature of about 900 to 950.degree. C. whereby it
is transformed into 100% austenite. In the austenitic state the
material has a tensile strength of about 200 MPa and an extension
degree of about 40%. After the heat treatment the austenite
material is rapidly moved into a processing tool for shaping the
material before it starts to oxidize. Usually the duration of the
shaping stage is about 8 to 10 seconds. During the shaping the
shaped material is subject to cooling during which the austenite is
transformed to marteniste. In other words a phase transformation
occurs during the cooling. When the shaped material leaves the
shaping tool, its temperature is about 150 to 200.degree. C.
[0003] Press hardening however means that the entire sheet of metal
will be hardened, which sometimes may not be desirable. It is also
a non flexible process.
[0004] However, more flexibility in hardening the metal sheet is
desirable.
[0005] Hence, there is a need for devices and methods which enable
optimized and selective hardening of a metal sheet material e.g.
based on material and the purpose of the finished product
comprising the metal sheet.
SUMMARY
[0006] An object of the present invention is to eliminate or
alleviate at least one of the drawbacks mentioned above, in
accordance with the appended claims.
[0007] An advantage of the present invention is that a process of
controlling the case hardness depth in a selective heat pattern
provided on a metal sheet comprising boron steel is provided.
[0008] By being able to control the case hardness depth of the
selective heat pattern the metal sheets may be tailor made in terms
of formability, and deformation capability etc.
[0009] According to an aspect a device for providing selective heat
treatment on a metal sheet comprising boron steel is provided. The
device comprises a laser source for emitting a laser beam onto the
metal sheet to provide a selective heat treatment thereon, wherein
a defined heat treated pattern is formed on the metal sheet by the
selective heat treatment. The device further comprises a control
unit arranged to control the operation of the laser source for
providing the defined heat treated pattern onto the metal sheet.
Moreover, the control unit is arranged to control the case hardness
depth of the defined heat treated pattern based on a temperature
parameter and a holding time parameter associated with the
operation of the laser source.
[0010] According to another aspect a method for providing a defined
heat treated pattern on a metal sheet comprising boron steel is
provided. The method comprises emitting a laser beam onto the metal
sheet to provide the selective heat treatment thereon. Moreover,
the method comprises controlling the operation of the laser source
for providing the defined heat treated pattern onto the metal
sheet. Furthermore, the method comprises controlling the case
hardness depth of the defined heat treated pattern based on a
temperature parameter and a holding time parameter associated with
the operation of the laser source.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Further objects, features and advantages will appear from
the following detailed description of embodiments, with reference
being made to the accompanying drawings, in which:
[0012] FIG. 1 is a schematic drawing showing a device for providing
a selective heat treatment on a metal sheet;
[0013] FIG. 2 is a schematic drawing showing an alternative device
for providing a selective heat treatment on a metal sheet;
[0014] FIG. 3 shows a block chart of a method;
[0015] FIG. 4 is a schematic drawing illustrating shaped metal
sheet part being provided with a grid pattern before or after
shaping;
[0016] FIG. 5 is a schematic drawing illustrating shaped metal
sheet part being provided with a grid pattern;
[0017] FIG. 6 is a schematic drawing illustrating shaped metal
sheet part being provided with a grid pattern before or after
shaping;
[0018] FIGS. 7a and 7b show two examples of the ability of
controlling the case hardness depth using a CO2 laser; and
[0019] FIGS. 8a and 8b show two examples of the ability of
controlling the case hardness depth using a fiber laser.
DETAILED DESCRIPTION
[0020] An idea of the present invention is to controllably conduct
selective heat treatment on the metal sheet based on the desired
properties of a ready-to-use component formed by the metal sheet.
The desired properties may relate to a desired metal sheet
formability or trimability of the metal sheet and/or required
in-use properties of the ready-to-use component comprising the
selective heat treated metal sheet.
[0021] The selective heat treatment includes heating the sheet
material according to a defined heat treatment pattern, forming a
grid pattern, followed by cooling and thereby generating a grid
pattern wherein the metal sheet is locally annealed or hardened
compared to the non-treated portions of the metal sheet.
[0022] It is an aim of the present invention to provide a solution
as how to control the hardening and/or annealing process, and in
particular relating to the ability of controlling the case hardness
depth in the areas associated with the defined heat treated
pattern.
[0023] The inventors have realized that the selective heat
treatment may be optimized for different applications by
controlling the selective heat treatment such that a specific case
hardness depth of in the metal sheet may be obtained.
[0024] The metal sheet may comprise boron steel. During the
selective heat treatment, the boron steel along the lines of the
specific heat pattern converts into austenite at elevated
temperature (.gtoreq.925.degree. C.). During subsequent cooling,
the austenite converts into martensite along the lines of the
specific heat pattern. The martensite acts as a hardened or
reinforcing structure of the metal sheet.
[0025] FIG. 1 illustrates a device 100 for providing a defined heat
treated pattern on a metal sheet comprising boron steel. The device
comprises a laser source 101 and a control unit 103 operatively
connected to the laser source 101. The control unit has processing
capabilities and may comprise a processor and memory.
[0026] Optionally, the device may comprise a movement device 105
arranged to move the metal sheet 104 in relation to the laser
source 101. The movement device 105 may for example be a conveyer
belt.
[0027] The selective heat treatment is performed by the laser
source emitting a laser beam 102 onto a metal sheet 104 in the
defined heat treated pattern.
[0028] The control unit 103 is arranged to control the operation of
the laser source 101 for providing the defined heat treated pattern
onto the metal sheet. Moreover, the control unit is further
arranged to control the case hardness depth of the defined heat
treated pattern based on a temperature parameter and a holding time
parameter associated with the operation of the laser source
101.
[0029] By being able to control the case depth parameter the
selectively heat treated metal sheet may be given tailor made
formability and reinforcing properties. For example, while a
smaller (in relation to the thickness of the metal sheet) case
hardness depth provide for increased subsequent formability the
abrasion resistance is reduced, whereas a larger case hardness
depth provides for increased abrasion resistance at the expense of
reduced subsequent formability properties.
[0030] The laser source 101 may for example be a carbon dioxide
(CO.sub.2) laser or a fiber laser.
[0031] The control unit 103 may control the case hardness depth
based on input, e.g. in the form of an input signal, comprising
information associated with a desired case hardness depth of the
defined heat treated pattern to be applied to the metal sheet
104.
[0032] In some embodiments, the temperature parameter may define a
temperature range for the case of the metal sheet during the
selective heat treatment, e.g. ranging from the transition
temperature of the metal sheet and up to just below the melting
temperature of the metal sheet. It should be appreciated that the
holding time when using a temperature closer to transition
temperature is longer than that when using a temperature closer to
the melting temperature.
[0033] For example, carbon and stainless steel have a phase
transition temperature of about 925.degree. C. and a melting
temperature of about 1700.degree. C.; aluminum has a transition
temperature of about 200.degree. C. and a melting temperature of
about 400.degree. C.; magnesium has a transition temperature of
about 100.degree. C. and a melting temperature of about 350.degree.
C. and titanium 925.degree. C.
[0034] In order to improve the throughput of the selective heat
treatment, a holding time as short as possible is normally
preferred.
[0035] The device may optionally comprise a temperature sensor (not
shown) for measuring the actual temperature in the defined heat
treated pattern. The temperature sensor may be operatively
connected to the control unit for providing the latter with present
temperature measurements. The temperature sensor may be an infrared
sensor, such as an infrared laser temperature sensor, being able to
accurately measure the temperature in the defined heat treated
pattern or adjacent areas remotely.
[0036] The temperature sensor may e.g. be placed adjacent to the
laser source at a distance from the metal sheet, and measuring the
temperature in a point along the defined heat treated pattern just
behind the point of the metal sheet currently being irradiated by
the laser beam 102 of the laser source 101.
[0037] The temperature sensor may be a thermal camera operatively
connected to the control unit. The thermal camera, e.g. an IR
camera, may be arranged to detect heat from both the grid lines of
the grid pattern and the non-treated areas adjacently arranged the
grid pattern.
[0038] Optionally, the control unit 103 may be configured to
measure the depth of the case hardness of the metal sheet. If the
depth of the case hardness is deemed to be too small for a desired
application the control unit 103 may operate the laser such that
the temperature is increased. For example, the depth of the case
hardness may be measured using a camera imaging the edge of the
metal sheet. The edge camera may be arranged to detect light in the
visual light spectrum, i.e. in the wavelength range of 400 to 700
nm. By means of an edge detection camera grid lines extending to
the edge of the metal sheet at an angle may be analyzed whereby the
depth of the case hardness at the grid lines may be measured and be
fed back to the control unit, which may adapt the operation
parameters accordingly.
[0039] In order to increase the temperature for the remainder of
the defined heat treated pattern, the power of the laser could be
increased, e.g. by means of a control signal sent from the control
unit to the laser source. Thus, the case hardness depth may also be
controlled by varying the power of the laser and the laser speed or
the speed of the sheet movement.
[0040] The temperatures to which the metal sheet may be elevated to
in order to accomplish the phase change, formability, trimability,
property change or improvement are e.g. in the range of 100 to
1700.degree. C. depending on the material of the metal sheet.
[0041] In some embodiments, the control unit 103 may additionally
or alternatively also control the holding time parameter, e.g. by
controlling the laser speed or the speed of the sheet movement, in
order to control the case hardness depth.
[0042] The holding parameter may determine a time period during
which each metal sheet portion of the defined heat treated pattern
is above a predefined temperature. The reason why a holding time
needs to be considered is because it takes some time for the
material of the metal sheet in the selectively heat treated region
to completely convert into austenite. The process of converting the
boron steel to austenite includes reforming the core and crystals
of the material in terms of size and shape.
[0043] For example, in a commonly known process of heat treating
whole metal sheets, i.e. where the entire metal sheet is to be
converted into austenite, requires a holding time of at least 5
minutes. Hence, each metal sheet blank requires a holding time of
at least 5 minutes.
[0044] An aim of the present invention is to reduce the holding
time parameter as far as possible, and this is possible by
conducting the selective heat treatment at temperatures higher than
those required to initiate the material phase transformation but
below the melting point of the metal sheet.
[0045] In general, the holding time parameter and the temperature
parameter are related to the case hardness depth as follows. A
lower temperature in the metal sheet, still above the minimum phase
change temperature, will require a longer holding time than that of
a higher temperature, in order to achieve the same case hardness
depth of the metal sheet. Hence, for temperatures above the minimum
phase change temperature, e.g. 950.degree. C. for boron steel to
convert into austenite, the temperature parameter and holding time
parameter are inversely proportional to one another, in order to
achieve the same case hardness depth
[0046] It should be appreciated that in order to control the case
hardness depth, the temperature in the metal sheet achieved by the
irradiating laser beam should never be at or above the melting
point of the metal sheet. Hence, the control unit is arranged to
control the operation of the laser source such that the metal sheet
is never subjected to temperatures above the melting temperature of
said metal sheet.
[0047] Calibration Mode
[0048] The control unit may operate in a calibration mode. In the
calibration mode a series of grid lines having a certain length are
formed in the metal sheet with different parameter settings. The
parameters as well as the input from any temperature sensor, such
as the heat camera, and/or the case hardness depth measuring camera
are then stored into the memory of the control unit. The parameters
may be stored in a calibration table. Based on receipt of a desired
case hardness depth, the control unit may be arranged to select
appropriate parameter values from the calibration table.
[0049] For example, the calibration may be executed by selectively
heat treating grid lines having a length of just a few centimeters
with a number of different parameter settings.
[0050] Optionally, the control unit may be arranged to extrapolate
suitable parameter values from parameter values stored in the
calibration table, e.g. when the specific desired case hardness
depth is not present in the calibration table.
[0051] Selective Heat Treatment Mode
[0052] The control unit may also operate in a normal operation
mode, or selective heat treatment mode, in which it controls the
operation of the laser source in order to conduct the selective
heat treatment of the metal sheet such that the resulting grid
pattern meets a desired case hardness depth.
[0053] In the selective heat treatment mode the temperature sensor,
e.g. the thermal camera, and/or the case hardness depth measuring
camera, are used to provide continuous or intermittent feedback to
the control unit, such that the desired case hardness depth is
achieved and maintained throughout the selective heat treatment
process.
[0054] The feedback loop from the temperature sensor and/or case
hardness depth measurement sensor provides the control unit with a
built-in treatment process monitoring functionality.
[0055] For example, based on input that the case hardness depth is
too shallow or small the control unit 103 may be configured to
control the laser source such that it illuminates the same spot of
the metal sheet 104 for a longer period of time.
[0056] Alternatively or additionally, in the event the metal sheet
104 is provided on the conveyer belt 105, the control unit 103 may
control the conveyor speed of the conveyer belt such that each part
of the defined heat treated pattern of the metal sheet 104 is
irradiated by the laser source 101 for a longer period of time.
Hence, by slowing the conveyor speed down the longer each point of
the metal sheet will be irradiated per time unit.
[0057] It should be noted that the laser source may be arranged to
move over the metal sheet to provide the grid pattern thereon.
Alternatively, or additionally the metal sheet may be moved in
relation to the laser source by means of the aforementioned
conveyor belt 105. Hence, in under some conditions both the laser
source may move over the metal sheet while the conveyor belt moves
the metal sheet.
[0058] Experiments have shown that is possible to controllably
achieve a specific case hardness depth in the metal sheet
independently of the laser beam incidence angle, i.e. the angle at
which the laser beam hits the surface of the metal sheet. This
finding makes the arrangement of the device, particularly when
providing selective heat treatment on already formed or shaped
metal sheets, significantly less complex.
[0059] The laser source 101 may in some embodiments be a carbon
dioxide (CO.sub.2) laser or a fiber laser.
[0060] The laser beam 102 may be set to a uniform distribution or a
Gaussian distribution which is well known. A Gaussian distribution
setting results in that the spot shape of the laser beam is
circular, while for a uniform distribution the spot shape of the
laser beam is square shaped. Experiments have showed that a
Gaussian distribution setting may be preferred for some
applications, see e.g. examples in relation to FIGS. 7a to 8b
below. When using a Gaussian distribution the transition between
the hardened grid line and the non-hardened adjacent areas is more
smooth, compared to that resulting from a uniform distribution,
which provides for an improved abrasion resistance and improved
flexibility for subsequent forming of the metal sheet.
[0061] Although, a Gaussian distribution (circular spot shape) has
shown to be preferred, depending on the application the control
unit 103, upon receipt of the input signal, may be further arranged
to control case hardness depth by adapting the circular laser focus
spot shape to a square shaped spot shape.
[0062] In some applications, the selective heat treatment is
conducted to improve the strength either before or after forming of
the metal sheet.
[0063] The specific heat pattern, e.g. grid pattern, may be said to
form a skeleton structure in the metal sheet. The grid pattern may
improve or facilitate subsequent forming of the sheet material. A
grid pattern or a portion of the grid pattern may also be designed
to allow for tailor-made deformation capabilities of the resulting
metal sheet component, see e.g. FIGS. 4 to 6.
[0064] The grid pattern, its shape and position in view of the
metal sheet is designed and carried out based on the identified
needs or requirements concerning forming, trimming and/or component
performance or properties. Hence, the design of the grid pattern
may be derived by reverse engineering through for instance crash
simulation.
[0065] In case boron steel sheet is used, the sheet material along
the lines of the grid pattern will convert into austenite during
the heating process. Moreover, the selective heat treatment may
comprise cooling the selectively heated sheet material, whereby the
austenite along the lines of the grid pattern is converted into
martensite. Here, the forming step may be executed immediately
after the selective heat treatment, such that parts of the sheet
material, in particular the martensite grid pattern, has a higher
temperature than the ambient temperature.
[0066] The cooling may be induced by actively cooling the metal
sheet, e.g. by an air cooling device.
[0067] Alternatively, the cooling is applied through the ambient or
still air, such that a passive cooling at a sufficient rate of the
defined heat treated pattern is achieved. The sufficient rate may
be around 27.degree. C. per second up to around 50.degree. C. per
second, since this is the known cooling rate required for the
austenite to convert into martensite. In fact, the passive cooling,
i.e. not using any cooling device, has shown to meet the sufficient
cooling rate of 27.degree. C. per second, since the selective heat
treatment is local, meaning that the adjacent areas to the grid
pattern are very cool, i.e. close to room temperature, in relation
to the temperature of the grid pattern, thereby cooling the heated
grid lines sufficiently rapid for the austenite to convert into
martensite.
[0068] It is also possible to cool the heat treated metal sheet
using by allowing the metal sheet to engage in relatively cooler
objects, such as tools, dies and jigs/fixtures or cooled tools
& dies, wherein the tools e.g. have heat exchanging channels
filled with a refrigerant fluid flowing there through, and
jigs/fixtures.
[0069] In order to apply the defined heat treated pattern to the
metal sheet a plurality of laser sources could be utilized, as is
illustrated in FIG. 2.
[0070] FIG. 2 illustrates an example embodiment of the device 200
for providing a selective heat treatment on the metal sheet.
[0071] The device 200 comprises two laser sources 201a, 201b
connected to a control unit 203. During the selective heat
treatment, the laser sources 201a, 201b emits a respective laser
beam 202a, 202b onto a metal sheet 104 according to a defined heat
treated pattern 205.
[0072] The laser sources 201a, 201b, the control unit 103 and the
metal sheet 104 may e.g. be the same laser source, control unit and
metal sheet as descried in conjunction with FIG. 1. The device 200
of FIG. 2 is illustrated without a conveyer belt, however it should
be appreciated that the device 200 of FIG. 2 may also be connected
to a conveyor belt in the same manner as in FIG. 1.
[0073] The laser sources 201a, 201b direct their respective laser
beams 202a, 202b such that they form the grid pattern 205. Thus,
only the metal sheet portions at the position of the grid pattern
205 will be heated and transform into martensite.
[0074] The first laser source 201a irradiates the sheet material
intermittently along a grid line extending over the width of the
metal sheet 204 coil or blank (the expression "intermittently" is
here meant to be understood as occurring at regular intervals). The
second laser unit 201b may be configured to provide at least one
grid line in the longitudinal (i.e. parallel to the direction of
movement indicated by the arrow) direction of the metal sheet 204.
As can be seen in FIG. 2, although no conveyor is shown the
relative movement between the laser sources and the metal sheet is
shown by the dashed arrows.
[0075] In its simplest form, the grid shown in FIG. 2 can be
applied by a single laser source, e.g. by the laser source as
described in FIG. 1. In more advanced systems, the grid pattern can
be created by a mirror-based pattern generator in laser
systems.
[0076] The control unit 103 may be arranged to control the
operation of the laser source(s) 201a, 201b for providing a defined
heat treated pattern onto the metal sheet and to further control
the case hardness depth of the defined heat treated pattern based
on a temperature parameter and a holding time parameter associated
with the operation of the laser source, wherein the laser speed,
i.e. the speed at which the laser beam travels over the metal
sheet, or the sheet movement speed is associated with both of these
said parameters. The holding time may be reduced by increasing the
relative speed between the laser source and the metal sheet. This
relative speed may be increased by either increasing the laser
speed or the metal sheet speed or both. Moreover, the shorter time
period during which the laser beam hits a certain point on the
metal sheet the lower the temperature increase in said point will
be. Hence, the relative speed between the laser beam and the metal
sheet also affects the temperature of the heat pattern.
[0077] The control unit 103 may control the case hardness depth
based on receipt of an input signal comprising information
associated with a desired case hardness depth of the defined heat
treated pattern applied to the metal sheet 104.
[0078] FIG. 3 illustrates a method 300 for according to some
embodiments. The method 300 comprises controlling 301 the operation
of a laser source emitting a laser beam (e.g. any of the laser
beams as described in conjunction with FIGS. 1 and 2) onto a metal
sheet (e.g. any of the metal sheets as described in conjunction
with FIGS. 1 and 2) to provide a defined heat treated pattern
thereon. The metal sheet may e.g. comprise boron steel.
[0079] The method further comprises controlling 302 the case
hardness depth of the defined heat treated pattern based on a
temperature parameter and a holding time parameter associated with
the operation of the laser source.
[0080] The method 300 may further comprise monitoring 303 the case
hardness depth to check whether it differs from a predetermined
level.
[0081] If it in 303 is determined that the case hardness depth is
below the depth threshold (yes path out of 303) it is an indication
that the depth needs to be increased and the method continues to
304 where at least one of a holding parameter or a temperature
parameter is increased.
[0082] Thus, either the temperature of the laser beam is increased,
e.g. by increasing the power of the laser, and/or the holding time
during which the grid pattern applied to the metal sheet is
subjected to the laser beam is increased resulting in that the case
hardness depth is increased. In general, it is more preferred to
keep the holding time to a minimum while adapting the temperature
to achieve the desired case hardness depth.
[0083] If it in 303 is determined that the case hardness depth is
above, or on, the depth threshold (no-path out of 303) then, that
is a indication that the depth should not be increased further and
the method continues to 305 where at least one of the temperature
parameter or the holding time parameter is decreased.
[0084] The method then returns to 303 where the case depth hardness
is monitored.
[0085] In some embodiments, step 303 of the method 300 may comprise
determining the case hardness depth based on receipt of an input
signal comprising information associated with a desired case
hardness depth of the defined heat treated pattern. Optionally, the
input signal may be created by a user typing in a desired case
hardness depth using a user interface operatively connected to the
control unit.
[0086] In some embodiments, the method may further comprise, upon
receipt of said input signal, controlling the depth of the case of
the metal sheet by adapting the power of the laser source such as
to result in a grid pattern temperature ranging between the
transition temperature and up to just below the melting temperature
of the metal sheet. For example, an increase in the power of the
laser source results in a stronger laser beam which results in a
deeper case depth.
[0087] The temperature parameter may determine a temperature range
for the case of the metal sheet during the selective heat
treatment.
[0088] The holding time parameter may determine a time period
during which each metal sheet portion of the defined heat treated
pattern is above a predefined temperature. The laser speed and/or
the sheet movement speed and the accomplished temperature control
the holding time.
[0089] The method may further comprise controlling the case
hardness depth by adapting the laser beam incidence angle onto the
metal sheet.
[0090] The laser beam has a circular spot shape.
[0091] The method may further comprise controlling the case
hardness depth by adapting the circular laser focus such that a
variable laser spot size is achieved.
[0092] The laser source is a carbon dioxide laser or a fiber
laser.
[0093] The selective heat treatment is applied to a first surface
of the metal sheet, which first surface is selected to coincide
with an outer surface after forming or shaping the metal sheet.
[0094] In case the purpose of the selective heat treatment is to
improve the formability or trimability, the grid pattern must be
applied before forming or trimming. For the application that the
formability and trimability of the metal sheet is to be improved,
the metal sheet is selectively heated up to a temperature below the
transition temperature. In this way the metal sheet is not hardened
but instead annealed.
[0095] FIGS. 4 to 6 each illustrates different applications where a
grid pattern has been applied to a metal sheet which then has been
formed or shaped into a finished part.
[0096] Turning to FIG. 5, the grid pattern shown may be produced by
a laser source arranged on a robotic arm allowing the laser source
to move relative to the flat metal sheet or the formed metal
sheet.
[0097] FIGS. 7a and 7b show two comparable cross-sectional
micrographs examples of the ability of controlling the case
hardness depth of a 1.0 mm thick boron steel Boloc02 metal sheet. A
CO.sub.2 laser unit is used. The purpose of the selective laser
heat treatment is to reinforce the sheet material strength. The
selective heat treatment was conducted on a flat (undeformed) metal
sheet. Laser spot shape and size is 6.24 mm, and the sheet is
heated to 950.degree. C., after which it is allowed to be still
air-cooled.
Practical Examples
[0098] FIGS. 7a to 8b show a number of practical examples where a
Gaussian distribution, i.e. circular spot laser beam shape, has
been used for the selective heat treatment.
[0099] In FIG. 7a, the laser power is 650 W and the scan speed is
500 mm/min resulting in a hardness depth of 0.6 mm. In FIG. 7b, the
laser power is 2300 W and the scan speed is 5000 mm/min and the
resulting hardening depth is 0.17 mm. In both cases, the initial
sheet hardness is 220 HV and the hardness in the hardened region is
470 HV.
[0100] The information shown in FIGS. 7a and 7b is of great
importance. It exemplifies the optimization of the laser heat
treatment process in order to achieve the wanted impacts on the
selectively laser heat treated component. It also shows the
possibility and opportunity to generate variety depending on the
needs and one has therefore access to more parameters to accomplish
the varying properties in different part locations.
[0101] FIGS. 8a and 8b show two comparable examples of the ability
of controlling the case hardness depth using a fiber laser as laser
source. The metal sheet is still a 1 mm thick boron steel Boloc02
graphitized for better coupling. The laser spot shape and size is
5.5 mm in both cases. The laser effect/power is 670 W in both FIGS.
8a and 8b. The laser (scan) speed is however, 1320 mm/min and 1500
mm/min respectively. Argon protection gas is used in both cases.
The case hardness depth is 0.9 mm in FIG. 8a, while it is 0.45 mm
in FIG. 8b. The initial hardness is 220 HV. It is increased to 490
HV in the hardened region in FIGS. 8a and 470 in the hardened
region in FIG. 8b.
[0102] It is possible to accomplish the same results as in FIG. 8a
by using other laser processing parameters. Temperature control,
e.g. 1500.degree. C. in the metal sheet and 25 mm/sec in laser
speed, yields the same results as in FIG. 8a. In other words, the
heat treatment parameters can be selected differently as long the
results are in compliance with those wanted.
[0103] Reference has been made herein to various embodiments.
However, a person skilled in the art would recognize numerous
variations to the described embodiments that would still fall
within the scope of the claims. For example, the method embodiments
described herein describes example methods through method steps
being performed in a certain order. However, it is recognized that
these sequences of events may take place in another order without
departing from the scope of the claims. Furthermore, some method
steps may be performed in parallel even though they have been
described as being performed in sequence.
[0104] In the same manner, it should be noted that in the
description of embodiments, the partition of functional blocks into
particular units is by no means limiting. Contrarily, these
partitions are merely examples. Functional blocks described herein
as one unit may be split into two or more units. In the same
manner, functional blocks that are described herein as being
implemented as two or more units may be implemented as a single
unit without departing from the scope of the claims.
[0105] Hence, it should be understood that the details of the
described embodiments are merely for illustrative purpose and by no
means limiting. Instead, all variations that fall within the range
of the claims are intended to be embraced therein.
* * * * *