U.S. patent application number 12/493390 was filed with the patent office on 2009-12-31 for differential heat shaping and hardening using infrared light.
Invention is credited to Johannes Boeke, Markus Pellmann.
Application Number | 20090320968 12/493390 |
Document ID | / |
Family ID | 40826012 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090320968 |
Kind Code |
A1 |
Boeke; Johannes ; et
al. |
December 31, 2009 |
DIFFERENTIAL HEAT SHAPING AND HARDENING USING INFRARED LIGHT
Abstract
A shaped part having at least two structural regions of
different ductility is made from one unitary blank of a hardenable
steel alloy by first heating the entire blank to an elevated
temperature below an AC.sub.3 point of the alloy. Then only first
regions of the blank are heated by a plurality of infrared lamps to
above the AC.sub.3 point of the alloy while maintaining the rest of
the blank below the AC.sub.3 point of the alloy. Finally the first
regions are hardened in a thermoshaping and hardening tool.
Inventors: |
Boeke; Johannes; (Blomberg,
DE) ; Pellmann; Markus; (Sassenberg, DE) |
Correspondence
Address: |
K.F. ROSS P.C.
5683 RIVERDALE AVENUE, SUITE 203 BOX 900
BRONX
NY
10471-0900
US
|
Family ID: |
40826012 |
Appl. No.: |
12/493390 |
Filed: |
June 29, 2009 |
Current U.S.
Class: |
148/546 ;
148/643 |
Current CPC
Class: |
C21D 1/34 20130101; C21D
1/673 20130101; C21D 9/48 20130101; C21D 1/185 20130101; C21D
2221/00 20130101 |
Class at
Publication: |
148/546 ;
148/643 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2008 |
DE |
102008030279.1 |
Claims
1. A method of making a shaped part having at least two structural
regions of different ductility from one unitary blank of a
hardenable steel alloy, the method comprising the steps of
sequentially: heating the entire blank with a plurality of infrared
lamps to an elevated temperature below an AC.sub.3 point of the
alloy; differentially heating only first regions of the blank to
above the AC.sub.3 point of the alloy while maintaining the rest of
the blank below the AC.sub.3 point of the alloy; hardening the
first regions in a thermoshaping and hardening tool.
2. The method defined in claim 1 wherein the entire blank is heated
to the elevated temperature below the AC.sub.3 point in a
pass-through furnace.
3. The method defined in claim 1 wherein the elevated temperature
is at most the AC.sub.1 point of the alloy.
4. The method defined in claim 1 wherein the elevated temperature
is above the AC.sub.1 point and below the AC.sub.3 point of the
alloy.
5. The method defined in claim 2 wherein the alloy consists
essentially of by weight percent: Carbon (C) 0.18% to 0.3% Silicon
(Si) 0.1% to 0.7% Manganese (Mn) 1.0% to 2.5% Phosphorus (P)
maximum 0.025% Chromium (Cr) up to 0.8% Molybdenum (Mo) up to 0.5%
Sulfur (S) maximum 0.01% Titanium (Ti) 0.02% to 0.05% Boron (B)
0.002% to 0.005% Aluminum (Al) 0.01% to 0.06% and remainder iron
and impurities resulting from melting.
6. The method defined in claim 1, further comprising the steps,
before heating the blank to the elevated temperature, coating the
blank; and completely alloying the coating of the blank.
7. The method defined in claim 6 wherein the coating is
aluminum.
8. The method defined in claim 1 wherein the first regions are
differentially heated by a plurality of infrared-light sources, the
method further comprising the step of: providing shields between
the regions at the sources.
9. A method of making a shaped part, the method comprising the
steps of sequentially: making an elongated workpiece blank of
alloyed sheet steel; heating the entire workpiece blank to an
elevated temperature at most equal to the AC.sub.1 point and below
an AC.sub.3 point of the alloy; heating only a center region of the
blank to above the AC.sub.3 point of the alloy before any part of
the blank cools below the AC.sub.1 point, the end regions remaining
below the AC.sub.3 point; and hardening the center in a
thermoshaping and hardening tool.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to method of making a
workpiece with regions of different ductility. More particularly
this invention concerns an alloy-steel workpiece.
BACKGROUND OF THE INVENTION
[0002] In the field of vehicle construction, more and more vehicle
parts made of high-strength and ultra-high-strength steel are being
employed in order to be able to satisfy criteria for light-weight
construction. This also applies to car body construction where, in
order to meet weight goals and safety requirements, for instance
structural and/or safety elements such as door intrusion beams, A
and B columns, bumpers, side rails, and cross rails are
increasingly being produced from thermoshaped and press-hardened
steel having tensile strengths greater than 1000 Mpa.
[0003] A method is known from GB 1,490,535 for press-shaping and
hardening a steel sheet that is relatively thin and of good
dimensional stability in which a sheet made of boron-alloyed steel
is heated to a temperature above its AC.sub.3 point and then in
less than 5 seconds is pressed into the final shape between two
indirectly cooled tools that change its shape significantly, and,
while still in the press is subjected to rapid cooling such that a
martensitic or bainitic structure is obtained. Using these measures
produces a finished product with good shape accuracy, good
dimensional stability, and high strength, and that is well suited
for structural and safety elements in vehicle construction. This
process is hereinafter referred to as thermoshaping and
press-hardening. Both preshaped parts and even flat panels can be
thermoshaped and press-hardened. In preshaped parts, the shaping
process can also be limited to shaping of a small percentage of the
final geometry or to calibration.
[0004] In different applications of motor vehicle engineering,
shaped parts are to have a high strength in certain regions while
in other regions they are to have higher ductility relative
thereto. In addition to reinforcing with additional sheet or
joining parts that have different strengths, also already known is
treating a part using heat treatments such that local regions have
higher strength or higher ductility.
[0005] It is known from US 2004/0060623 for instance to produce a
hardened metal part having at least two regions with different
ductility. A plate or preshaped shaped part is heated to an
austenitization temperature in a heater and then transported along
a transport path to a hardening process. During transport, first
regions of the plate or shaped part that have higher ductility
properties in the final part are cooled. The method is optimized
for mass production in that the first regions are quenched from a
predetermined cooling start temperature that is greater than the
.gamma.-.alpha. transformation temperature and in that the
quenching is terminated when a predetermined cool temperature is
attained, specifically prior to any transformation into ferrite
and/or perlite taking place or after an only slight transformation
into ferrite or perlite has taken place. Then the workpiece is
maintained approximately under an isothermal condition for
converting the austenite to ferrite and/or perlite. During this,
the hardening temperature (.sub.THE) in the second region, which
has lower ductility properties in the final part in comparison, is
just high enough for sufficient martensite formation in the second
regions during a hardening process. Then the hardening process is
performed. In this method, more thermal energy is added to the
first regions of the plate or shaped part than is necessary, and
then thermal energy is removed in a second process step, which is
also linked to expenditure of energy. The method therefore has a
relatively poor energy balance.
[0006] DE 101 08 926 C1 discloses a thermal treatment process for
changing the physical properties of a metal article. The article is
irradiated, at least in a predetermined surface section, with
electromagnetic radiation from an emitter having a radiator
temperature of 2900 K or more in the near infrared range with a
high power density. The material of the surface layer absorbs
predetermined treatment temperature as a function of the material
parameters. Then the irradiated surface region is actively cooled
and thus hardened and tempered. However, completely heating an
article that has a large surface area from room temperature to
hardening temperature using this method would be too uneconomical
for an industrial thermoshaping line.
[0007] U.S. Pat. No. 7,540,993 discloses a method for producing a
shaped part that has at least two regions with different ductility
from a semifinished product made of hardenable steel by heating in
a continuous furnace followed by a hardening process. During
transport through a continuous furnace the semifinished product to
be heated simultaneously passes through at least two zones in the
continuous furnace that are adjacent one another in the travel
direction and that have different temperature levels and thus are
heated differently so that in a subsequent hardening process at
least two structural regions are created that have different
ductility. The continuous furnace here is consequently provided
with at least two longitudinally extending zones that are mutually
adjacent in the longitudinal workpiece-advance direction, that are
separated from one another by a partition such that a workpiece
passing through the furnace has parts in both zones so separate
temperature control is possible in each zone. However, this
multizone furnace is a special furnace for parts that are to be
heated zone-wise.
OBJECTS OF THE INVENTION
[0008] It is therefore an object of the present invention to
provide an improved differential heating method for steel-alloy
workpieces.
[0009] Another object is the provision of such an improved
differential heating method for steel-alloy workpieces that
overcomes the above-given disadvantages, in particular that can use
a conventional thermoshaping line as economically as possible in
the press cycle for producing a differentially hardened part.
SUMMARY OF THE INVENTION
[0010] A shaped part having at least two structural regions of
different ductility is made from one unitary blank of a hardenable
steel alloy by first heating the entire blank to an elevated
temperature below an AC.sub.3 point of the alloy. Then heating only
first regions of the blank by a plurality of infrared lamps to
above the AC.sub.3 point of the alloy while maintaining the rest of
the blank below the AC.sub.3 point of the alloy. Finally the first
regions are hardened in a thermoshaping and hardening tool.
[0011] The first heater according to the invention preferably
comprises a conventional pass-through or tunnel-type furnace.
Differentially hardened parts can be produced in a conventional
thermoshaping line in this manner using the inventive method. Both
preshaped parts and even flat plates can be heated using the
inventive method, both being referred to hereinafter as blanks. In
the case of preshaped parts, the shaping process can also be
limited to shaping a small percentage of the final geometry or to
calibration of the shape.
[0012] In thermoshaping and press-hardening, a defined amount of
heat must be applied to the blank. All of the regions that are to
undergo as close to a complete structural change to martensite as
possible due to the hardening must be heated in advance to a
temperature that is greater than or equal to the AC.sub.3 point of
the alloy. These are referred to as first regions hereinafter.
Regions that are not to be hardened or are not to be completely
hardened, referred to as second regions hereinafter, must not be
heated to a temperature above the AC.sub.3 point. For the
press-hardening process it would be sufficient if the second
regions were room temperature. This would also be the best variant
in terms of energy, but at room temperature steel is significantly
less malleable than heated steel. Therefore, at least for more
complex deep-drawn parts, it is necessary for the shaping process
that the steel be heated even in the second regions, especially
since common thermoshaping steel springs back after cold-shaping,
which has a negative effect on the tolerances that are to be
maintained. In addition, there is the fact that if the temperature
gradient between the first regions and the second regions is too
great, stresses are produced in the transition region after
hardening. In order to prevent formation of martensite in the
second regions after hardening, in one preferred embodiment the
second regions are heated to a temperature up to a maximum of the
AC.sub.1 point of the alloy. Once the AC.sub.1 point has been
exceeded, a partial structural change begins that after hardening
can also lead to partial martensite formation, which is not
desired. Conversely, however, heating with the infrared lamps
should not last too long. Therefore the start temperature for the
lamp heating using infrared should be as high as possible.
Consequently the entire part is preferably heated to a homogeneous
temperature up to a maximum of the AC.sub.1 point of the alloy in a
continuous furnace and then is transferred to the infrared lamp
field in order to heat first regions to above the AC.sub.3 point.
While this is happening, the second regions are not irradiated with
infrared at all or are merely maintained at their temperature. In
this manner heating by means of infrared is performed rapidly
enough to ensure the production sequence in the press cycle. If
heating the first regions by means of infrared to above the
AC.sub.3 point is slower than the press cycle, two or more infrared
lamp fields must be used. It is therefore an advantage of the
inventive method that it is possible to retain the conventional
continuous furnaces in a conventional production line for the
thermoshaping and to be able to simply and economically retrofit
the conventional line for production of an only differentially
hardened part. In addition, in an existing production line, it is
possible to construct the heating furnace simpler and more
economically overall if the furnace only has to withstand reach
temperatures up to AC.sub.1 and not above the AC.sub.3 point in
continuous operation.
[0013] In another preferred embodiment, the blank is heated overall
to a homogenous temperature below the AC.sub.3 point, but greater
than the AC.sub.1 point of the alloy and is then transferred to an
infrared lamp field in which the first regions are heated to above
the AC.sub.3 point. Then after hardening a mixed structure occurs
in the second areas, and this mixed structure settles between the
properties of the initial structure and the properties of the hard
structure. This mixed structure can be advantageous for certain
purposes. The part parameters can therefore be flexibly adjusted as
needed by controlling the power of the infrared lamps.
[0014] The method is suitable in particular for thermoshaping an
uncoated boron-alloy steel that constitutes, as expressed in weight
percent: [0015] Carbon (C) 0.18% to 0.3% [0016] Silicon (Si) 0.1%
to 0.7% [0017] Manganese (Mn) 1.0% to 2.5% [0018] Phosphorus (P)
maximum 0.025% [0019] Chromium (Cr) up to 0.8% [0020] Molybdenum
(Mo) up to 0.5% [0021] Sulfur (S) maximum 0.01% [0022] Titanium
(Ti) 0.02% to 0.05% [0023] Boron (B) 0.002% to 0.005% [0024]
Aluminum (Al) 0.01% to 0.06% and [0025] remainder iron and
impurities resulting from melting.
[0026] A blank made of this steel is first heated homogeneously to
at least 400.degree. C., preferably to about 700.degree. C., and
then is heated in the first regions to a temperature of about
930.degree. C. by means of infrared lamps. The second regions are
maintained at approximately 700.degree. C. while this heating takes
place. Immediately following the heating, the blank is fitted to a
thermoshaping and hardening tool and shaped and hardened in first
regions. A differentially hardened, dimensionally accurate,
thermoshaped part with defined properties in the respective regions
is thus obtained.
[0027] However, the method can also be employed for a coated metal
workpiece such as for instance thermoform steel coated with
aluminum or zinc. However, in particular a thermoform steel coated
with a layer containing aluminum must initially be heated to a
temperature above the AC.sub.3 point of the alloy and fully alloyed
in order to form a so-called intermetal phase. In order to use the
inventive method described herein in a cost-effective manner, a
thermoform steel coated with aluminum must therefore first be fully
alloyed in a separate work step. It would be best for this work
step to be performed by the steel manufacturer when the coil is
produced.
BRIEF DESCRIPTION OF THE DRAWING
[0028] The above and other objects, features, and advantages will
become more readily apparent from the following description,
reference being made to the accompanying drawing in which:
[0029] FIG. 1 is a schematic diagram of an inventive thermoshaping
line for an uncoated steel workpiece;
[0030] FIG. 2 is a schematic diagram of an inventive thermoshaping
line for a coated steel workpiece;
[0031] FIG. 3 is a large-scale view of a detail of the infrared
lamp heater of FIGS. 1 and 2;
[0032] FIG. 4 shows the hardness distribution in an B-column
according to the invention;
[0033] FIG. 5 is a schematic top view of the infrared lamp station
70; and
[0034] FIG. 6 shows a heating curve for the first region.
SPECIFIC DESCRIPTION
[0035] As seen in FIG. 1 a thermoshaping line 1 has a
workpiece-supply coil 2 of uncoated thermoform steel-alloy sheet
that is unwound and cut to create a shaped blank 4 at a cutting
station 3. The shaped blank 4 can be selectively cold preshaped in
a molding station 5 and/or can be cut. As a rule, cold-shaping is
deep-drawing at room temperature, and trimming is done as close to
the final contours as possible. The shaping station 5 is optional
and depends on the complexity of the part's shape, and there may be
no shaping station 5 at all.
[0036] Then the shaped blank 4 is transferred in a travel direction
that is left to right in the drawing directly to a heating station
6. In the heating station 6, the shaped blank 4 is homogeneously
heated to a temperature that is below the AC.sub.3 point and is
then immediately transferred to an infrared lamp station 7. The
infrared lamp station 7 is shown as a separate station here.
However, the infrared lamps can also for instance be integrated
into the heating station 6, for instance at its downstream end. In
the infrared lamp station 7, a first region of the shaped blank 4
is heated to a temperature above the AC.sub.3 point of the steel
alloy to produced a differentially heat-treated B-column 41. Second
regions remain at a temperature that is below the AC.sub.3
point.
[0037] In the illustrated embodiment in FIG. 1, the second regions
are at each end of the treated blank 41 and the first region is in
the center of the shaped blank 4. The shaped blank 41 preheated in
this manner is then advanced to a force-cooled shaping and
hardening tool 8 and is there thermoshaped and differentially
hardened.
[0038] FIG. 2 shows an embodiment of a thermoshaping line 10 for
coated steel sheet. A coil 20 of thermoform steel coated with an
alloy containing aluminum is continuously unwound and rewound after
moving through a heater 9. In the heater 9, the coated thermoform
steel is homogeneously heated to a temperature above the AC.sub.3
point so that the coating is completely alloyed and forms with the
base metal a so-called intermetal phase. The heated coated steel is
not quenched at this point, however, so that it does not harden,
because then its resistance to deformation would be too high for
further processing. When it leaves the heater 9, the completely
alloyed coated steel is re-wound onto a second coil 21.
[0039] The coated steel is then continuously unwound from this coil
21 and cut into a coated shaped blank 40 at a cutting station 3.
The molding station 5 for cold preshaping is not used because the
intermetal phase that occurred during the complete alloying process
cannot be cold shaped without cracking. Therefore the shaped blank
40 is transferred directly to the heating station 6. In the heating
station 6, the coated shaped blank 40 is homogeneously heated to a
temperature that is below the AC.sub.3 point and is then
immediately transferred to the infrared lamp station 7. The
infrared lamp station 7 is shown as a separate station here.
However, the infrared lamps can also for instance be integrated
into the heating station 6, for instance in the end area. In the
infrared lamp station 7, the first region of the shaped blank 40 is
heated to a temperature above the AC.sub.3 point of the alloy. The
second regions remain at a temperature below the AC.sub.3 point. In
the illustrated embodiment in FIG. 2, the second regions are at
each end of the shaped blank 40 and the first region is in the
center of the shaped blank 40.
[0040] The shaped blank 40 preheated in this manner is then
supplied to a force-cooled shaping and hardening tool 8 and is
thermoshaped and differentially hardened in the station 8.
[0041] FIG. 3 provides a detailed view of the infrared lamp station
7 in FIGS. 1 and 2. Rod-shaped infrared lamps 71 are attached to an
overhead support 75. The infrared lamps 71 are controlled in the
temperature fields 72 and 74 such that they maintain the two end
regions of the preshaped and preheated part 41 that are on a
support plate 76 at 700.degree. C. In the temperature field 73, the
rod-shaped infrared lamps are controlled such that they heat the
center of the part 41 to 930.degree. C. In this FIG. 3, the
temperature fields 72, 73, and 74 are separated from one another by
shields or partitions 77 and 78. The shields 77 and 78 make it
easier to control the temperature distribution in the part 41 and
to adjust the hardness values more precisely in the finished
part.
[0042] As shown in FIG. 4 after the thermoshaping and hardening, a
differentially hardened B column 42 has been created from the blank
41 in FIG. 3. The B column 42 is relatively ductile in the head
area 43 and in the foot 44. The B column has been hardened in the
center region 47 and a mixed structure was created in the
transition regions 45 and 46 from the hardened region to the
unhardened regions.
[0043] FIG. 5 is a schematic top view of another embodiment 70 of
an infrared lamp station. The heated shaped blank 4 is positioned
beneath spot-like infrared lamps 710. The shaped blank 4 is
maintained at a temperature of 700.degree. C. in the head region 43
and in the foot region 44, both constituting the "second" regions.
The shaped blank 4 is heated to 930.degree. C. in the center region
47 constituting the "first" region. The temperature drops from
930.degree. C. to 700.degree. C. in the transition regions 45 and
46.
[0044] FIG. 6 shows a heating curve 110 for a first region in a
sheet. The temperature is shown in .degree. C. over time in
seconds. The curve area 11 shows the continuous heating of the
sheet in a continuous furnace. The entire sheet is homogeneously
heated from room temperature to about 700.degree. C. in just under
200 seconds. Then at curve point 12 the sheet is transferred to a
position beneath an infrared lamp field and within about 30 seconds
it is heated to just under 1000.degree. C. Heating concludes at
point 13
* * * * *