U.S. patent application number 10/544206 was filed with the patent office on 2006-07-06 for method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure and band thus obtained.
Invention is credited to Antoine Moulin.
Application Number | 20060144482 10/544206 |
Document ID | / |
Family ID | 32696392 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060144482 |
Kind Code |
A1 |
Moulin; Antoine |
July 6, 2006 |
Method of producing a cold-rolled band of dual-phase steel with a
ferritic/martensitic structure and band thus obtained
Abstract
The invention relates to a method of producing a cold-rolled
band of dual-phase steel with a ferritic/martensitic structure. The
inventive method consists in hot rolling a slab having a chemical
composition which comprises, by weight,
0.01%.ltoreq.C.ltoreq.0.1%., 0.05%.ltoreq.Mn.ltoreq.1%,
0.01%.ltoreq.Cr.ltoreq.1%, 0.01%.ltoreq.Si.ltoreq.0.5%,
0.001%.ltoreq.P.ltoreq.0.2%, 0.01%.ltoreq.Al.ltoreq.0.1%,
N.ltoreq.0.01%, the remainder being iron and impurities resulting
from the preparation thereof. The method also comprises the
following subsequent steps consisting in: hot winding the band
obtained at a temperature of between 550 and 850.degree. C.; cold
rolling the band with a reduction ratio of between 60 and 90%;
annealing the band continuously in the intercritical region;
cooling said band to ambient temperature, in one or more steps, the
rate of cooling between 600.degree. C. and ambient temperature
being between 100.degree. C./s and 1500.degree. C/s; and,
optionally, tempering same at a temperature of less than
300.degree. C. The aforementioned annealing and cooling operations
are performed such that the end band comprises between 1 and 15%
martensite. The invention also relates to the steel band thus
formed.
Inventors: |
Moulin; Antoine; (Metz,
FR) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Family ID: |
32696392 |
Appl. No.: |
10/544206 |
Filed: |
January 30, 2004 |
PCT Filed: |
January 30, 2004 |
PCT NO: |
PCT/FR04/00209 |
371 Date: |
March 10, 2006 |
Current U.S.
Class: |
148/603 |
Current CPC
Class: |
C21D 2211/005 20130101;
C21D 2211/008 20130101; C21D 8/0273 20130101; C21D 8/0236 20130101;
C21D 1/185 20130101 |
Class at
Publication: |
148/603 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 5, 2003 |
FR |
03/01358 |
Claims
1. A process for producing a cold-rolled ferritic/martensitic
dual-phase steel strip, wherein a slab, the chemical composition of
which comprises, by weight: 0.010%.ltoreq.C.ltoreq.0.100%
0.050%.ltoreq.Mn.ltoreq.1.0% 0.010%.ltoreq.Cr.ltoreq.1.0%
0.010%.ltoreq.Si.ltoreq.0.50% 0.001%.ltoreq.P.ltoreq.0.20%
0.010%.ltoreq.Al.ltoreq.0.10% N.ltoreq.0.010% the balance being
iron and impurities resulting from the smelting, is hot rolled,
said process then comprising the steps consisting in: coiling the
hot-rolled strip obtained at a temperature of between 550 and
850.degree. C.; then cold rolling the strip with a reduction ratio
of between 60 and 90%; then annealing the strip continuously in the
intercritical range; and cooling it down to the ambient temperature
in one or more steps, the cooling rate between 600.degree. C. and
the ambient temperature being between 100.degree. C./s and
1500.degree. C./s; and optionally tempering it at a temperature
below 300.degree. C., the annealing and cooling operations being
carried out in such a way that the strip finally contains from 1 to
15% martensite.
2. The process as claimed in claim 1, wherein the chemical
composition of the steel comprises: 0.020%.ltoreq.C.ltoreq.0.060%
0.300%.ltoreq.Mn.ltoreq.0.500% 0.010%.ltoreq.Cr.ltoreq.1.0%
0.010%.ltoreq.Si.ltoreq.0.50% 0.010%.ltoreq.P.ltoreq.0.100%
0.010%.ltoreq.Al.ltoreq.0.10% N.ltoreq.0.010% the balance being
iron and impurities resulting from the smelting.
3. The process as claimed in either of claims 1 and 2, wherein the
strip is hot rolled at a temperature above 850.degree. C.
4. The process as claimed in any one of claims 1 to 3, wherein the
strip is hot rolled at a temperature of between 550 and 750.degree.
C.
5. The process as claimed in any one of claims 1 to 4, wherein the
strip is cold rolled with a reduction ratio of between 70 and
80%.
6. The process as claimed in any one of claims 1 to 5, wherein the
continuous annealing of the cold-rolled strip comprises a
temperature rise phase followed by a soak phase at a predetermined
temperature.
7. The process as claimed in claim 6, wherein the soak temperature
is between Ac.sub.1 and 900.degree. C.
8. The process as claimed in claim 7, wherein the soak temperature
is between 750 and 850.degree. C.
9. The process as claimed in any one of claims 1 to 8, wherein the
cooling down to the ambient temperature comprises a first, slow
cooling step between the soak temperature and 600.degree. C.,
during which the cooling rate is less than 50.degree. C./s,
followed by a second cooling step at a higher rate, of between
100.degree. C./s and 1 500.degree. C./s, down to the ambient
temperature.
10. The process as claimed in claim 9, wherein the second cooling
step is carried out by water quenching.
11. The process as claimed in any one of claims 1 to 8, wherein the
cooling is carried out in a single operation at a cooling rate of
between 100.degree. C./s and 1500.degree. C./s.
12. The process as claimed in claim 11, wherein the cooling is
carried out by water quenching.
13. A cold-rolled ferritic/martensitic dual-phase steel strip, the
chemical composition of which comprises, by weight:
0.010%.ltoreq.C.ltoreq.0.100% 0.050%.ltoreq.Mn.ltoreq.1.0%
0.010%.ltoreq.Cr.ltoreq.1.0% 0.010%.ltoreq.Si.ltoreq.0.50%
0.001%.ltoreq.P.ltoreq.0.20% 0.010%.ltoreq.Al.ltoreq.0.10%
N.ltoreq.0.010% the balance being iron and impurities resulting
from the smelting, the strip furthermore containing between 1% and
15% martensite.
14. The steel strip as claimed in claim 13, the chemical
composition of which furthermore comprises:
0.020%.ltoreq.C.ltoreq.0.060% 0.300%.ltoreq.Mn.ltoreq.0.500%
0.010%.ltoreq.Cr.ltoreq.1.0% 0.010%.ltoreq.Si.ltoreq.0.50%
0.010%.ltoreq.P.ltoreq.0.100% 0.010%.ltoreq.Al.ltoreq.0.10%
N.ltoreq.0.010% the balance being iron and impurities resulting
from the smelting.
15. The steel strip as claimed in either of claims 13 and 14, which
has a tensile strength R.sub.m of greater than 450 MPa.
16. The steel strip as claimed in claim 15, which has a tensile
strength R.sub.m of greater than 500 MPa.
17. The steel strip as claimed in claim 16, further which has a
tensile strength R.sub.m of greater than 600 MPa.
18. The steel strip as claimed in any one of claims 13 to 17, which
has a mean anisotropy coefficient r of greater than 1.1.
19. The steel strip as claimed in claim 18, further which has a
mean anisotropy coefficient r of greater than 1.3.
20. The steel strip as claimed in any one of claims 13 to 19, which
furthermore contains between 1% and 10% martensite.
21. The steel strip as claimed in claim 20, which furthermore
contains between 5% and 8% martensite.
22. The use of a steel strip as claimed in any one of claims 13 to
21 for the production of automobile parts by deep drawing.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a process for producing a
cold-rolled ferritic/martensitic dual-phase steel strip and to a
strip that can be obtained by this process, which is more
particularly intended for the production of automobile parts by
deep drawing.
PRIOR ART
[0002] Ultrahigh-strength steels have been developed in recent
years, especially so as to meet the specific requirements of the
automobile industry, which are in particular the reduction in
weight, and therefore in thickness, of the parts and the
improvement in safety afforded by the increase in fatigue strength
and impact behavior of the parts. These improvements must also not
degrade the formability of the steel sheet used for producing the
parts.
[0003] Thus, dual-phase steels have been developed in which the
structure is ferritic/martensitic, which make it possible to
achieve a tensile strength R.sub.m of more than 400 MPa but which
do not have good drawability characteristics, since their mean
anisotropy coefficient r is close to 1. Moreover, their
galvanizability is poor, since they contain large amounts of
silicon or other elements deleterious to good wetting of the
surface of the strip by the molten zinc.
[0004] Also known are steels with a single-phase structure, which
have a high mean anisotropy coefficient r but have only moderate
mechanical properties, with a tensile strength R.sub.m not
exceeding 400 MPa.
[0005] As examples, mention may be made of low-interstitial steels
and aluminum-killed reparkerized steels. Attempts at enhancing the
conventional hardening mechanisms for these types of steel fail to
appreciably improve their mechanical properties. Furthermore, this
steel must be capable of being galvanized.
SUMMARY OF THE INVENTION
[0006] The object of the present invention is to remedy the
drawbacks of the steels of the prior art by proposing a steel strip
capable of deep drawing and having at the same time excellent
mechanical properties and excellent anisotropy characteristics.
[0007] For this purpose, the first subject of the invention is a
process for producing a cold-rolled ferritic/martensitic dual-phase
steel strip, characterized in that a slab, the chemical composition
of which comprises, by weight: [0008] 0.010%.ltoreq.C.ltoreq.0.100%
[0009] 0.050%.ltoreq.Mn.ltoreq.1.0% [0010]
0.010%.ltoreq.Cr.ltoreq.1.0% [0011] 0.010%.ltoreq.Si.ltoreq.0.50%
[0012] 0.001%.ltoreq.P.ltoreq.0.20% [0013]
0.010%.ltoreq.Al.ltoreq.0.10% [0014] N.ltoreq.0.010% the balance
being iron and impurities resulting from the smelting, is hot
rolled, said process then comprising the steps consisting in:
[0015] coiling the hot-rolled strip obtained at a temperature of
between 550 and 850.degree. C.; then
[0016] cold rolling the strip with a reduction ratio of between 60
and 90%; then
[0017] annealing the strip continuously in the intercritical range;
and
[0018] cooling it down to the ambient temperature in one or more
steps, the cooling rate between 600.degree. C. and the ambient
temperature being between 100.degree. C./s and 1500.degree. C./s;
and
[0019] optionally tempering it at a temperature below 300.degree.
C.,
the annealing and cooling operations being carried out in such a
way that the strip finally contains from 1 to 15% martensite.
[0020] In a preferred method of implementation, the chemical
composition of the steel furthermore comprises, by weight: [0021]
0.020%.ltoreq.C.ltoreq.0.060% [0022] 0.300%.ltoreq.Mn.ltoreq.0.500%
[0023] 0.010%.ltoreq.Cr.ltoreq.1.0% [0024]
0.010%.ltoreq.Si.ltoreq.0.50% [0025] 0.010%.ltoreq.P.ltoreq.0.100%
[0026] 0.010%.ltoreq.Al.ltoreq.0.10% [0027] N.ltoreq.0.010% the
balance being iron and impurities resulting from the smelting.
[0028] The process according to the invention may also include the
following features, by themselves or in combination:
[0029] the strip is hot rolled at a temperature above 850.degree.
C.;
[0030] the strip is hot coiled at a temperature of between 550 and
750.degree. C.;
[0031] the strip is cold rolled with a reduction ratio of between
70 and 80%;
[0032] the continuous annealing of the cold-rolled strip comprises
a temperature rise phase followed by a soak phase at a
predetermined temperature;
[0033] the soak temperature is between Ac.sub.1 and 900.degree.
C.;
[0034] the soak temperature is between 750 and 850.degree. C.;
[0035] the cooling down to the ambient temperature comprises a
first, slow cooling step between the soak temperature and
600.degree. C., during which the cooling rate is less than
50.degree. C./s, followed by a second cooling step at a higher
rate, of between 100.degree. C./s and 1500.degree. C./s, down to
the ambient temperature.
[0036] The second subject of the invention is a cold-rolled
ferritic/martensitic dual-phase steel strip, the chemical
composition of which comprises, by weight:
[0037] In a preferred embodiment, the composition of the strip is
the following: [0038] 0.020%.ltoreq.C.ltoreq.0.060% [0039]
0.300%.ltoreq.Mn.ltoreq.0.500% [0040] 0.010%.ltoreq.Cr.ltoreq.1.0%
[0041] 0.010%.ltoreq.Si.ltoreq.0.50% [0042]
0.010%.ltoreq.P.ltoreq.0.100% [0043] 0.010%.ltoreq.Al.ltoreq.0.10%
[0044] N.ltoreq.0.010% the balance being iron and impurities
resulting from the smelting.
[0045] The steel according to the invention may also include the
following features, by themselves or in combination:
[0046] it has a tensile strength R.sub.m of greater than 450
MPa;
[0047] it has a tensile strength R.sub.m of greater than 500
MPa;
[0048] it has a tensile strength R.sub.m of greater than 600
MPa;
[0049] it has a mean anisotropy coefficient r of greater than
1.1;
[0050] it has a mean anisotropy coefficient r of greater than
1.3;
[0051] it furthermore contains between 1% and 10% martensite;
[0052] it furthermore contains between 5% and 8% martensite.
[0053] Finally, the third subject of the invention is a steel strip
according to the invention for the production of automobile parts
by deep drawing.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0054] The process according to the invention consists in hot
rolling a slab of specific composition and then in coiling the
hot-rolled strip obtained at a temperature of between 550 and
850.degree. C.
[0055] This high-temperature coiling operation is favorable to the
development of what is called a texture, that is to say an
anisotropic structure. This is because such a coiling operation
makes it possible for the Fe.sub.3C cementite precipitates to
coalesce and to reduce the amount of carbon going back into
solution during the anneal, this being detrimental to the
development of the recrystallization texture.
[0056] The process then consists in cold rolling the strip with a
reduction ratio of between 60 and 90% and then in annealing the
strip continuously in the intercritical range.
[0057] The intercritical anneal allows most of the carbide phases
formed during the coiling after the recrystallization to be
redissolved. The fact that the austenization and the dissolution of
the carbide phases take place after the recrystallization makes it
possible to retain the carbon trapped during the recrystallization
and to free it once the recrystallized ferrite texture has
developed. The texture will therefore be unaffected by the carbon
in solid solution, as is the case with low-temperature coiling, but
is only impaired by the isotropic character of the martensite
formed.
[0058] The process then consists in cooling the strip down to the
ambient temperature, in one or more steps, the cooling rate between
600.degree. C. and the ambient temperature being between
100.degree. C./s and 1500.degree. C./s, and optionally in tempering
it at a temperature below 300.degree. C.
[0059] This rapid cooling step allows martensite to form in the
structure of the steel, thereby achieving very good mechanical
properties. However, measures must be taken to ensure that too much
martensite does not form, as martensite is isotropic and therefore
reduces the mean anisotropy coefficient r.
[0060] Water quenching allows substantial proportions of carbide
phases to be formed in the composition in question. It is possible
to reduce the amount of martensitic phase formed by lowering the
soak temperature toward lower values in the intercritical range, or
else by carrying out a slow cooling operation before the
quench.
[0061] It is also possible to reduce the difference in hardness
between the ferritic matrix and the martensitic phase, by cooling
the strip more slowly or by performing a short tempering operation,
lasting around one minute, on the martensitic phase formed after
the water quench.
[0062] It should be noted that this tempering operation is in no
case an averaging treatment, as is found in the prior art. This is
because these averaging treatments, which are generally carried out
between 300 and 500.degree. C., have in particular the effect of
suppressing the martensite, which is an essential element of the
present invention. The tempering optionally carried out according
to the invention consists in precipitating some of the carbon in
solid solution trapped in the martensite, without reducing the
proportion of this martensite. The maximum temperature of this
tempering operation is 300.degree. C., preferably 250.degree. C.
and more particularly preferably 200.degree. C.
[0063] The composition according to the invention includes carbon
with a content of between 0.010% and 0.100%. This element is
essential for obtaining good mechanical properties but it must not
be present in too great an amount, as it would cause an excessive
proportion of martensitic phase to be formed.
[0064] It also includes manganese with a content of between 0.050%
and 1.0%. Manganese improves the yield strength of the steel, but
greatly reduces its ductility. This is why its content is
limited.
[0065] The composition also includes chromium with a content of
between 0.010% and 1.0%, which helps in the desired martensite
formation.
[0066] The composition also includes silicon with a content of
between 0.010% and 0.50%. This greatly improves the yield strength
of the steel, but slightly reduces its ductility and degrades its
coatability.
[0067] The composition also includes phosphorus with a content of
between 0.001% and 0.20%, which hardens the microstructure without
affecting its texture.
[0068] The composition also includes aluminum with a content of
between 0.010% and 0.10%, which prevents aging by nitrogen
trapping.
EXAMPLES
[0069] By way of nonlimiting examples, and so as to better
illustrate the invention, two grades of steel were produced. Their
compositions, in thousandths of a percent, are given in the
following table. TABLE-US-00001 C Mn Cr Si P Al N A 60 600 70 70 20
56 5 B 43 373 76 13 22 56 5.7
[0070] The balance of the compositions consists of iron and
inevitable impurities resulting from the smelting.
ABBREVIATIONS EMPLOYED
[0071] R.sub.e: yield strength in MPa;
[0072] Rm: tensile strength in MPa;
[0073] r: anisotropy coefficient;
[0074] P: plateau;
[0075] % m: proportion of martensite.
[0076] After production, the two grades were austenized at
1250.degree. C. for one hour, so as to dissolve the aluminum
nitrides. The slabs were then hot rolled in such a way that the
end-of-rolling temperature was above 900.degree. C., the value of
AR.sub.3 for both grades being about 870.degree. C.
[0077] The hot-rolled strips were then cooled by water quenching,
at a cooling rate of around 25.degree. C./s, until the coiling
temperature was reached. Grade A was coiled at 720.degree. C.,
while one specimen of grade B was coiled at 550.degree. C. and
another at 720 .degree. C.
[0078] The various specimens were then cold rolled so as to achieve
a reduction ratio of 75%, then they underwent an annealing
treatment at a soak temperature of 750.degree. C. in the case of
some specimens and 800.degree. C. in the case of the others. The
cooling down to the ambient temperature was then carried out at a
rate of around 25.degree. C./s by water quenching.
[0079] Next, the mechanical properties and the anisotropy
characteristics of the steels obtained were measured.
[0080] The results are collated in the following table.
TABLE-US-00002 T.sub.coil T.sub.soak R.sub.m Grade (.degree. C.)
(.degree. C.) Direction R.sub.e (MPa) (MPa) P (%) r mean r % m A
720 800 T 420 711 0 1.10 0.98 14 L 405 713 0 1.11 45.degree. 425
720 0 0.85 750 T 443 713 0 1.26 1.02 12 L 438 717 0 1.13 45.degree.
451 736 0 0.84 B 720 800 T 432 656 0 1.46 1.27 8 L 430 697 0 1.60
45.degree. 436 668 0 1.01 750 T 454 662 0 2.04 1.37 7 L 457 690 0
1.41 45.degree. 461 677 0 1.01 550 800 T 455 677 0 1.47 1.21 6 L
446 667 0 1.44 45.degree. 472 687 0 0.97 750 T 475 680 0.3 1.46
1.09 5 L 463 668 0.4 1.25 45.degree. 482 697 0.3 0.83
[0081] The overall anisotropy of a steel is determined by the mean
normal anisotropy coefficient r: r = r T + r L + 2 .times. r 45 4
##EQU1## where r.sub.T denotes the value of r measured in the
direction transverse to the rolling direction of the strip, r.sub.L
denotes the value of r measured in the longitudinal or rolling
direction of the strip and r.sub.45.degree. denotes the value of r
measured at 45.degree. to the rolling direction of the strip.
[0082] For a coiling temperature of 720.degree. C., FIG. 1 shows
the relationship between the mean coefficient r and the content of
martensite formed %m for grades A and B. It may be seen that the
higher the martensite content, the more anisotropic the steel.
[0083] It may also be seen that the higher the martensite content,
the higher the mechanical properties.
[0084] As an illustration, FIG. 2 shows the microstructure obtained
with grade A, coiled at 720.degree. C. and then annealed at
750.degree. C. in order finally to obtain 12% martensite. The
ferrite and the martensite formed can be clearly distinguished in
the figure.
* * * * *