U.S. patent application number 11/741414 was filed with the patent office on 2007-11-08 for process for producing shaped steel parts.
This patent application is currently assigned to DaimlerChrysler AG. Invention is credited to Martin Brodt, Dieter Lange, Ralf Mehrholz, Timo Wieland.
Application Number | 20070256762 11/741414 |
Document ID | / |
Family ID | 38565097 |
Filed Date | 2007-11-08 |
United States Patent
Application |
20070256762 |
Kind Code |
A1 |
Brodt; Martin ; et
al. |
November 8, 2007 |
PROCESS FOR PRODUCING SHAPED STEEL PARTS
Abstract
A process for producing a shaped part of a pre-form of
hardenable and heat formable steel sheet, by means of heat
treatment with cold deforming tools, in which by the heat forming a
hardened component is produced, which exhibits a martensitic and/or
banitic microstructure, wherein the hardened component is tempered
subsequent to heat forming so that a shaped part is formed which
exhibits, at least in areas, a higher yield strength and a
pronounced break limit compared to the hardened component.
Inventors: |
Brodt; Martin; (Weil der
Stadt, DE) ; Lange; Dieter; (Magstadt, DE) ;
Mehrholz; Ralf; (Stuttgart, DE) ; Wieland; Timo;
(Weinstadt, DE) |
Correspondence
Address: |
AKERMAN SENTERFITT
P.O. BOX 3188
WEST PALM BEACH
FL
33402-3188
US
|
Assignee: |
DaimlerChrysler AG
Stuttgart
DE
|
Family ID: |
38565097 |
Appl. No.: |
11/741414 |
Filed: |
April 27, 2007 |
Current U.S.
Class: |
148/533 ;
148/598; 148/620 |
Current CPC
Class: |
C21D 8/02 20130101; C21D
1/673 20130101; C21D 1/25 20130101 |
Class at
Publication: |
148/533 ;
148/598; 148/620 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2006 |
DE |
102006019567.1-24 |
Claims
1. A process for producing a shaped part from a pre-form of
hardenable and heat deformable steel sheet metal, by means of heat
forming with cold deforming tools, in which by hot forming a
hardened component is produced, exhibiting martensitic and/or
banitic microstructure, wherein the hardened component is tempered
subsequent to hot forming, so that a shaped component is produced,
which in comparison to the hardened component condition, exhibits
at least in areas a higher yield strength and/or pronounced yield
strength.
2. The process according to claim 1, wherein temperature and
duration of tempering are so selected, that the yield strength is
increased by at least 20%.
3. A process for producing a shaped part from a pre-form of
hardenable and hot deformable steel sheet metal, including the
steps: a) heating the preform to an austenitising temperature; b)
press hardenening in a cold tool as a result of which, by quencing,
at least partially a martensitic and/or banitic microstructure is
produced, c) wherein an annealing of the shaped pre-form at
temperatures below 400.degree. C. with formation of a shaped part
with increased yield strength and/or break elongation occurs in
comparison to the hardened component.
4. The process according to claim 1, wherein the pre-form is a
chrome-molybdenum steel.
5. The process according to claim 3, wherein the chrome-molybdenum
steel is 25CrMo4, 34CrMo4 or 42CrMo4.
6. The process according to claim 1, wherein the pre-form is a
boron alloyed case hardened and heat treated steel.
7. The process according to claim 7, wherein the boron alloyed case
hardened and heat treated steel is 17MnB3, 22MnB5 or 27MnCr5-2.
8. The process according to claim 1, wherein the pre-form is a flat
plate, a cold deformed steel pre-form or cut or trend pre-form.
9. The process according to claim 1, wherein the annealing occurs
at a temperature in the range of 250 to 400.degree. C.
10. The process according to claim 1, wherein the annealing occurs
at a temperature of from 300 to 330.degree. C.
11. The process according to claim 1, wherein the hardened pre-form
is cut to its final contour prior to or subsequent to step c).
12. The process according to claim 1, wherein the surface of the
hardened shaped part is cleansed prior to annealing, in particular,
by a powder blasting process.
13. The process according to claim 1, wherein during annealing in
step c) at the same time also a surface treatment for formation of
a defined corrosion protection and/or friction resistance for the
shaped part occurs.
14. The process according to claim 13, wherein the surface
treatment is galvanizing or thin layer zinking.
15. The process according to claim 1, wherein the pre-form is a
collection of multiple heat deformable steel sheets, of which at
least one is comprised of a hardenable steel.
16. The process according to claim 1, wherein the shaped part is
used to produce shell parts for the hollow beams or carriers
integrated in the passenger cell of a vehicle undercarriage.
17. The process according to claim 1, wherein from the shaped part
shell parts are produced for a hollow beam integrated in the
undercarriage of a vehicle.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The invention concerns a process for producing a shaped
component from a pre-form or semi-manufactured part of hardenable
and hot formable steel sheet metal, by means of hot working with
cold deforming tools, wherein as a result of the hot forming a
hardened component is produced, which exhibits a martensitic and/or
bainitic microstructure. The invention in particular concerns a
process for press-hardening of steel sheets for production of
automobile parts with high toughness.
[0003] 2. Related Art of the Invention
[0004] For weight reduction in body construction and in body
components the current trend is to reduce sheet metal thickness.
For this, it is necessary that the conventional steels are replaced
with hard and hardly hardened steel materials. The savings in
materials must be compensated for by an increased toughness of the
materials. With respect to the requirements of tensile strength,
steel 22MnB5 is suited for example, which belongs to the class of
the ultra highly hardened steels with a stiffness of 2000 MPa and
more.
[0005] Press hardening is a particularly economical process for the
series production of shaped components. This applies in particular
for sheet-like pre-forms or semi-manufactured parts, since here,
due to the low thickness, a through-hardening of the entire sheet
is made possible. Press hardening is known for example from DE 198
15 022 A1, in which the pre-form is first cold formed by a drawing
process, thereupon heated and then press hardened in a hot forming
tool.
[0006] From DE 197 43 802 C1 a process for press hardening of sheet
blanks is known. Therein a metallic shaped part for vehicle body
components, which has areas exhibiting a higher ductility, is
produced from a boron-manganese steel alloy. The composition of the
steel, expressed it in weight percent, is: 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%, chrome (Cr) 0.1% to 0.8%, molybdenum
(Mo) 0.1% to 0.5%, sulfur (S) maximum 0.1%, titanium (Ti) 0.02% to
0.05%, boron (B) 0.02% to 0.05%, aluminum (Al) 0.01% to 0.6%. The
sheet blanks are heated to temperatures of about 900.degree. C. and
formed in a cold pressed tool wherein aging occurs. Subsequently
individual areas of the shaped component are subjected to a partial
thermal treatment at temperatures of between 600.degree. C. and
900.degree. C.
[0007] Press hardened components are in general very brittle, so
that the possibility of their employment in motor vehicle bodies,
in particular in crash structures, is very limited as long as they
are in this condition. A material characteristic with a pronounced
yield strength and high elastic limit behavior Re/Rm would be
needed to come close to the requirements of car body manufacturing.
Likewise, for the component design for a passenger cell, besides
the construction geometry, the component-material-yield strength is
also determinative, and not the maximal component-material-tensile
strength.
[0008] It is thus the task of the invention to provide an
economical manufacturing process for high strength steel shaped
components, in which the material exhibits both a high strength as
well as a high ductility.
SUMMARY OF THE INVENTION
[0009] The task is inventively solved by a process for production
of a shaped component of a blank or pre-form of hardenable and hot
deformable steel sheet metal, by means of hot forming with cold
deforming tools, wherein by the hot forming a hardened component is
produced, which exhibits a martensitic and/or bainitic
microstructure, when the hardened component is tempered after the
hot forming, so that a shaped component is produced which, in
comparison to the hardened component condition, exhibits in areas a
higher yield strength.
[0010] In a further embodiment, the invention is solved by a
process for production of a shaped component of a blank or pre-form
of hardenable and hot formable steel sheet metal, including the
steps: [0011] a) heating the semi-finished part to an austenitising
temperature and; [0012] b) press hardening in a cold tool, wherein
by quenching at least partially a martensitic and/or bainitic
microstructure is produced; and [0013] c) annealing the deformed
semi-finished product at temperatures below 400.degree. C. with
formation of a shaped part with increased yield strength and/or
tensile strength compared to the hardened component.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the following the invention will be described in greater
detail with reference to the figures which show in:
[0015] FIG. 1. a tension/elongation diagram for component A
according to the invention, and
[0016] FIG. 2. a tension/elongation diagram for comparative
Component B.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In accordance with the invention it is proposed that a hot
formed and hardened component is tempered in a cold tool, so that a
shaped part is produced, which in comparison to the hardened
component condition exhibits at least in areas a higher and a
pronounced yield strength. By tempering of the component and
improvement of the mechanical characteristics takes place. Therein
it is important that the yield strength is increased and a
pronounced yield strength is achieved. Thereby the component is not
only highly hard, but also sufficiently ductile for use in vehicle
body construction.
[0018] A pronounced yield strength is achieved when the
tension-elongation curve reflecting a draw test clearly deviates
following a linear area (Hookish area) and exhibits a more or less
pronounced short almost level progression. The yield strength
(elastic limit) (Rm) is defined by the location of the deviation
point. In materials without pronounced elastic limit, frequently
the Rp0.2-limit is used for the constructive determination or
interpretation.
[0019] It is further envisioned in accordance with the invention
that the press hardened component is subject to a thermal treatment
below 400.degree. C., which leads to a shaped component with higher
yield strength and/or break elongation in comparison to the
hardened component. This thermal treatment is to be understood as
annealing at low temperatures. The temperatures selected in
accordance with the invention lay significantly below the
temperature level conventional for annealing high and high strength
steels. By the inventive thermal treatment a component with high
strength at simultaneously high ductility is produced, which meets
the requirements of vehicle body construction and in particular
also is satisfactory in crash structures in automobiles.
[0020] Preferably the temperature and the duration of the tempering
or annealing is so selected that the yield strength is increased by
at least 20%. Therein the breaking elongation of the shaped part
can also be increased or, at worst, be only insubstantially
reduced.
[0021] In the crash structures in the case of deformation it is
desired that the components have high energy absorption during
deformation. This is achieved by the combination of strength and
ductility achievable with the inventive process.
[0022] In a preferred embodiment of the invention the
semi-manufactured part is formed of a chrome-molybdenum steel. The
content of Cr and Mo preferably lies, for Cr, at 0.8 to 1.3% and,
for Mo, at 0.13 to 0.4%, in which the content of carbon lies at 0.2
to 0.5%.
[0023] Suitable examples of these steels include 25CrMo4, 34CrMo4
or 42CrMo4, as well as 25CrMoS4, 34CrMoS4 and 42CrMoS4. For these
steels, the manufacturers recommend hardening temperatures in the
range of 840 to 880.degree. C. and annealing temperatures at 540 to
680.degree. C. The indicated annealing temperatures may lead to an
improvement in ductility, however at the same time they also lead
to an unacceptable reduction in strength or, as the case may be,
yield strength. For the inventive temperatures of annealing below
400.degree. C. no cause is observed for embrittlement, but rather
an increase in lateral contraction, yield strength and break
elongation. Tempering is preferably carried out in the temperature
range of 250.degree. C. to 400.degree. C. For these steels the
annealing temperature particularly preferably lies in the range of
250 to 350.degree. C.
[0024] In a preferred further development of the invention the
similar manufactured component is formed of a boron alloyed
case-hardened steel or heat-treated (quenched and tempered) steel.
Suitable representatives of these steels are, for example, 17MnB3,
22MnB5, or 27MNCr5-2. Particularly preferably the annealing
temperature for these steels lies in the range of 300 to
330.degree. C.
[0025] The time required for annealing depends in particular upon
the material thickness of the component. For example, for material
thickness of about 1 mm, 2 to 10 minutes are suitable. If the
holding temperature is too long then depending upon the steel alloy
negative influences can be exercised on the component or, as the
case may be, on the material characteristics.
[0026] For the inventive manufacturing process sheet-like
semi-manufactured parts are particularly preferably suited.
Preferably the semi-manufactured component is comprised of a flat
plate of an already cold deformed steel blank or of a cut component
semi-manufactured part. The material thickness typically lies in
the range of 0.8 to 3 mm.
[0027] By the inventive deformation process, in particular profile
members or beams or hollow beams for automobile body and
undercarriage are produced.
[0028] It is useful that the hardened pre-form or semi-manufactured
component is cut to the end contour or shape only directly prior to
or after the annealing or, as the case may be, tempering. For this,
the surface of the hardened shaped part is preferably cleaned prior
to annealing. A contamination of this surface during annealing is
to be avoided if possible, so that the follow up treatment of the
surface following annealing can be minimized. A preferred cleaning
process is particle blasting and dry cleaned by means of
blasting.
[0029] In a further preferred embodiment of the invention a surface
treatment or formation of a defined corrosion protection and/or
wear protection for the shaped part is carried out during annealing
(in step c)), preferably simultaneously also during tempering.
Therein the temperature of the annealing, which can be for example
250 to 400.degree. C., can be taken advantage of or used for
burning in of a coating, in particular corrosion protection
coating.
[0030] Particularly preferred is when during annealing or, as the
case may be, tempering, a galvanizing or thin layer zinking is
carried out as the surface treatment. Herein the semi-finished
product is preferably comprised of multiple heat treatable steel
sheets, of which at least one is a hardenable steel.
[0031] The process is preferably used for manufacture of a form
part for a hollow beam integrated in a passenger cell of a motor
vehicle body or for a hollow beam or support member integrated in
the undercarriage of a vehicle. Particularly preferred is when the
hollow beam is provided in the crash structure or the deformation
structure of automobiles.
EXAMPLE
[0032] For comparative purposes a number of pre-forms of a
22MnB5-steel were subjected to an inventive heat formation or, as
the case may be, press hardening, thereafter annealing (Component
A) and on the other hand only hot forming or as the case may be
press hardening (Component B). The material thickness of the sheet
metal plates lies at approximately 1.0 mm.
For annealing (Component A) a temperature of 320.degree. C. and a
duration of approximately 4 to 6 minutes was selected.
[0033] For the Component A the following average values resulted:
TABLE-US-00001 Yield Strength 1226 MPa Tensile Strength 1335 MPa
Tear Strength 1000 MPa Stretch Load Limit 2.4% Elongation Breakage
4.1% Contraction at Fracture 29.8%
[0034] For component B the following average values resulted:
TABLE-US-00002 Yield Strength 923 MPa Tensile Strength 1659 MPa
Tear Strength 1351 MPa Stretch Load Limit 3.2% Elongation Breakage
5.0% Contraction at Fracture 18.3%
[0035] It has been shown that by the inventive process a
substantial increase in yield strength can be achieved. For user as
auto body components the measured reduction in tensile strength is
insignificant, since this plays only a subordinate role in the
design of the construct. The break elongation decreases only
insubstantially from 5 to 4.1% as a result of the inventive
process. In contrast, the reduction in tear sensitivity increases
in comparison to the contraction at fracture. Here also the
condition with subsequent annealing or tempering shows, with 29.8
instead of 18.3%, a significant improvement in performance compared
to only heat treated and hardened condition.
[0036] The course of tensioning of the tensile samples of
Components A or B is reproduced in the form of tension/elongation
diagrams in FIG. 1 for Component A and in FIG. 2 for Component
B.
[0037] For the Component A significant yield points or elastic
limits are recognizable. The curves clearly deviate following the
linear (Hookish) area, in a short almost horizontal progression.
The subsequent long area of the plastic deformation is likewise
comparatively level.
[0038] In contrast to this, the curves in FIG. 2 for Component B
show no deflection and no recognizable elastic limit.
* * * * *