U.S. patent application number 13/985643 was filed with the patent office on 2013-12-12 for method for manufacturing high-strength steel sheet parts subject in use to fatigue stresses.
This patent application is currently assigned to Sistemi Sospensioni S.p.A.. The applicant listed for this patent is Guido Borgna, Piero Monchiero, Andrea Santini. Invention is credited to Guido Borgna, Piero Monchiero, Andrea Santini.
Application Number | 20130327106 13/985643 |
Document ID | / |
Family ID | 43976469 |
Filed Date | 2013-12-12 |
United States Patent
Application |
20130327106 |
Kind Code |
A1 |
Santini; Andrea ; et
al. |
December 12, 2013 |
METHOD FOR MANUFACTURING HIGH-STRENGTH STEEL SHEET PARTS SUBJECT IN
USE TO FATIGUE STRESSES
Abstract
The manufacturing method comprises the steps of: carrying out
one or more forming operations so as to give the desired geometry
to the part; and subjecting the part thus formed to a single heat
treatment having only a stress relieving treatment, which is
preferably carried out at a temperature in the range from
530.degree. C. to 580.degree. C. for a time in the interval from 45
to 60 minutes and is followed by cooling of the part in air. By
virtue of the formed part being subjected to a stress relieving
heat treatment, the residual stress state due to the initial
forming process and to the bead welding, if any, is eliminated or
at least significantly reduced.
Inventors: |
Santini; Andrea; (Torino,
IT) ; Borgna; Guido; (Torino, IT) ; Monchiero;
Piero; (Torino, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Santini; Andrea
Borgna; Guido
Monchiero; Piero |
Torino
Torino
Torino |
|
IT
IT
IT |
|
|
Assignee: |
Sistemi Sospensioni S.p.A.
Corbetta, Milano
IT
|
Family ID: |
43976469 |
Appl. No.: |
13/985643 |
Filed: |
February 17, 2012 |
PCT Filed: |
February 17, 2012 |
PCT NO: |
PCT/EP12/52757 |
371 Date: |
August 15, 2013 |
Current U.S.
Class: |
72/54 ;
72/342.5 |
Current CPC
Class: |
B60G 7/001 20130101;
B21C 37/08 20130101; C21D 8/02 20130101; C21D 8/0205 20130101; C21D
9/0068 20130101; F16L 9/02 20130101; C21D 1/30 20130101; B21D 31/00
20130101; B23K 2101/06 20180801; C21D 9/50 20130101; C21D 9/08
20130101; B60G 2206/722 20130101; C21D 9/46 20130101; C21D 8/105
20130101 |
Class at
Publication: |
72/54 ;
72/342.5 |
International
Class: |
B21D 31/00 20060101
B21D031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 18, 2011 |
IT |
TO2011A000139 |
Claims
1. Method for manufacturing sheet metal parts of high-strength
steel which are subject in use to fatigue, the method comprising in
the order the steps of: a) carrying out one or more forming
operations on a sheet of high-strength steel so as to provide the
part with the desired geometry; and b) subjecting the part thus
formed to a single heat treatment having only a stress relieving
treatment, which is carried out keeping the formed part at a
temperature in the range from 530.degree. C. to 580.degree. C. for
a time in the range from 45 to 60 minutes and then leaving the part
to cool in air.
2. Method according to claim 1, wherein the forming step a) is
carried out using a sheet of high-strength low-alloy steel.
3. Method according to claim 1, wherein the forming of the part
carried out at step a) comprises at least one of the following
operations: tube making, shaping, cold-forming and
hydro-forming.
4. Method according to claim 1, further comprising the step of
carrying out at least one bead welding on the part, wherein such a
welding step is carried out before the stress relieving step b).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a "national phase" application based
upon International Patent Application PCT/EP2012/052757 filed on
Feb. 17, 2012, which, in turn, claims priority to Italian Patent
Application TO2011A 000139 filed on Feb. 18, 2011.
BACKGROUND OF INVENTION 1. Field of Invention
[0002] The present invention relates to a method for manufacturing
high-strength steel sheet parts suitable for fatigue applications,
i.e. parts which are subject in use to fatigue stresses. In
particular, although not exclusively, the invention relates to the
manufacturing of high-strength steel sheet parts intended to be
used on vehicle suspension systems. In the following description
and claims, the expression "high-strength" is to be intended as
referred to a steel having a yield strength greater than 420
MPa.
[0003] 2. Description of Related Art
[0004] As is known, plastic forming operations carried out on steel
sheets leave stress states in the material, the so-called residual
stresses, which turn out to be of the same order of magnitude as
the yield strength of the material. The same occurs when a weld
bead is made on a steel sheet: depending on how much the welded
joint is free to deform after cooling, residual stresses are
generated in the material which are great in the vicinity of the
weld bead and decrease as the distance from the weld bead
increases. These residual stress states of the material remain in
the finished part throughout its service life. If the finished part
is requested to bear fatigue stresses (as is the case, for
instance, with vehicle suspension parts), then the residual stress
state usually reduces the ability of the part to meet the
prescribed requirements in terms of fatigue life. The higher the
quality of the steel of which the part is made, the larger the
effect of reduction in the fatigue life of the part. This is due to
the fact that the fatigue limit is less dependent on the yield
strength of the material than the residual stress level, which on
the contrary is directly linked to the yield strength of the
material. In order to compensate the reduction in the fatigue life
due to the residual stress state in the material, either the
thickness of the material is increased (with resulting impact on
the weight and cost of the part) or special materials are adopted
(for instance, boron steel) which are subjected to heat treatment,
namely to quenching followed by tempering, such a heat treatment
allowing to obtain high mechanical properties and at the same time
to reduce (or even eliminate) the residual stress state due to the
forming process and to the quenching treatment.
[0005] In case of metal sheet parts for vehicle suspension systems
obtained by forming tubular blanks, the manufacturing method
comprises first of all a so-called tube making phase, i.e. a phase
of production of a tubular blank, which typically consists in the
following operations: [0006] 1) producing ribbons having a width
equal to the required circumference of the tube starting from a
coil of cold- or hot-rolled steel sheet; [0007] 2) unwinding the
ribbons and producing the tube continuously by carrying out in
sequence forming operations; [0008] 3) welding the tube
longitudinally (i.e. along a direction parallel to the axis of the
tube) without addition of material, and also scarfing the tube, if
necessary; and [0009] 4) straightening the tube.
[0010] The tube blank thus obtained is then formed (typically by
cold-stamping and/or hydro-forming) so as to be given the desired
final geometry. For instance, in case of a cross-member for a
twist-beam axle rear suspension for a motor-vehicle, the tube blank
is bent and/or squashed so that its own longitudinal axis is given
the desired course and/or its cross-section is given the desired
shape. In case of use of normal structural steels (for instance
Fe510D) or of steels for heat treatment (for instance 20MnB5), a
normalizing treatment is performed before forming in order to
reduce the high residual stress state generated by the tube making
process, which is typically slightly less than the yield strength
of the material. Moreover, in case of steels for heat treatment the
formed part is finally subjected to a quenching and tempering
treatment, in order to reach the high mechanical properties
required by the specific mission of the part. A shot-peening
operation, if necessary, may follow the quenching and tempering
treatment in order to further increase the fatigue life of the
part. In case of use of normal structural steels, the parts thus
obtained have low mechanical properties, but low production costs,
whereas the use of steels for heat treatment allows to obtain parts
having very high mechanical properties, but high production
costs.
[0011] A method for manufacturing a steel tube is known from EP 2
045 348, wherein the tube is first subjected to hot-bending (at a
temperature of about 1000.degree. C.) and finally to a quenching
and tempering treatment. A similar method is known from JP 7
090375, since according to this document the steel tube is also
first hot-formed and then subjected to a quenching and tempering
treatment. U.S. Pat. No. 3,533,157 discloses a method for
manufacturing tubes and mentions, as the only heat treatment used
in the method, the normalizing treatment, which involves, as is
known, heating up to a temperature higher (typically by about
50.degree. C.) than the critical zone or critical interval of the
steel.
SUMMARY OF INVENTION
[0012] It is the object of the present invention to provide a
method for manufacturing steel sheet parts subject in use to
fatigue stresses, such as for instance parts for vehicle suspension
systems, which allows to ensure that high mechanical properties are
obtained, which are much higher than those obtainable using normal
structural steels, but which requires less time and lower costs
than those required in case of use of steels for heat
treatment.
[0013] To this end, the method of the present invention includes
the steps of carrying out one or more forming operations on a sheet
of high-strength steel so as to provide the part with the desired
geometry; subjecting the part thus formed to a single heat
treatment having only in a stress-relieving treatment, which is
carried out keeping the formed part at a temperature in the range
from 530.degree. C. to 580.degree. C. for a time in the range from
45 to 60 minutes and then leaving the part to cool in air.
[0014] In short, the invention is based on the idea of using a
high-strength steel, in particular a high-strength low-alloy steel
(which, as everyone knows, is a kind of steel not suitable for
quenching), and of carrying out, after the forming step, a stress
relieving treatment as the only heat treatment, instead of a
quenching and tempering treatment. The stress relieving treatment
consists, in per-se-known manner, in keeping the part for a given
time (for instance, 45 to 60 minutes) at a temperature lower than
the critical interval of the steel (for instance, at a temperature
comprised in the range from 530.degree. C. to 580.degree. C.) and
then leaving the part to cool in air. The stress relieving
treatment allows to reduce, if not even to completely eliminate,
the residual stresses generated in the material as a result of
processing operations (not of heat treatments) carried out in the
previous steps of the manufacturing method (tube making, in case of
a part obtained from a tube blank, and forming), and therefore to
obtain parts having high fatigue life characteristics. In this
connection, it is to be noted that in those parts which are
intended to be used on vehicle suspension systems the fatigue life
is one of the most important requirements. A further advantage is
that the quenching treatment is avoided and therefore the
associated times and costs are saved. Moreover, the stress
relieving treatment requires temperatures and times lower than
those required by the tempering treatment (typically between
590.degree. C. and 610.degree. C. for about 2 hours). The savings
in terms of cycle-times and costs (both processing costs and
apparatus costs) with respect to the use of steels for heat
treatment and to the carrying out of a quenching and tempering
treatment at the end of the forming steps are therefore evident.
Moreover, the quenching and tempering treatment significantly
changes the metallographic structure of the material and therefore
requires special quality controls on the finished part to check the
treatment has been correctly carried out. The method according to
the invention, on the contrary, does not require these special
controls, since it does not provide for subjecting the already
formed part to a quenching and tempering treatment, but only to a
stress relieving treatment.
[0015] For the purposes of the present invention, the term
"forming" is used to designate any plastic forming process carried
out on the sheet metal, such as in particular a tube making
process, a shaping process, a cold-stamping process or a
hydro-forming process. Irrespective of the specific process (or of
the specific processes, as several forming processes of different
kind could be carried out one after the other) used to form the
part starting from a sheet of high-strength steel, according to the
invention a stress relieving treatment is carried out as the only
heat treatment after the part has been formed, so as to reduce as
much as possible, if not even to completely eliminate, the residual
stress state generated in the material as a result of the
forming.
[0016] Further features and advantages of the method according to
the invention will be more evident from the following detailed
description of a preferred mode for carrying out the method.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
[0017] The manufacturing method according to the invention provides
first of all for obtaining a part, such as for instance a vehicle
suspension part, by means of one or more forming operations
starting from a sheet of high-strength steel, in particular of
high-strength low-alloy steel, such as for instance S600MC steel.
The part may be for instance a cross-member for a twist-beam axle
suspension, a longitudinal or transverse arm for an independent
suspension or for an interconnected suspension, a frame for a front
suspension, etc. In case of a tubular or profiled part, such as for
instance a cross-member for a twist-beam axle suspension, the
method will include first a tube making process (or, more
generally, a shaping process) such as the one described in the
introductory part of the description of the present application, so
as to obtain a tube blank which will be then subjected to the
outright forming process. The forming process may be performed by
cold-stamping and/or by hydro-forming. During the forming process,
other operations on the part, such as for instance bead welding
operations, may obviously be also carried out. Also making welding
beads generates residual stresses in the material.
[0018] After the forming process (and the bead welding, if any),
the part thus obtained is subjected to a stress relieving heat
treatment in order to reduce to the minimum the residual stress
state generated in the material as a result of the plastic
deformation undergone during the forming process, and also as a
result of the tube making or shaping process or as a result of the
bead welding. The stress relieving treatment is for instance
carried out keeping the already formed part at a temperature
comprised for instance in the range from 530.degree. C. to
580.degree. C. for a time comprised for instance in the range from
45 to 60 minutes and then leaving the part to cool in air. The
above-indicated temperature and time ranges for the stress
relieving treatment are to be considered as applicable to
high-strength low-alloy steels, such as for instance S550MC and
S700MC steels, and might therefore change in case of use of
high-strength steels of different kind.
[0019] The Applicant has experimentally noticed that the fatigue
life of formed tubes made of S700MC steel is increased by about ten
times if the tube is subjected to a stress relieving heat treatment
after being stamped. Similar fatigue tests have been carried out by
the Applicant on formed tubes made of S550MC steel and have given
as a result a fatigue life increased by at least two/three times
(the tests have been stopped for time reasons and no breakages have
been detected) with respect to the one of similar tubes not
subjected to a stress relieving treatment.
[0020] In the light of the preceding description, it is evident
that the idea of subjecting a formed part of high-strength steel
sheet only to a stress relieving heat treatment after the forming
process allows to obtain parts with high mechanical properties, in
particular high fatigue life, in less time and at lower costs than
those required in case the part is subjected to a quenching and
tempering treatment after the forming process. The method according
to the invention is therefore particularly suitable for being used
for the manufacturing of parts for vehicle suspension systems, as
these parts must meet very strict requirements in terms of fatigue
life and must be produced with the lowest times and costs
possible.
[0021] The present invention has been described in an illustrative
manner. It is to be understood that the terminology which has been
used is intended to be in the nature of words of description rather
than of limitation. Many modifications and variations of the
present invention are possible in light of the above teachings.
Therefore, within the scope of the appended claims, the present
invention may be practiced other than as specifically
described.
* * * * *