U.S. patent application number 16/076923 was filed with the patent office on 2019-02-14 for method and device for producing hardened steel components.
The applicant listed for this patent is voestalpine Metal Forming GmbH, voestalpine Stahl GmbH. Invention is credited to JOHANNES HASLMAYR, SIEGFRIED KOLNBERGER, HARALD SCHWINGHAMMER, ANDREAS SOMMER, BENEDIKT TUTEWOHL.
Application Number | 20190048432 16/076923 |
Document ID | / |
Family ID | 58185488 |
Filed Date | 2019-02-14 |
![](/patent/app/20190048432/US20190048432A1-20190214-D00000.png)
![](/patent/app/20190048432/US20190048432A1-20190214-D00001.png)
![](/patent/app/20190048432/US20190048432A1-20190214-D00002.png)
United States Patent
Application |
20190048432 |
Kind Code |
A1 |
HASLMAYR; JOHANNES ; et
al. |
February 14, 2019 |
METHOD AND DEVICE FOR PRODUCING HARDENED STEEL COMPONENTS
Abstract
The invention relates to a method for press hardening sheet
steel components in which a blank is detached from a sheet steel
band composed of a hardenable steel alloy and the blank is then
austenitized, in that it is heated to a temperature greater than
Ac.sub.3 and is then inserted into a forming tool and formed in the
forming tool, and during the forming, is cooled at a speed greater
than the critical hardening speed, characterized in that in order
to inhibit microcracks of the second type from being produced
during the forming and hardening process in the sheet metal blanks
that are to be formed, oxygen is supplied adjacent to the positive
radii and/or drawing edges; the invention also relates to a device
for performing this method.
Inventors: |
HASLMAYR; JOHANNES; (Linz,
AT) ; KOLNBERGER; SIEGFRIED; (Pasching, AT) ;
SCHWINGHAMMER; HARALD; (Pasching, AT) ; SOMMER;
ANDREAS; (Abtsgmund, DE) ; TUTEWOHL; BENEDIKT;
(Durlangen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
voestalpine Stahl GmbH
voestalpine Metal Forming GmbH |
LINZ
KREMS AN DER DONAU |
|
AT
AT |
|
|
Family ID: |
58185488 |
Appl. No.: |
16/076923 |
Filed: |
February 7, 2017 |
PCT Filed: |
February 7, 2017 |
PCT NO: |
PCT/EP2017/052603 |
371 Date: |
August 9, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/673 20130101;
C21D 9/48 20130101 |
International
Class: |
C21D 1/673 20060101
C21D001/673 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2016 |
DE |
102016102322.1 |
Claims
1. A method for press hardening sheet steel components in which a
blank is detached from a sheet steel band composed of a hardenable
steel alloy and the blank is then austenitized, the method
comprising: heating the blank to a temperature greater than
Ac.sub.3; inserting the blank into a forming tool having a mold
cavity; and forming the blank in the forming tool, wherein during
the forming, the blank is cooled at a speed greater than the
critical hardening speed, characterized in that in order to avoid
microcracks of second type from forming in the sheet metal blanks
to be formed during the forming and a subsequent hardening, an
oxygen-containing fluid reservoir is present adjacent to positive
radii and/or drawing edge and/or in other contact regions outside
of the positive radii and/or drawing edges.
2. The method according to claim 1, characterized in that entry of
oxygen takes place by means of at least one recess provided in the
forming tool adjacent to the drawing edges and/or positive radii,
which are dimensioned so that deep drawing is not negatively
affected and the recess forms a reservoir for oxygen-containing
fluids or else oxygen-containing fluids can be supplied via this
recess.
3. The method according to claim 2, characterized in that the entry
of oxygen-containing fluid is ensured by means of the air that is
present in the recess.
4. The method according to claim 2, characterized in that the
recess is supplied with fluids or oxygen or oxygen-containing
fluids from the forming tool side, or the recess or mold cavity is
supplied with fluids, in particular oxygen or an oxygen-containing
fluid, between two forming procedures.
5. The method according to claim 2, characterized in that vacuum is
applied to the recess.
6. The method according to claim 1, characterized in that
oxygen-containing fluid is supplied continuously.
7. A device for performing the method according to claim 1, having
two faulting tool halves; the two forming tool halves cooperate in
order to deep-draw a blank and are embodied so that they can move
toward and away from each other; depending on a desired forming
contour, at least one positive radius (1) or one drawing edge
region (1) is provided with a drawing edge (2), wherein a recess
(5) is provided in a surface (4) that is situated after the drawing
edge (2) or the positive radius (1) in the drawing direction and/or
in other contact regions outside of the positive radii (1) and/or
drawing edge (2).
8. The device according to claim 7, characterized in that the
recess (5) is dimensioned so that the remaining thickness of the
drawing edge (2) between a surface, which adjoins the drawing edge
(2), and the recess (5) corresponds approximately to its
radius.
9. The device according to claim 7, characterized in that the
recess (5) between the drawing edge (2) and a forming tool surface
(4) has a height, which corresponds to approximately 25 to 35 mm at
a depth of 5 to 9 mm or is embodied as a groove (6), which has a
height between the surface (4) and the drawing edge (2), which
totals approximately 8 to 12 mm, with a depth of 5 to 9 mm or in
the region of the wall (4) adjacent to the drawing edge (2), a
plurality of recesses in the form of grooves (7) extending in the
drawing direction; and the grooves (7) or slots (7) have a slot
width of 4 to 8 mm and a slot spacing of 7 to 11 mm so that
remaining bridge pieces have a width of 1 to 5 mm.
10. The device according to claim 9, characterized in that the
recesses (5), the grooves (6), or the slot (7) are supplied from
the rear, i.e. from the tool side, with an oxygen-containing fluid
by means of supply openings and correspondingly drilled lines.
Description
[0001] The invention relates to a method and device for producing
hardened steel components.
[0002] Hardened steel components, particularly in vehicle body
construction for motor vehicles, have the advantage that due to
their outstanding mechanical properties, it is possible to achieve
a particularly stable passenger compartment without having to use
components that are much more massive at normal strengths and must
therefore be embodied as much heavier.
[0003] To produce hardened steel components of this kind, steel
types are used that can be hardened by means of a quench hardening.
Steel types of this kind include, for example, boron-alloyed
manganese carbon steels, the most widely-used of these being
22MnB5. But other boron-alloyed manganese carbon steels are also
used for this purpose.
[0004] In order to produce hardened components from these types of
steel, the steel material must be heated to the austenitization
temperature (>Ac.sub.3) and it is necessary to wait until the
steel material is austenitized. Depending on the desired degree of
hardness, partial or complete austenitization can be achieved in
this connection.
[0005] If after the austenitization, such a steel material is
cooled at a speed that is above the critical hardening speed, then
the austenitic structure converts into a martensitic, very hard
structure. In this way, it is possible to achieve tensile strengths
R.sub.in of up to over 1500 MPa.
[0006] Currently, two different procedural approaches are commonly
used for producing steel components.
[0007] In so-called form hardening, a sheet steel blank is detached
from a steel band, for example cut out or stamped out from it, and
then--using a conventional, for example five-step, deep drawing
process--is deep drawn to produce the finished component. This
finished component in this case is dimensioned somewhat smaller in
order to compensate for a subsequent thermal expansion during the
austenitization.
[0008] The component produced in this way is austenitized and then
inserted into a form hardening tool in which it is pressed, but is
not formed or is only formed to a very slight extent and by means
of the pressing, the heat flows out of the component and into the
press tool, specifically at the speed greater than the critical
hardening speed.
[0009] The other procedural approach is so-called press hardening
in which a blank is detached from a sheet steel band, for example
cut out or stamped out from it, then the blank is austenitized and
the hot blank is formed at a temperature below 782.degree. C. in a
preferably one-stage step and at the same time, is cooled at a
speed greater than the critical hardening speed.
[0010] In both cases, it is possible to use blanks provided with
metallic anticorrosion coatings e.g. with zinc or a zinc-based
alloy. Form hardening is also referred to as the indirect process
and press hardening is referred to as the direct process. The
advantage of the indirect process is that it is possible to achieve
more complex tool geometries.
[0011] The advantage of the direct method is that a higher material
utilization ratio can be achieved. But the achievable component
complexity is lower, especially with the one-stage forming
process.
[0012] In press hardening, however, it is disadvantageous that
microcracks form in the surface, particularly with galvanized sheet
steel blanks.
[0013] In this connection, a distinction is drawn between
first-order microcracks and second-order microcracks.
[0014] First-order microcracks are attributed to so-called liquid
metal embrittlement. The theory is that during the forming, i.e. as
tensile stresses are being exerted on the material, liquid zinc
phases interact with still existing austenite phases, causing
microcracks with depths of up to a few hundred .mu.m to be produced
in the material.
[0015] The applicant has succeeded in suppressing these first-order
microcracks by actively or passively cooling the material--in the
time between the removal from the heating furnace and the start of
the hot-forming process--to temperatures at which liquid zinc
phases are no longer present. This means that the hot-forming takes
place at temperatures below approximately 750.degree. C.
[0016] Up to this point, it has not been possible to control the
second-order microcracks in hot-forming despite pre-cooling and
they occur even at hot-forming temperatures below 600.degree. C.
The crack depths in this case amount to a few tens of .mu.m.
[0017] Neither first-order microcracks nor second-order microcracks
are accepted by users since they constitute potential sources of
damage.
[0018] With the previous methods, it has not yet been possible to
ensure a production of components without second-order
microcracks.
[0019] DE 10 2011 055 643 A1 has disclosed a method and a forming
tool for hot-form press hardening components made of sheet steel,
particularly made of galvanized workpieces composed of sheet steel.
In this case, the female dies used for the hot-forming and press
hardening--in their drawing edge region that is defined by a
positive drawing radius--should be liquid-coated with a material or
provided with an insert piece, which has a thermal conductivity
that is at least 10 W/(m.times.K) less than the thermal
conductivity of the section of the female die that is adjacent to
the drawing edge region and that comes into contact with the
workpiece as the latter is being hot-formed and press hardened. The
material that is applied to the surface of the drawing edge region
facing the workpiece or of the insert piece that has been put into
position should have a transverse dimension extending across the
drawing edge that is in a range of 1.6 to 10 times the positive
drawing radius of the female die. This should improve the flow
properties of workpieces made of sheet steel during the hot-forming
and should thus significantly reduce the risk of the occurrence of
cracks in the hot-forming of workpieces made of sheet steel,
preferably made of galvanized steel blanks. Such a tool, however,
does not make it possible to avoid microcracks of the second
type.
[0020] DE 10 2011 052 773 A1 has disclosed a tool for a press
hardening tool in which the mold surface of the tool is
microstructured in some regions by two micro-cavities that are
introduced into the mold surface. This step is intended to four
restrict the effective contact area for the forming of a blank
between the mold surface with a blank to the surface portions
situated between the cavities. This is intended to reduce the
friction.
[0021] DE 10 2004 038 626 B3 has disclosed a method for producing
hardened components out of sheet steel in which before or after the
forming of the formed part, a required final trimming of the formed
part and any necessary punching procedures or the production of a
hole pattern is carried out and the formed part is then heated at
least in some areas to a temperature that enables an
austenitization of the steel material; the component is then
transferred to a form hardening tool and a form hardening is
carried out in the form hardening tool in which the component is
cooled and thus hardened at least in some areas by the contact and
pressing of the component; and the component is supported by the
form hardening tool in the region of the positive radii and is
preferably held by two clamps in the region of the trim edges and
in regions in which the component is not clamped, the component is
at least spaced apart from a mold half by means of a gap. This
measure makes it possible to clamp the component in a
distortion-free manner and to set different hardness gradients by
means of different hardening speeds.
[0022] The object of the invention is to avoid microcracks of the
second type in directly hot-formed, i.e. press hardened,
components.
[0023] The object attained with a method having the features of
claim 1.
[0024] Advantageous modifications are characterized in the
dependent claims.
[0025] Another object of the invention is to create a device with
which sheet steel blanks can be hot-formed and hardened in the
press hardening process and in which microcracks are avoided.
[0026] The object is attained with a device having the features of
claim 6. Advantageous modifications are characterized in the
dependent claims that depend on this claim.
[0027] The inventors have realized that microcracks of the second
type are produced when, in regions under tensile strain, the zinc
vapor that occurs arrives at the steel in a sufficient
concentration, so-called vapor metal embrittlement (VME). Zinc
vapor is produced due to the tearing of the zinc/iron layer that
occurs in the stretching during the forming process. A sufficient
concentration particularly occurs in those regions in which direct
contact of the sheet metal with the tool prevails or the sheet
metal is a very small distance from the tool. A very small distance
as defined by the invention is being less than 0.5 mm.
[0028] According to the invention, second-order microcracks should
be avoided, while retaining the largest possible working window
with regard to the material and temperature and ensuring an
inexpensive implementation. With at least the same residence time,
there should be no increase in cycle time or reduction in
throughput during component production.
[0029] According to the invention, in the regions under tensile
strain (elongation edge fiber), the zinc vapor that occurs is
either conveyed away by gas flows (convection) or more precisely,
blown away, or is sufficiently diluted. Alternatively or in
addition to this, through an influx of fluids, zinc can be quickly
transformed into a stable compound such as zinc oxide or ZnI.sub.2.
In addition, the protection of the steel from second-order
microcracks can also be achieved by producing a protective layer
such as an oxide layer by supplying a fluid. All of the measures
described above have respectively demonstrated that microcracks are
significantly reduced.
[0030] Gaseous oxygen-containing fluids such as air or oxygen are
particularly preferable because they cannot excessively contaminate
the tool and in addition, a possibly unwanted massive cooling
action of the kind that can occur, for example, by means of water
can be more easily regulated by tempering the fluid.
[0031] In this case, the avoidance of second-order microcracks is
ensured by the fact that in the region of the positive radii, i.e.
in the drawing edge region of the female die and/or male die, after
the drawing edge or other contact regions situated outside of the
positive radii/drawing edge in the drawing direction, a recess is
provided, which is dimensioned so that on the one hand, the deep
drawing is not negatively affected or the blank or workpiece
becomes wavy and on the other hand, is dimensioned so that the
outflow of heat that is necessary for the hardening is likewise not
negatively affected to a significant degree.
[0032] The recesses, however, are dimensioned so that they
constitute a reservoir for oxygen in such a way that a sufficient
amount of oxygen travels to the blank that is being drawn and to
the material in order to supply oxygen for oxidation to the zinc
phases or zinc/iron phases that are being released.
[0033] The recess functions as a fluid reservoir in particular for
oxygen, but this reservoir can also contain other fluids such as
water or also nitrogen. If these reservoirs are filled with an
inert gas or are also continuously flushed with an inert gas, then
they do not function by means of oxidation, but rather by diluting
or carrying away the zinc vapor that occurs.
[0034] If need be, the recesses can advantageously be continuously
supplied from the tool side with oxygen-containing fluids during
the forming, for example via suitable entry openings,
advantageously permitting a flow cushion to form. In addition,
after the removal of a workpiece from the mold and before the
insertion of another blank, the mold cavity can be flushed with a
fluid, in particular oxygen-containing one, which is then present
in the recesses. Examples of an oxygen-containing fluid include air
as well as water, i.e. they can be supplied in both liquid and
gaseous form.
[0035] It has turned out that these recesses, even when their
dimensions are only relatively small, effectively prevent the
formation of second-order microcracks through oxidation of zinc
phases or zinc/iron phases.
[0036] The invention will be explained by way of example based on
the drawings. In the drawings:
[0037] FIG. 1 shows the tool region adjacent to a drawing edge with
a recess according to the invention;
[0038] FIG. 2 shows the drawing edge region of a tool with a
different embodiment of the recess according to the invention;
[0039] FIG. 3 is a partially cut-away side view of the drawing edge
region of a tool with a slot arrangement according to the
invention;
[0040] FIG. 4 is a top view of the arrangement according to FIG.
3.
[0041] The drawing edge region 1 or the region of a positive radius
1 is positioned on a forming tool and has two surfaces 3, 4
oriented toward the workpiece, which meet in the region of a
drawing edge or positive radius 2.
[0042] A recess 5 according to the invention is provided in a
surface 4 situated after the drawing edge 2 in the drawing
direction. The recess 5 in this case is dimensioned so that the
remaining thickness of the drawing edge 2 between the surface 3 and
the recess 5 corresponds approximately to its radius in order to
offer a sufficient supporting action for the material that is to be
drawn.
[0043] Naturally, other recesses can be provided, which are
positioned in regions in which the sheet metal contacts the tool;
these contact regions are defined by means of a maximum distance of
approx. 0.5 mm between the sheet metal and the tool.
[0044] Between the drawing edge 2 and the surface 4, the recess 5
has a height that is approximately 25 to 35 mm, with a depth of 5
to 9 mm.
[0045] In another advantageous embodiment (FIG. 2), instead of
providing a large-area recess 5 adjacent to the drawing edge 2 and
leaving it with the thickness described above, a groove 6 is
introduced into the surface 4. The groove 6 in this case has a
height between the surface 4 and the drawing edge 2 that totals
approximately 8 to 12 mm, with a depth of 5 to 9 mm.
[0046] In a further embodiment, instead of a continuous recess 5 in
the region of the wall 4 adjacent to the drawing edge 2, a
plurality of grooves 7 is provided, which extend in the drawing
direction; for example, the grooves 7 or slots 7 have a slot width
of 4 to 8 mm and a slot spacing of 7 to 11 mm so that the remaining
bridge pieces have a width of 1 to 5 mm. The grooves 7 or slots 7
in this case likewise have a depth of 5 to 9 mm.
[0047] It has surprisingly turned out that with the above-mentioned
geometries, the relatively small quantity of fluid inside the
recesses 5, 6, 7--possibly even despite the presence of the bridge
pieces 4--is sufficient in order to effectively prevent the
formation of microcracks of the second type through the provision
of oxygen.
[0048] In an advantageous embodiment (not shown), the recesses 5,
the groove 6, and the slots 7 are supplied from the rear, i.e. from
the tool side, with an oxygen-containing fluid by means of supply
openings and correspondingly drilled lines in order, if need be, to
further increase the partial pressure of oxygen in the region of
the recesses 5, grooves 6, and slots 7.
[0049] In order to keep the oxygen content at a high level in the
recesses 5, grooves 6, and slots 7 during continuous processing,
the mold cavity can also be flushed with an oxygen-containing fluid
so that at all times, there is a sufficient oxygen reservoir in the
recesses 5, grooves 6, and slots 7.
[0050] Primarily in the direct press hardening process, 20MnB8,
22MnB8, and other manganese/boron steels are also used in addition
to 22MnB5.
[0051] Consequently, steels of the following alloy composition are
suitable for the invention (all indications in mass %):
TABLE-US-00001 C Si Mn P S Al Cr Ti B N [%] [%] [%] [%] [%] [%] [%]
[%] [%] [%] 0.20 0.18 2.01 0.0062 0.001 0.054 0.03 0.032 0.0030
0.0041
and the rest made up of iron and smelting-induced impurities; in
such steels, particularly the alloy elements boron, manganese,
carbon, and optionally chromium and molybdenum, are used as
transformation-delaying agents.
[0052] Steels of the following general alloy composition are also
suitable for the invention (all indications in mass %):
TABLE-US-00002 Carbon (C) 0.08-0.6 Manganese (Mn) 0.8-3.0 Aluminum
(Al) 0.01-0.07 Silicon (Si) 0.01-0.8 Chromium (Cr) 0.02-0.6
Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02
Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo)
<1
and the rest made up of iron and smelting-induced impurities.
[0053] The following steel configurations have turned out to be
particularly suitable (all indications in mass %).
TABLE-US-00003 Carbon (C) 0.08-0.35 Manganese (Mn) 1.00-3.00
Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr)
0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B)
0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum
(Mo) <1
and the rest made up of iron and smelting-induced impurities.
* * * * *