Title of Invention

A PROCESS FOR MANUFACTURING BAKE HARDENING STEEL SHEET AND A BAKE HARDENING SHEET

Abstract The invention relates to a process for manufacturing bake hardening steels and to steel sheet and part obtained by implementing it. The technical problem to be solved consists in providing a steel having good mechanical properties, a bake hardening value above 40 Mpa and presenting an improved resistance to natural aging. This is obtained by smelting a steel comprising : 0.03 ≤ C ≤ 0.06 0.50 ≤ Mn ≤ 1.10 0.08 ≤ Si ≤ 0.20 0.015 ≤ Al ≤ 0.070 N ≤ 0.007 Ni ≤ 0.040 Cu ≤ 0.040 P ≤ 0.035 S ≤ 0.015 Mo ≤ 0.010 Ti ≤ 0.005 and boron in an amount such that 0.64 ≤ B/N ≤ 1.60 then casting a slab of this steel, hot rolling it above Ar3, coilinh it between 500 - 700°C, cold rolling it with a reduction of 50 - 80%, continuously annealing it during less than 15 minutes and skin-passing it at a reduction ratio of 1.2 - 2.5%.
Full Text A Process For Manufacturing Bake Hardening Steel Sheet
And A Bake Hardening Sheet
The present invention relates to a process for manufacturing bake
hardening steel sheet and a bake hardening sheet as well as to steel sheet
and parts obtained by implementing this process.
This steel sheet and these steel parts may include an
anticorrosion coating, such as that obtained by hot dip
galvanizing or by electrogalvanizing. The steel sheet
is more particularly intended for the manufacture of
visible parts for automobiles, such as hoods for
example, whereas the parts, which are thicker than the
sheet, are more particularly intended for the
production of structural parts, again for automobiles.
This is because visible parts for automobiles must be
produced from a material which can be processed easily
by drawing and has, on completion of this processing
operation, good indentation resistance and is as light
as possible so as to reduce vehicle consumption.
Now, these various characteristics are contradictory -
a material, has good drawability when its yield strength
is low, but good indentation resistance requires it to
have a high yield strength and to be of great thickness.
Bake hardening (BH) steels have therefore been
developed that are characterized by a low yield
strength before forming, so that they are easily
drawable. However, once drawn, then coated with paint
and subjected to a bake heat treatment (at 170°C for 20
minutes), it is found that BH steel sheet or parts have
a yield strength that has increased considerably,
giving them good indentation resistance.
In the case of structural parts, this property of
hardening as the coating is being baked is in
particular put to advantage in order to reduce the
thickness, and therefore the weight, of these parts.
From a metallurgical standpoint, these property
modifications can be explained by the behavior of the
carbon in solid solution in the steel. This carbon has
a natural tendency of being fixed on the dislocations
in the steel, until they are saturated, thereby
hardening the steel. By controlling the amount of
carbon in solid solution and the density of
dislocations present in the steel during the process,
it is therefore possible to harden the steel when so
desired, by creating new dislocations that are
saturated with carbon, which remains in solid solution
and which migrates under the effect of thermal
activation. However, the presence of too large a
quantity of carbon in solid solution should be avoided,
as it could then cause aging of the steel in the form
of inopportune hardening before drawing, which would go
counter to the intended aim.
Bake hardening steels are known, the composition of
which includes manganese and silicon and an appreciable
amount of phosphorus, in the region of 0.1% by weight.
These steels have good mechanical properties and a bake
hardening (BH) value, i.e. an increase in yield
strength after baking, of about 45 MPa, but they
undergo considerable natural aging.
The object of the present invention is to provide bake
hardening steels having good mechanical properties,
which have a bake hardening (BH) value of at least
40 MPa and are less sensitive to natural aging than the
steels of the prior art.
For this purpose, a first subject of the present
invention is a process for manufacturing bake hardening
steel sheet comprising:
the smelting of a steel, the composition of
which comprises, expressed in % by weight:

it being understood that the steel also contains boron
in an amount such that:

the balance of the composition consisting of iron and
impurities resulting from the smelting;
- the casting of a slab of this steel, this slab
then being hot rolled in order to obtain a sheet, the
end-of-rolling temperature being above that of the Ar3
point;
- the coiling of said sheet at a temperature of
between 500 and 700°C; then
- the cold rolling of said sheet with a reduction
ratio of 50 to 80%;
- a continuous annealing heat treatment which is
carried out for a time of less than 15 minutes; and
- a skin pass which is carried out with a
reduction ratio of between 1.2 and 2.5%.
In a first preferred method of implementation, the
continuous annealing heat treatment comprises:
- a reheat of the steel until it reaches a
temperature of between 750 and 850°C;
- an isothermal soak;
- a first cooling operation down to a temperature
of between 380 and 500°C; and
- an isothermal soak; and then
- a second cooling operation down to the ambient
temperature.
In a second preferred method of implementation, first
cooling operation comprises a slow first part carried
out at a rate of less than 10°C/s, followed by a rapid
second part carried out at a rate of between 20 and
50°C/s.
The process may also comprise the following variants,
taken individually or in combination:
- the manganese content and the silicon content
of the steel are such that:

- the manganese content of the steel is between
0.55 and 0.65% by weight and the silicon content of the
steel is between 0.08 and 0.12% by weight;
- the manganese content of the steel is between
0.95 and 1.05% by weight and the silicon content of the
steel is between 0.16 and 0.20% by weight;
- the nitrogen content of the steel is less than
0.005% by weight; and
- the phosphorus content of the steel is less
than 0.015% by weight.
The carbon content of the composition according to the
invention is between 0.03 and 0.06% by weight, as this
element substantially lowers the ductility. However, it
must have a minimum content of 0.03% by weight in order
to avoid any aging problem.
The manganese content of the composition according to
the invention must be between 0.50 and 1.10% by weight.
Manganese improves the yield strength of the steel
while greatly reducing its ductility. Below 0.50% by
weight, aging problems are observed, whereas above
1.10% by weight the ductility is reduced excessively.
The silicon content of the composition according to the
invention must be between 0.08 and 0.20% by weight.
Silicon greatly improves the yield strength of the
steel, while slightly reducing its ductility, but it
substantially increases its aging tendency. If its
content is below 0.08% by weight, the steel does not
have good mechanical properties, whereas if it exceeds
0.20% by weight surface appearance problems arise,
striping defects appearing on the surface.
In a preferred embodiment of the invention, the ratio
of the manganese content to the silicon content is
between 4 and 15 so as to avoid any problem of
embrittlement in flash welding. This is because, if the
ratio lies outside these values, the formation of
embrittling oxides is observed during this welding
operation.
The main function of the boron is to fix the nitrogen
by early precipitation of boron nitrides. It must
therefore be present in a sufficient amount to prevent
an excessive amount of nitrogen remaining free, without
however too greatly exceeding the stoichiometric
quantity, since the free residual amount could pose
metallurgical problems and cause coloration of the
edges of the coil. For information, it should be
mentioned that strict stoichiometry is achieved for a
B/N ratio of 0.77.
The aluminum content of the composition according to
the invention is between 0.015 and 0.070% by weight,
without this being of critical importance. The aluminum
is present in the grade according to the invention
owing to the smelting process during which this element
is added in order to deoxidize the steel. However, the
content must not exceed 0.070% by weight as problems of
aluminum oxide inclusions would then be encountered,
these being deleterious to the mechanical properties of
the steel.
Phosphorus is limited in the steel according to the
invention to a content of less than 0.035% by weight,
preferably less than 0.015% by weight. Phosphorus
allows the yield strength of the grade to be increased,
but at the same time it increases its aging tendency in
the heat treatments, which explains it limitation. It
also impairs the ductility.
The titanium content of the composition must be less
than 0.005% by weight, the sulfur content must be less
than 0.015% by weight, the nickel content must be less
than 0.040% by weight, the copper content must be less
than 0.040% by weight and the molybdenum content must
be less than 0.010% by weight. These various elements
constitute in fact the residual elements resulting from
the smelting of the grades that are usually encountered.
Their contents are limited as they are capable of
forming inclusions that reduce the mechanical
properties of the grade. Among these residual elements
may also be niobium, which is not added to the
composition but may be present in trace amounts, that
is to say with a content of less than 0.004%,
preferably less than 0.001%, and particularly
preferably equal to 0.
A second subject of the invention is a bake hardening
sheet that can be obtained by the process according to
the invention and that has a yield strength of between
260 and 360 MPa, a tensile strength of between 320 and
460 MPa, a BH2 value of greater than 40 MPa, and
preferably greater than 60 MPa, and a yield plateau of
less than or equal to 0.2%.
The present invention will be illustrated by the
following examples, the table below giving the
composition of the various steels tested, in % by
weight, among which heats 1 to 4 are in accordance with
the present invention, while heat 5 is used as
comparison.

The balance of the composition of heats 1 to 5 consists,
of course, of iron and possibly impurities resulting
from the smelting.
Measurement of the increase in yield strength after
baking
To quantify the possible increase in yield strength of
the steel after baking, conventional tests were carried
out that simulate the actual use during which a sheet
is drawn and then baked.
A test piece is therefore subjected to a uniaxial
tensile strain of 2% and then undergoes a heat
treatment for 170°C for 20 minutes.
During this process, the following are measured in
succession:
- the yield strength Re0 of the test piece cut
from the steel sheet that has undergone continuous
annealing; then
- the yield strength Re2% of the test piece that
has undergone uniaxial tensile strain of 2%; and then
- the yield strength ReHT after 170 °C heat
treatment for 20 minutes.
The difference between Re0 and Re2% is used to calculate
the work hardening WH, whereas the difference between
Re2% and ReHT gives the bake hardening denoted, for this
conventional test, by BH2.
Abbreviations employed
A: elongation at break in %
Re: yield strength in MPa
Rm: tensile strength in MPa
n: work hardening coefficient
P: yield plateau in %
Example 1
Slabs were manufactured from heats 1 to 4, the slabs
then being hot rolled at a temperature above Ar3. For
these heats, the end-of-rolling temperature was between
854 and 880°C. The sheets thus obtained were coiled at
a coiling temperature between 580 and 620°C for these
heats, and then they were cold rolled with a reduction
ratio varying from 70 to 76%.
The sheets were then subjected to a continuous
annealing operation having the following steps:
- reheating of the sheet until a temperature of
750°C was reached, at a reheating rate of 6°C/s; then
- a soak at this temperature for 50 seconds;
- slow cooling down to 650°C, at a cooling rate
of 4°C/s; then
- rapid cooling down to 400°C, at a cooling rate
of 28°C/s;
- a soak at this temperature for 170 seconds; and
then
- cooling down to the ambient temperature, at a
cooling rate of 5°C/s.
Next, test pieces were cut from these sheets and their
yield strengths Re0 measured. Next, these test pieces
were subjected to a uniaxial tensile strain of 2% and
their yield strength Re2% and their other mechanical
properties were measured. Next, they were subjected to
a conventional heat treatment at 170°C for 20 minutes
and their new yield strengths ReHT were measured. Their
BH2 values were then calculated.

This shows that heats 1 to 3 according to the invention
had good mechanical properties and a good BH2 value,
and exhibited little or no yield plateau.
New test pieces were then cut from the sheets that had
undergone continuous annealing, and these were
subjected to a heat treatment at 75°C for 10 hours.
This heat treatment is equivalent to natural aging of
6 months at room temperature. The following results
were obtained:

This shows that, after simulating 6 months of natural
aging, heats 1 to 3 according to the invention do not
exhibit a plateau extension unacceptable to the Z
appearance (this being less than or equal to 0.2%).
Example 2
Slabs were manufactured from heats 1 to 5 and then hot
rolled, the end-of-rolling temperature being 850/880°C.
The sheets thus obtained were coiled at a coiling
temperature of 580/620°C and then cold rolled with a
reduction ratio varying from 70-7 6% for these heats.
The sheets were then subjected to a continuous
annealing operation having the following steps:
- reheating of the sheet until a temperature of
820°C was reached, at a reheating rate of 7°C/s; then
- a soak at this temperature for 30 seconds;
- slow cooling down to 650°C, at a cooling rate
of 6°C/s; then
- rapid cooling down to 470°C, at a cooling rate
of 45°C/s;
- a soak at this temperature for 20 seconds; and
then
- cooling down to ambient temperature, at a
cooling rate of 11°C/s.
Next, test pieces were cut from these sheets and their
yield strengths Re0 measured. Next, these test pieces
were subjected to a uniaxial tensile strain of 2% and
their yield strengths Re2% and their other mechanical
properties were measured. Next, they were subjected to
a conventional heat treatment at 170°C for 20 minutes
and their new yield strengths ReHT were measured. Their
BH2 values were then calculated.

This shows that heats 1 to 4 according to the invention
have good mechanical properties and a very good BH2
value, and exhibit little or no yield plateau, unlike
heat 5 which has a 1.2% plateau.
New test pieces were then cut from the sheets that had
undergone the continuous annealing, and these were
subjected to a heat treatment at 75°C for 10 hours.
This heat treatment is equivalent to natural aging of
6 months at room temperature. The following results
were obtained:
This shows that, after simulating 6 months of natural
aging, heats 1 to 4 according to the invention do not
exhibit a plateau unacceptable to the Z appearance
(less than or equal to 0.2%), unlike heat 5 which has a
plateau of 1.8%.
WE CLAIM :
1. A process for manufacturing bake hardening steel
sheet comprising:
the smelting of a steel, the composition of
which comprises, expressed in % by weight:
0.03 = C = 0.06
0.50 = Mn = 1.10
0.08 = Si = 0.20
0.015 = Al = 0.070
N = 0.007
Ni = 0.040
Cu = 0.040
P = 0.035
S Mo = 0.010
Ti = 0.005
it being understood that the steel also contains boron
in an amount such that:

the balance of the composition consisting of iron and
impurities resulting from the smelting;
- the casting of a slab of this steel, this slab
then being hot rolled in order to obtain a sheet, the
end-of-rolling temperature being above that of the Ar3
point;
- the coiling of said sheet at a temperature of
between 500 and 700°C; then
- the cold rolling of said sheet with a reduction
ratio of 50 to 80%;
- a continuous annealing heat treatment which is
carried out for a time of less than 15 minutes; and
- a skin pass which is carried out with a
reduction ratio of between 1.2 and 2.5%.
2. The process as claimed in claim 1, wherein said continuous annealing heat
treatment comprises :
- a reheat of the steel until it reaches a
temperature of between 750 and 850°C;
- an isothermal soak;
- a first cooling operation down to a temperature
of between 380 and 500°C; and
- an isothermal soak; and then
- a second cooling operation down to the ambient
temperature.
3. The process as claimed in either of claims 1 and 2, wherein said first cooling
operation comprises a slow first part carried out at a rate of less than 10°C/s,
followed by a rapid second part carried out at a rate of between 20 and 50°C/s,
4. The process as claimed in any one of claims 1 to 3, wherein , in addition, the
manganese content and the silicon content of the steel are such that:

5. The process as claimed in any one of claims 1 to 4, wherein, in addition, the
manganese content of the steel is between 0.55 and 0.65% by weight and the
silicon content of the steel is between 0.08 and 0.12% by weight.
6. The process as claimed in any one of claims 1 to 4, wherein, in addition, the
mangenese content of the steel is between 0.95 and 1.05% by weight and the
silicon content of the steel is between 0.16 and 0.20% by weight.
7. The process as claimed in any one of claims 1 to 6, wherein, in addition, the
nitrogen content of the steel is less than 0.005% by weight.
8. The process as claimed in any one of claims 1 to 7, wherein, in addition, the
phosphorus content of the steel is less than 0.015% by weight.
9. A bake hardening sheet that can be obtained by the process as claimed in
any one of claims 1 to 8, wherein, it has a yield strength of between 260 and 360
Mpa, a tensile strength of between 320 and 460 Mpa, a BH2 value of greater than
40 Mpa and a yield plateau of less than or equal to 0.2%.
10. The sheet as claimed in claim 9, which has a BH2 value of greater than 60
Mpa.
11. The sheet as claimed in claims 9 or 10, in the form of a cut part thereof, the
latter being painted and baked at less than 200°C.

The invention relates to a process for manufacturing bake hardening steels and to
steel sheet and part obtained by implementing it. The technical problem to be solved
consists in providing a steel having good mechanical properties, a bake hardening
value above 40 Mpa and presenting an improved resistance to natural aging. This is
obtained by smelting a steel comprising :
0.03 ≤ C ≤ 0.06
0.50 ≤ Mn ≤ 1.10
0.08 ≤ Si ≤ 0.20
0.015 ≤ Al ≤ 0.070
N ≤ 0.007
Ni ≤ 0.040
Cu ≤ 0.040
P ≤ 0.035
S ≤ 0.015
Mo ≤ 0.010
Ti ≤ 0.005
and boron in an amount such that 0.64 ≤ B/N ≤ 1.60
then casting a slab of this steel, hot rolling it above Ar3, coilinh it between 500 -
700°C, cold rolling it with a reduction of 50 - 80%, continuously annealing it during
less than 15 minutes and skin-passing it at a reduction ratio of 1.2 - 2.5%.

Documents:

795-kolnp-2005-abstract.pdf

795-kolnp-2005-assignment.pdf

795-kolnp-2005-claims.pdf

795-KOLNP-2005-CORRESPONDENCE-1.1.pdf

795-kolnp-2005-correspondence.pdf

795-kolnp-2005-description (complete).pdf

795-kolnp-2005-examination report.pdf

795-kolnp-2005-form 1.pdf

795-kolnp-2005-form 18.pdf

795-KOLNP-2005-FORM 27.pdf

795-kolnp-2005-form 3.pdf

795-kolnp-2005-form 5.pdf

795-KOLNP-2005-FORM-27-1.1.pdf

795-KOLNP-2005-FORM-27.pdf

795-kolnp-2005-gpa.pdf

795-kolnp-2005-reply to examination report.pdf

795-kolnp-2005-specification.pdf


Patent Number 236047
Indian Patent Application Number 795/KOLNP/2005
PG Journal Number 38/2009
Publication Date 18-Sep-2009
Grant Date 17-Sep-2009
Date of Filing 03-May-2005
Name of Patentee USINOR.
Applicant Address IMMEUBLE "LA PACIFIC", 11/13 COURS VALMY, LA DEFENSE 7, F-92800 PUTEAUX
Inventors:
# Inventor's Name Inventor's Address
1 MARSAL JOEL 27C ROUTE DE GANDREN, F-57570 BEYREN LES SIERCK
2 KIRCH FERNANDE 30 RUE DE LA TOURNAILLE, F-57300-AY SUR MOSELLE
3 MESCOLINI DOMINIQUE 64, RUE MAZELLE, F-57000 METZ
PCT International Classification Number C21D 8/04
PCT International Application Number PCT/FR2003/002985
PCT International Filing date 2003-10-10
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 02/12753 2002-10-14 France