U.S. patent application number 16/074303 was filed with the patent office on 2021-06-24 for method and device for producing hardened steel components.
This patent application is currently assigned to voestalpine Stahl GmbH. 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 | 20210189513 16/074303 |
Document ID | / |
Family ID | 1000005503882 |
Filed Date | 2021-06-24 |
United States Patent
Application |
20210189513 |
Kind Code |
A1 |
HASLMAYR; Johannes ; et
al. |
June 24, 2021 |
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 |
|
|
Assignee: |
voestalpine Stahl GmbH
Linz
AT
voestalpine Metal Forming GmbH
Krems an der Donau
AT
|
Family ID: |
1000005503882 |
Appl. No.: |
16/074303 |
Filed: |
February 7, 2017 |
PCT Filed: |
February 7, 2017 |
PCT NO: |
PCT/EP2017/052604 |
371 Date: |
July 31, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B21D 28/26 20130101;
C21D 1/673 20130101; C21D 9/0062 20130101; B21D 22/022 20130101;
C21D 1/613 20130101 |
International
Class: |
C21D 1/673 20060101
C21D001/673; B21D 22/02 20060101 B21D022/02; B21D 28/26 20060101
B21D028/26; C21D 9/00 20060101 C21D009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2016 |
DE |
10 2016 102 324.8 |
Claims
1. A method for press hardening sheet steel components comprising:
cutting a blank out from a sheet steel band composed of a
hardenable steel alloys; austenitizing the blank, by heating it to
a temperature greater than Ac.sub.3, inserting it into a forming
tool, and forming in the forming tool, and during the forming,
cooling it 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 blank to be formed during the
forming and hardening process, oxygen is supplied in and/or
adjacent to the positive radii and/or drawing edges and/or in
contact regions.
2. The method according to claim 1, characterized in that the entry
of oxygen by means of inserts (1) made of oxygen-storing materials
are provided in the forming tool adjacent to or in the region of
the drawing edges and/or positive radii, which are dimensioned so
that deep drawing is not negatively affected and the inserts (1)
form a reservoir for oxygen.
3. The method according to claim 2, characterized in that inserts
(1) made of sintered metals, porous ceramics, or impervious
ceramics are used.
4. The method according to claim 2, characterized in that the
inserts (1) are supplied with oxygen or oxygen-containing fluids
from the forming tool side, or the inserts (1) or mold cavity are
flooded with oxygen or an oxygen-containing fluid between two
forming procedures.
5. A device for the press hardening or hot forming and hardening of
sheet steel components, having two forming 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 or one drawing edge region is provided with a drawing edge
(2), a ceramic insert being positioned in lieu of a metallic
drawing edge (2), wherein the it is inserted into the respective
forming tool half in a form-fitting way.
6. The device according to claim 5, characterized in that in the
ceramic insert, a recess (7) is provided, which 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.
7. The device according to claim 6, 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 (8), 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 slots (9) extending in the
drawing direction; and the slots (9) 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.
8. The device according to claim 7, characterized in that the
recess (7), the groove (8), or the slots (9) 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.
9. The method according to claim 3, characterized in that the
inserts (1) are supplied with oxygen or oxygen-containing fluids
from the forming tool side, or the inserts (1) or mold cavity are
flooded with oxygen or an oxygen-containing fluid between two
forming procedures.
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 steel 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.m 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 cut out
from a steel band 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 cut out from a sheet steel band, then the blank
is austenitized and the hot blank is formed in a 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
anticorrosion coatings such as zinc. 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 with a lower component
complexity.
[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 this by cooling
the material--in the time between the removal from the heating
furnace and the insertion into the forming tool--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 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, however, it has not 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, at least in some areas, 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 is 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 5. 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), through an influx of
oxygen-containing fluids, the zinc vapor or free zinc that occurs
is 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. The
measures described above have 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] According to the invention, in the tool--preferably in the
region of the positive radii or adjacent to the positive
radii--inserts are used, which permit an entry of oxygen when the
sheet metal blank is being deformed, i.e. when the blank material
is flowing. In addition, inserts can also be provided at narrow
points or contact regions of the sheet metal blank with the tool,
these contact regions being defined as the regions in which the
distance of the sheet metal to the tool is at most 0.5 mm.
[0032] To this end, the corresponding material naturally has to be
supported in the region of the positive radii because these are the
edges that produce the deformation and initiate the flow of
material.
[0033] Adjacent to these edges and spaced apart from them so that
the inserts are not damaged, the inserts have means that enable an
entry of oxygen. These means can, for example, be sintered metal
inserts or porous ceramic inserts in which, after the move away
from each other and the workpiece hardens and before a new blank is
inserted, enough oxygen is stored that it can be imparted to
released zinc or released zinc phases.
[0034] Furthermore, the inserts have surfaces that are left open so
that the material, after it has flowed past the edge, is spaced
apart from the insert.
[0035] In an advantageous embodiment, this left-open region is
embodied as slotted so that a minimum support of the material is
possible, but the entry of oxygen is ensured.
[0036] In all of these cases, there can also be fluid connection
lines, which feed into the open regions or into the regions that
that are filled with sintered metal or porous ceramic so that a
sufficient amount of oxygen is supplied. In the simplest case, this
can be air or also water vapor, for example.
[0037] Materials that inherently have a high oxygen diffusion
capacity such as certain ceramics can also be embodied in a massive
form and are acted on with oxygen-containing fluids either while
the press is open or from the rear and store this oxygen until it
can be imparted to released zinc iron phases or released zinc.
[0038] These inserts can be embodied on both the female die and the
male die.
[0039] A charging with oxygen can also be carried out by flooding
the mold cavity, for example with water vapor or with the mediums
already mentioned above.
[0040] The invention will be explained by way of example based on
the drawings. In the drawings:
[0041] FIG. 1 shows an example of a tool insert in a massive
embodiment;
[0042] FIG. 2 shows a tool insert with a recess;
[0043] FIG. 3 shows another tool insert with a recess;
[0044] FIG. 4 is a sectional side view of a slotted tool
insert;
[0045] FIG. 5 shows the slotted tool insert in a view from the
forming surface.
[0046] For example, an insert 1 is made of a ceramic and in
particular, of an oxide ceramic. The ceramic insert extends along
drawing edges 2 and is used in the tool in lieu of the metallic
drawing edge 2; it has a back side 3 and an underside 4 with which
it is inserted in a form-fitting way into a recess in the metallic
tool. In addition, the ceramic insert 1 has a top side 6 and
mold-front side 5, the mold-front side 5 and top side 6 preferably
being flush with the corresponding surfaces of the tool.
[0047] This ceramic insert can be embodied as massive or impervious
and hard or porous and hard.
[0048] In the region of the surfaces 3 or 4, leading from the
metallic forming tool and corresponding to the latter, a gas
connection (not shown) can be provided, if the ceramic is embodied
as oxygen-conducting or porous, which brings a sufficient
concentration of oxygen through the insert 1 to the region of the
surfaces 5 and the drawing edge 2.
[0049] In another advantageous embodiment (FIG. 2), a recess 7 is
produced in the region of the surface 5 adjacent to the drawing
edge 2. For example, the recess 7 has a depth of 5 to 10 mm,
whereas the insert as a whole has a height between the surfaces 4
and 6 of 35 to 50 mm and a width between the surfaces 3 and 5 of 15
to 30 mm, for example.
[0050] Preferably, the drawing edge 2 in this case is embodied so
that the thickness of the drawing edge in front of the recess 7
corresponds approximately to its radius.
[0051] In another advantageous embodiment, in lieu of a recess 7
adjacent to the drawing edge 2 (FIG. 3), there is only a groove 8
extending parallel to the surface 6 that has a depth, for example,
of 5 to 8 mm, with the height of the groove 8 between the drawing
edge 2 and the surface 5 being 8 to 12 mm.
[0052] According to the invention, it has turned out that such a
groove 8 with these dimensions stores enough oxygen in the form of
a gas after the demolding of a component and the insertion of a new
blank to ensure the sufficient oxygen supply during the
forming.
[0053] In another advantageous embodiment (FIGS. 4, 5), the surface
5 is embodied with slots 9, which extend from a surface 4 in the
direction of the drawing edge 2, but the drawing edge 2 still has a
thickness that corresponds to its radius.
[0054] The slot width in this case is 4 to 8 mm, with a slot
spacing of 7 to 11 mm so that a bridge piece width of 2 to 5 mm is
achieved with a slot depth of 5 to 9 mm. Here, too, it has turned
out that the bridge piece width does not negatively influence the
oxygen supply.
[0055] In another advantageous embodiment (not shown), the recesses
7 or the groove 8 or the slots 9 are filled with a porous ceramic
material or a porous sintered metal material; on the back side 3 of
the insert, supply openings for oxygen-containing fluids can be
provided and/or the sintered metal inserts or ceramic inserts are
charged with oxygen between the forming procedures, for example by
flooding the mold cavity with water vapor, or the ceramic and/or
the sintered metal has a high enough oxygen affinity that during
the forming procedures, oxygen is absorbed, which during the
drawing procedure, is imparted to released zinc iron or zinc
phases.
[0056] The invention has the advantage that relatively simple
measures can be used to effectively prevent the formation of
second-order microcracks; also, existing forming tools can be
retrofitted by milling out the positive radius regions and/or the
drawing edges inserting correspondingly shaped inserts.
[0057] 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.
[0058] Primarily in the direct press hardening process, 20MnB8,
22MnB8, and other manganese/boron steels are also used in addition
to 22MnB5.
[0059] 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.
[0060] 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.
[0061] 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.
* * * * *