U.S. patent application number 13/243295 was filed with the patent office on 2012-01-26 for process for manufacturing a cold rolled trip steel product.
This patent application is currently assigned to Arcelor France S.A.. Invention is credited to Joachim ANTONISSEN, Liesbeth BARBE, Serge CLAESSENS, Bruno DE COOMAN, Marijke DE MEYER, Sigrid JACOBS, Lucia TOSAL-MARTINEZ, Sven VANDEPUTTE.
Application Number | 20120018058 13/243295 |
Document ID | / |
Family ID | 32338263 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120018058 |
Kind Code |
A1 |
ANTONISSEN; Joachim ; et
al. |
January 26, 2012 |
PROCESS FOR MANUFACTURING A COLD ROLLED TRIP STEEL PRODUCT
Abstract
The present invention is related to a process comprising a cold
rolling step, for the production of uncoated, electro-galvanised or
hot dip galvanised TRIP steel products, hot rolling a slab of a
specific composition, wherein the finishing rolling temperature is
higher than the Ar3 temperature, to form a hot-rolled substrate,
cooling said substrate to a coiling temperature (CT) between
500.degree. C. and 680.degree. C., coiling said substrate at said
coiling temperature, pickling said substrate to remove the oxides,
cold rolling said substrate to obtain a reduction of thickness,
with a minimum reduction of 40%.
Inventors: |
ANTONISSEN; Joachim;
(Oosteeklo, BE) ; BARBE; Liesbeth; (Denderleeuw,
BE) ; DE MEYER; Marijke; (Sledinge, BE) ;
TOSAL-MARTINEZ; Lucia; (Dusseldorf, DE) ; CLAESSENS;
Serge; (Deurne, BE) ; VANDEPUTTE; Sven;
(Ruisbroek, BE) ; JACOBS; Sigrid; (Merelbeke,
BE) ; DE COOMAN; Bruno; (San Van Gen, NL) |
Assignee: |
Arcelor France S.A.
Saint Denis
FR
|
Family ID: |
32338263 |
Appl. No.: |
13/243295 |
Filed: |
September 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10539758 |
Dec 23, 2005 |
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PCT/BE2003/000188 |
Nov 6, 2003 |
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13243295 |
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Current U.S.
Class: |
148/518 ;
148/603 |
Current CPC
Class: |
C22C 38/06 20130101;
C21D 8/0236 20130101; C21D 2211/005 20130101; C22C 38/02 20130101;
C22C 38/04 20130101; C23C 2/40 20130101; C23C 2/02 20130101; C21D
8/0278 20130101; C21D 8/04 20130101; C21D 2211/002 20130101; C21D
8/0226 20130101; C21D 8/0273 20130101 |
Class at
Publication: |
148/518 ;
148/603 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C25D 5/36 20060101 C25D005/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2002 |
EP |
02447265.6 |
Claims
1. A process for manufacturing a cold rolled TRIP steel product,
comprising the steps of: preparing a steel slab having the
following composition: C: between 1300 ppm and 2600 ppm, Mn:
between 10000 ppm and 22000 ppm, Al: between 8000 ppm and 15000
ppm, Si: between 2000 ppm and 6000 ppm, P: between 400 and 1000
ppm, S: maximum 120 ppm, N: maximum 200 ppm, Ti: maximum 1000 ppm,
Nb: maximum 1000 ppm, V: maximum 1000 ppm, B: maximum 10 ppm. the
remainder being substantially iron and incidental impurities, hot
rolling said slab, wherein the finishing rolling temperature is
higher than the Ar3 temperature, to form a hot-rolled substrate,
cooling said substrate to a coiling temperature (CT) between
500.degree. C. and 680.degree. C., coiling said substrate at said
coiling temperature, pickling said substrate to remove the oxides,
cold rolling said substrate to obtain a reduction of thickness,
with a minimum reduction of 40%.
2. The process according to claim 1, further comprising the steps
of: soaking said substrate at a temperature between 760.degree. C.
and 850.degree. C., cooling said substrate with a cooling rate
higher than 2.degree. C./s to a temperature in the range
360.degree. C. to 450.degree. C., holding said substrate in said
temperature range for a time less than 700 s, cooling said
substrate to room temperature at a cooling rate higher than
1.degree. C./s. subjecting said substrate to a skinpass reduction
of maximum 1.5%.
3. The process according to claim 2, further comprising an
electrolytic zinc coating step.
4. The process according to claim 1, further comprising the
following processing steps: soaking said substrate at a temperature
between 760.degree. C. and 850.degree. C., cooling said substrate
with a cooling rate higher than 2.degree. C./s to the temperature
of a Zn-bath, holding said substrate in the temperature range
between 490.degree. C. and 460.degree. C. for less than 200
seconds, hot dip galvanising said substrate in said Zn-bath,
cooling said substrate to room temperature at a cooling rate higher
than 2.degree. C./s.
5. The process according to claim 4, further comprising the step of
subjecting said substrate to a skinpass reduction of maximum
1.5%.
6. The process according to claim 1, wherein the carbon content of
said composition is between 1300 ppm and 1900 ppm.
7. The process according to claim 1, wherein the carbon content of
said composition is between 1350 ppm and 1900 ppm.
8. The process according to claim 1, wherein the carbon content of
said composition is between 1400 ppm and 1900 ppm.
9. The process according to claim 1, wherein the carbon content of
said composition is between 1700 ppm and 2300 ppm.
10. The process according to claim 1, wherein the carbon content of
said composition is between 2000 ppm and 2600 ppm.
11. The process according to claim 6, wherein said composition
comprises: Mn: between 13000 ppm and 22000 ppm, Al: between 8000
ppm and 14000 ppm, Si: between 2500 ppm and 4500 ppm, P: between
600 and 1000 ppm, S: maximum 120 ppm, N: maximum 150 ppm, Ti:
maximum 200 ppm, Nb: maximum 100 ppm, V: maximum 100 ppm, B:
maximum 5 ppm.
12. The process according to claim 11, wherein the aluminium
content of said composition is between 9000 ppm and 13000 ppm.
Description
FIELD OF THE INVENTION
[0001] The present invention is related to a steel composition
comprising phosphor, to be used for the production of TRIP steel
products. The invention is equally related to the process of
production of said products, and to the end products
themselves.
STATE OF THE ART
[0002] In the automotive industry there is a need for weight
reduction, which implies the use of higher strength materials in
order to be able to decrease the thickness of the parts without
giving up safety and functional requirements. Ultra high strength
steel (UHSS) sheet products and in particular TRIP steel products,
showing a remarkable combination of high strength and good
formability, can provide the solution for this problem.
Additionally, an increased corrosion resistance of these steel
sheet products by means of electro or hot dip galvanising, is
frequently asked for.
[0003] Several documents are describing such UHSS products.
EP-A-1096029 is related to the production of a tempered martensite
TRIP steel, whose chemical composition is silicon-manganese based
and contains (in wt %) 0.05-0.20% C, 0.3-1.8% Si and 1.0-3.0% Mn as
well as one or more of the following additions (in wt %): 0.05-1%
Cr+Mo, .ltoreq.0.003% B, 0.01-0.1% Ti+Nb+V and .ltoreq.0.01%
Ca+REM. The cold rolling production process consists of three
consecutive annealing steps. In the first step, the sheet is
completely austenised during at least 5 seconds and subsequently
rapidly cooled (>10.degree. C./s) below the Ms (Martensite
start) temperature in order to produce lath martensite. The second
and third step are combined in a continuous annealing or
galvanising line and consist of reheating the sheet in the
intercritical region (Ac1<T<Ac3) during 5 to 120 seconds,
cooling (>5.degree. C./s) to 500.degree. C. or lower and than
subjecting the sheet to a galvanising or galvannealing treatment.
There are two main drawbacks associated with this invention. The
first one being the additional annealing step that is required to
produce the lath martensite starting micro-structure. This extra
process step will not only increase the total processing cost, but
it will also complicate logistics as well as weldability at the
entrance of the continuous annealing or hot dip galvanising line.
Cracks that are initiated in the weld or heat affected zone, will
easily propagate into the hard and brittle martensitic structure,
leading to a high risk on complete rupture of the weld between two
coils. The second major drawback is related to the rather high Si
content in these steels. From about 0.5% Si on, these high Si
contents are well known to provoke problems as to surface quality
because of the presence of Si-oxides which after pickling create a
surface with irregular and very high roughness. Moreover, in view
of corrosion protection, hot dip galvanising of such a high
Si-containing substrate in general leads to insufficient surface
appearance for automotive applications, with moreover a very high
risk on the presence of bare spots on the surface.
[0004] EP-A-0922782 also describes the production of a cold rolled
Si--Mn based TRIP steel which contains (in wt %) 0.05-0.40% C,
1.0-3.0% Si, 0.6-3.0% Mn, 0.02-1.5% Cr, 0.01-0.20% P and 0.01-0.3%
Al. As opposed to the previous invention, this product does not
require the use of an additional annealing step. Cr is added to the
analysis in order to retard the bainite formation and promote
acicular ferrite and martensite formation as it is thought by the
inventors that bainite is detrimental to the crushing behaviour in
Si--Mn based TRIP steels. P is added to avoid pearlite formation
and to increase the strength of the ferritic phase. The maximal P
content is limited to 0.2% because of weldability. The high Si
content in this invention will however again impair hot dip
galvanisability resulting in an insufficient surface appearance and
a very high risk on bare spots. The occurrence of red scale which
is difficult to remove, on the hot strip, due to the higher Si
content, is also expected to cause processing difficulties.
[0005] EP-A-0796928 describes the production of an Al-based Dual
Phase steel which contains (in wt %) 0.05-0.3% C, 0.8-3.0% Mn,
0.4-2.5% Al and 0.01-0.2% Si. Additionally the steel can contain
one of the following elements (in wt %) <0.05% Ti, <0.8% Cr,
<0.5% Mo, <0.5% Ni, <0.05% Nb and <0.08% P. After cold
rolling with a reduction rate higher than 40%, the material is
intercritically annealed at temperatures between 740 and
850.degree. C. and subsequently cooled at a cooling rate of 10 to
50 K/s to the Zn-bath temperature. As compared to both previous
analyses, the latter, nearly Si-free analysis, readily allows the
steel to be hot dip galvanised and avoids the formation of the
detrimental red scale. However, Al, as opposed to Si, does not
produce a strong solid solution strengthening effect. This implies
the use of rather high Al-contents in order to reach medium high
strength levels (e.g. Rm=700 MPa). These high Al levels are however
known to cause stickers during continuous casting and impair
weldability due to the presence of Al-oxides in the welded area.
This is especially detrimental for the crash behaviour of welded
structures. In order to avoid the casting problems an adapted very
fine casting powder is required that could cause health problems.
Steelmaking plants are therefore in general not willing to produce
this kind of compositions because the workers have to carry masks
and a lot of special precautions have to be taken.
[0006] EP-A-1170391 describes the production of a low carbon
(<0.08 wt %), low silicon (<0.5 wt %) and low aluminium
(<0.3 wt %) TRIP steel obtained by adding a nitriding step to
the processing (0.03-2 wt % N). The Al and Si contents have to be
kept low in order to avoid nitride precipitation and thus loss of
free N. Furthermore the Si content is preferably lower than 0.2 wt
% because of hot dip galvanisability. The carbon content is kept
very low because of weldability and because of the fact that the
presence of nitrogen in the steel also stabilises the retained
austenite. This nitrogen is incorporated in the steel sheet either
during or immediately after hot finish rolling, during
recrystallisation annealing, during intercritical annealing or via
a combination of one or more of these processes. All of them
require the steel sheet to be held for 2 sec to 10 min. in an
atmosphere containing not less than 2% ammonia in the temperature
range 550-800.degree. C. It is clear that this nitriding step makes
processing a lot more difficult and requires complicated technical
modifications to existing installations. At the moment this process
is internationally not considered to be industrially feasible.
Furthermore the very low alloying content of this steel grade, does
not allow to reach tensile strength levels above 650 MPa.
[0007] U.S. Pat. No. 5,470,529 deals with the production of cold
rolled TRIP steels based upon a wide variety of combined Al--Si
analyses. The carbon content range is set as 0.05-0.3 wt %, but
more preferably is 0.1-0.2 wt %. The Si-content is kept below 1.0
wt % in order to avoid red scale formation, but more preferably is
in the range 0.2-0.9 wt %. Manganese is added in 0.005 to 4.0 wt %,
but more preferably 0.5-2.0 wt %. As compared to traditional Si--Mn
TRIP compositions, part of the Si is replaced by Al for various
reasons. Like Si, Al also avoids cementite precipitation during
bainitic holding. This enables to use lower Si-levels and thus
avoid red scale formation. Furthermore the addition of Al increases
the Ar3 temperature, leading to an increased carbon concentration
in the austenite phase that is formed during intercritical
annealing. This in turn stabilises the retained austenite and makes
the steel less susceptible to stress-induced transformation in low
stress regions, leading to an improved hole expansion ratio.
Therefore the Al-range is set as 0.1-2.0 wt % and more preferably
as 0.5-1.5 wt %. However, as Al and Si are both ferrite
stabilizers, their sum is limited in order to avoid
over-stabilizing the retained austenite. The Al+Si content should
be in the range 0.5-3.0 wt % and more preferably in the range
1.5-2.5 wt %. In this invention P is considered as an incidental
impurity that should be limited as much as possible. The P-limit is
set at 0.1 wt % or less and preferably less than 0.02 wt %. Cu is
added to the analysis to facilitate the removal of red scale, to
improve the corrosion resistance of the as cold rolled product and
to improve the wettability by molten Zn. Therefore the Cu-range is
0.1-2.0 wt % and more preferably 0.1-0.6 wt %. To avoid hot
shortness problems when using Cu, Ni is added as well. For
economics its content is limited to 1.0 wt % and preferably 0.5 wt
%. The following constraints also apply: Ni(wt %)>Cu(wt %)/3
when Cu>0.5 wt % and Mn+Ni>0.5 wt %. Cr may be added as well
to stabilise the retained austenite and to further improve
corrosion resistance. It is added in the range 0.5-5.0 wt % and
more preferably in the range 0.6-1.6 wt %. To increase tensile
strength further Ti, Nb and V might be added as well. Their upper
limit is preferably 0.05 wt % for Nb and Ti and 0.10 wt % for V.
Although the maximum Si-content in this invention is limited to
<1 wt % in order to avoid red scale formation, most of the cold
rolled example steels have a Si-content in the range 0.5-1.1 wt %.
The latter is considered to give rise to hot dip galvanising
difficulties (bad wettability by molten Zn) and a deteriorated
surface appearance (bare spots). None of these example steels
contained micro-alloying additions as in high Si-TRIP steels, these
are known to markedly increase the hot strip hardness, leading to
strongly increased cold rolling forces. The low Si (0.2-0.4 wt %)
example steels on the other hand showed a high yield stress
(570-590 MPa) and only moderate ultimate tensile strength (700 MPa)
and total elongation values (A50.ltoreq.30%). In the latter steels
no P was added additionally. A large disadvantage of these
compositions is the necessity of adding Cu and Ni, elements which
are considered as impurities in bulk flat carbon steel production.
If a steelmaking plant has to cast this, extra logistic problems
with scrap recycling occur. Moreover, the use of Ni, Cu and Cr
makes the alloying cost much more expensive.
[0008] EP-A-1154028 describes the manufacturing of a P-alloyed
low-Al, low-Si TRIP steel, which contains (in wt %): 0.06-0.17% C,
1.35-1.80% Mn, 0.35-0.50% Si, 0.02-0.12% P, 0.05-0.50% Al, max.
0.07% Nb, max. 0.2% V, max. 0.05% Ti, max. 30 ppm B and 100-350 ppm
N. When the carbide forming elements Ti, Nb or V are added, the
carbon content is preferably 0.16 wt %. The amount of residual
austenite is limited to a maximum of 10%. The combination of low
Si-content and rather low C-content, results in tensile strength
values which are quite low (<600 MPa). When micro-alloying
elements are added, the strength level is markedly improved (800
MPa), but elongations drop drastically (A80<17%). Elongation
values are in all cases rather low, which can be explained by the
limited addition of Al and C, rendering the retained austenite less
stable.
[0009] L. Barbe et al have studied the influence of phosphorus
additions to several TRIP compositions, ranging from pure Si-TRIP
steels over combined Al--Si TRIP-steels to pure Al-TRIP steels
("Effect of phosphorus on the properties of a cold rolled and
intercritically annealed TRIP-aided steel", Int. Conf. on
TRIP-Aided High Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002).
They found that the TRIP-aided steel composition which contained
(in wt %) 0.24% C, 1.66% Mn, 0.6% Al, 0.4% Si and 0.073% P resulted
in an excellent combination of mechanical properties (A80=28.4% and
UTS=788 MPa). Lab-experiments performed by the inventors of the
present invention have however shown that Al-additions as low as
0.6 wt %, render the obtained mechanical properties very sensitive
to process parameter variations such as line speed and overageing
temperature. This can lead to a non-compatibility between different
galvanising lines (with e.g. different lengths of the levelling
zone around 490-460.degree. C.) or even to strongly
thickness-dependent mechanical properties. This can be explained by
the insufficient stabilisation of the retained austenite and the
shift of the optimum overageing time to noticeably longer times due
to the too low Al-additions.
[0010] O. Yakubovsky et al have studied the stress-strain behaviour
and bake hardening behaviour of several TRIP compositions, ranging
from pure Si-TRIP steels over combined Al--Si TRIP-steels to pure
Al-TRIP steels ("Stress-strain behaviour and bake hardening of TRIP
and TRIP-aided multiphase steels", Int. Conf. on TRIP-Aided High
Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002). In all cases the
carbon content was limited to (in wt %) 0.15% and the manganese
content to 1.5%. Amongst the steels studied was also a (in wt %)
0.25-0.45% Si, 1.5-2.0% Al and 0.05-0.10% P TRIP steel. No
mechanical properties were mentioned in the article for the latter
composition. Based upon the tensile strength versus carbon content
relationship that was established from industrial production and
laboratory research by the inventors of the present invention, the
proposed chemical composition is insufficiently alloyed in carbon
to reach tensile strengths in the range 700-850 MPa. Furthermore
the high Al-content requires the use of an adapted very fine
casting powder that can give rise to health problems. Furthermore
the weldability can be impaired due to the presence of Al-oxides in
the welded area, a consequence of the high Al-contents.
[0011] S. Papaefthymiou et al. studied the microstructure
development and mechanical behaviour of two Al--Si-TRIP steels that
were uni-axially deformed to different strain levels directly after
intercritical annealing ("Microstructure development and mechanical
behaviour of Al-containing TRIP-steels", Int. Conf. on TRIP-Aided
High Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002). As a
consequence of this special treatment and because of the
non-standard sample geometry, the mechanical properties mentioned
in the article are not comparable. The two Al containing TRIP
steels that were studied can be divided in a low-Al and a high-Al
steel. These contain (in wt %): low-Al steel: 0.19% C, 1.5% Mn,
0.26% Si, 0.086% P and 0.52% Al; high-Al steel: 0.17% C, 1.46% Mn,
0.26% Si, 0.097% P and 1.81% Al. As already explained before, the
low-Al steel will suffer from mechanical properties that are very
sensitive to process parameter variations such as line speed and
overageing temperature. This can lead to a non-compatibility
between different galvanising lines or even to strongly
thickness-dependent mechanical properties. The high-Al steel on the
other hand again requires the use of an adapted casting powder that
can give rise to health problems. Furthermore the weldability will
be impaired due to the presence of Al-oxides in the welded
area.
[0012] A. Pichler et al. ("Correlation between thermal treatment,
retained austenite stability and mechanical properties of
low-alloyed TRIP steels", Int. Conf. on TRIP-Aided High Strength
Ferrous Alloys, Ghent, Jun. 19-21, 2002) studied the influence of
different annealing process parameters on the retained austenite
stability and mechanical properties of a low-alloyed TRIP-steel
that contained (in wt %): .about.0.2% C, .about.1.6% Si+Al,
.about.1.5% Mn, <0.5% Cr+Mo, <0.04% P, <0.01% S and
<0.05% Ti+Nb. As no specifications were given related to the
Al/Si ratio in this analysis, it is not possible to conclude about
the galvanisability. The mentioned P-content is insufficient for
obtaining the desired mechanical properties (high tensile strength
in combination with high elongation), whilst maintaining a good
weldability (sufficient carbon content reduction).
AIMS OF THE INVENTION
[0013] It is the aim of the present invention to provide a high
strength, low Si, high Al, P-alloyed TRIP steel composition for the
production of steel products, produced by cold rolling that can
readily be galvanised.
[0014] It is a further aim to propose a process for the
manufacturing of such products, and to propose finally the products
themselves, wherein said composition, said process and said
products do not suffer from prior art problems.
SUMMARY OF THE INVENTION
[0015] The present invention is related to a cold rolled Al--Si
P-alloyed TRIP steel composition intended to be used as uncoated,
electro-galvanised or hot dip galvanised material. Said composition
is characterised by the following contents: [0016] C: between 1300
ppm and 2600 ppm [0017] Mn: between 10000 ppm and 22000 ppm [0018]
Al: between 8000 ppm and 15000 ppm [0019] Si: between 2000 ppm and
6000 ppm [0020] P: between 400 and 1000 ppm [0021] S: maximum 120
ppm [0022] N: maximum 200 ppm [0023] Ti: maximum 1000 ppm [0024]
Nb: maximum 1000 ppm [0025] V: maximum 1000 ppm [0026] B: maximum
10 ppm the remainder being substantially iron and incidental
impurities.
[0027] The novelty and inventive step of this composition lies in
the specific combination of elements P, Si, Al and C. In
particular, adding P in excess of prior art levels, whilst limiting
the maximum Si- and Al-content, allows to decrease the C-content
for reaching a certain strength level, in combination with better
weldability.
[0028] Three specific embodiments are related to the same chemical
composition, but having three different subranges for carbon which
are related to the strength level that is aimed at: [0029] UTS
(Ultimate Tensile strength) 590 MPa: carbon between 1300 ppm and
1900 ppm. Two particular embodiments are characterized by a carbon
content of 1350 ppm to 1900 ppm and 1400 ppm to 1900 ppm
respectively. [0030] UTS.gtoreq.690 MPa: carbon between 1700 ppm
and 2300 ppm [0031] UTS.gtoreq.780 MPa: carbon between 2000 ppm and
2600 ppm
[0032] Likewise three specific embodiments are related to the same
subranges for carbon content which are related to the strength
level that is aimed at, but having furthermore the following
specific chemical composition: [0033] Mn: between 13000 ppm and
22000 ppm [0034] Al: between 8000 ppm and 14000 ppm [0035] Si:
between 2500 ppm and 4500 ppm [0036] P: between 600 and 1000 ppm
[0037] S: maximum 120 ppm [0038] N: maximum 150 ppm [0039] Ti:
maximum 200 ppm [0040] Nb: maximum 100 ppm [0041] V: maximum 100
ppm [0042] B: maximum 5 ppm the remainder being substantially iron
and incidental impurities.
[0043] Likewise three more specific embodiments are related to the
same subranges for carbon content which are related to the strength
level that is aimed at, but having furthermore the following
specific sub-range for aluminium: 9000-1300 ppm. The subranges for
the other alloying elements are kept the same as in paragraph
[0017].
[0044] The present invention is equally related to a process for
manufacturing a cold rolled TRIP steel product, comprising the
steps of: [0045] preparing a steel slab having a composition
according to the invention, [0046] hot rolling said slab, wherein
the finishing rolling temperature is higher than the Ar3
temperature, to form a hot-rolled substrate, [0047] cooling said
substrate to a coiling temperature (CT) between 500.degree. C. and
680.degree. C., [0048] coiling said substrate at said coiling
temperature, [0049] pickling said substrate to remove the oxides,
[0050] cold rolling said substrate to obtain a reduction of
thickness, with a minimum reduction of 40%.
[0051] According to a first embodiment, the process of the
invention further comprises the steps of: [0052] soaking said
substrate at a temperature between 760.degree. C. and 850.degree.
C., [0053] cooling said substrate with a cooling rate higher than
2.degree. C./s to a temperature in the range 360.degree. C. to
450.degree. C., [0054] holding said substrate in said temperature
range for a time less than 700 s, [0055] cooling said substrate to
room temperature at a cooling rate higher than 1.degree. C./s.
[0056] subjecting said substrate to a skinpass reduction of maximum
1.5%.
[0057] According to a second embodiment, the process of the
invention further comprises an electrolytic zinc coating step.
[0058] According to a third embodiment, the process of the
invention further comprises the following processing steps after
the cold rolling step: [0059] soaking said substrate at a
temperature between 760.degree. C. and 850.degree. C., [0060]
cooling said substrate with a cooling rate higher than 2.degree.
C./s to the temperature of a Zn-bath, [0061] holding said substrate
in the temperature range between 490.degree. C. and 460.degree. C.
for less than 200 seconds. [0062] hot dip galvanising said
substrate in said Zn-bath, [0063] cooling said substrate to room
temperature at a cooling rate higher than 2.degree. C./s.
[0064] The process comprising a hot dip galvanising step may
further comprise the step of subjecting said substrate to a
skinpass reduction of maximum 1.5%.
[0065] The invention is equally related to a steel product produced
according to the process of the invention and having a
microstructure comprising 30-75% ferrite, 10-40% bainite, 0-20%
retained austenite and possibly 0-10% martensite.
[0066] The invention is equally related to a steel product produced
according to the process of the invention and having a carbon
content between 1300 ppm and 1900 ppm. Said product has a yield
strength between 320 MPa and 480 MPa, a tensile strength above 590
MPa, an elongation A80 higher than 26% and a n-value (this is the
strain hardening coefficient, calculated between 10% and uniform
elongation) higher than 0.2.
[0067] The invention is further related to a steel product produced
according to the process of the invention and having a carbon
content between 1700 and 2300 ppm. Said product has a yield
strength between 350 MPa and 510 MPa, a tensile strength above 700
MPa, an elongation A80 higher than 24% and a n-value (calculated
between 10% and uniform elongation) higher than 0.19.
[0068] The invention is further related to a steel product produced
according to the process of the invention and having a carbon
content between 2000 ppm and 2600 ppm. Said product has a yield
strength between 400 MPa and 600 MPa, a tensile strength above 780
MPa, an elongation A80 higher than 22% and a n-value (calculated
between 10% and uniform elongation) higher than 0.18.
[0069] The invention is also related to a steel product produced
according to the process of the invention and having a carbon
content between 2000 and 2600 ppm. Said product has a yield
strength between 450 MPa and 700 MPa, a tensile strength above 980
MPa, an elongation A80 higher than 18% and a n-value (calculated
between 10% and uniform elongation) higher than 0.14.
[0070] A steel product according to the invention may have a bake
hardening BH2 higher than 40 MPa in both longitudinal and
transversal directions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0071] According to the present invention, a steel composition is
proposed for the production of a P-alloyed Al--Si TRIP steel
product. Application of the broadest chemical composition ranges
which are indicated, will be able, in combination with the right
process parameters, to result in products having a desired TRIP
microstructure, good weldability as well as excellent mechanical
properties, with very high values of the product of tensile
strength and total elongation (this value is characteristic for a
high energy absorption potential in case of a crash). The preferred
ranges are related to more narrow ranges of mechanical properties,
for example a guaranteed minimum tensile strength of 780 MPa, or to
more stringent requirements on weldability (maximum of C-range, see
next paragraph).
[0072] C: between 1300 ppm and 2600 ppm. A first preferred subrange
is 1300-1900 ppm. A second preferred subrange is 1700-2300 ppm. A
third preferred subrange is 2000-2600 ppm. The minimum carbon
content per sub-range is needed in order to ensure the strength
level as carbon is the most important element for the
hardenability. The maximum of the claimed range per sub-range is
related to weldability. The effect of carbon on mechanical
properties is illustrated by exemplary composition A, E and F and
reference compositions B, C and D (tables 1, 3-8). The effect of
carbon content on spot weldability is illustrated by reference
compositions B, C and D (table 2). Two specific subranges for
carbon are characteristic for two specific embodiments: 1350-1900
ppm and 1400-1900 ppm. These subranges are aimed at ensuring an
Ultimate Tensile Strength of at least 600 MPa.
[0073] Mn: between 10000 ppm and 22000 ppm, preferably between
13000-22000 ppm. Manganese acts as an austenite stabiliser and thus
decreases the Ms temperature of the retained austenite. Furthermore
Mn suppresses pearlite formation and also contributes to the
overall strength level of the steel by solid solution hardening.
Adding excess Mn results on the other hand in insufficient ferrite
formation upon cooling from the soaking temperature and thus to
insufficient carbon concentration in the retained austenite,
rendering the latter less stable. Too much Mn will also increase
the hardness of the weld and will enhance the formation of
detrimental banded microstructures.
[0074] Al: between 8000 ppm and 15000 ppm, preferably between
8000-14000 ppm and most preferably between 9000-13000 ppm.
Aluminium is added because, to an even stronger degree than Si, it
is a ferrite stabiliser and thus enhances the ferrite formation
during soaking and during cooling from the soaking temperature,
thereby stabilising the retained austenite. The latter is
stabilised even more by the fact that Al also suppresses the
precipitation of carbon from the retained austenite during the
overageing stage. Unlike Si, Al has no detrimental effect on
galvanisability. Al-contents above 15000 ppm are however known to
require the use of an adapted very fine casting powder that can
cause health problems. Furthermore weldability can deteriorate due
to the presence of Al-oxides in the welded area. A minimum Al
content is however required to allow the material to be processed
on different hot dip galvanising lines with different lengths of
the levelling zones and to ensure a high process robustness.
[0075] Si: between 2000 ppm and 6000 ppm, preferably between
2500-4500 ppm. Silicon has essentially the same function as Al,
albeit slightly less pronounced. That is: Si is a ferrite
stabiliser and prevents carbide precipitation during the overageing
stage, thereby stabilising the retained austenite at room
temperature. Besides this, Si also contributes to the overall
strength level of the steel by solid solution hardening. The
maximum Si-content is however limited as Si is well known to
provoke problems as to surface quality because of the presence of
Si-oxides which after pickling create a surface with irregular and
very high roughness. Moreover, in view of corrosion protection, hot
dip galvanising of high Si-containing substrates in general leads
to insufficient surface appearance for automotive applications,
with moreover a high risk on the presence of bare spots on the
surface.
[0076] P: between 400 ppm and 1000 ppm, preferably between 600-1000
ppm. Phosphorous is added primarily to allow the carbon content to
be decreased to obtain improved weldability, while maintaining the
same tensile strength level. Furthermore, P in combination with Si,
is known to enhance the retained austenite stability by suppressing
carbide precipitation during the overageing stage. In this respect
P additions below 400 ppm do not allow a sufficiently large
reduction of C-content. When adding more than 1000 ppm P, the risk
on segregation defects increases and weldability is again
deteriorated.
[0077] S: maximum 120 ppm. The S-content has to be limited because
a too high inclusion level can deteriorate the formability.
[0078] N: maximum 200 ppm, preferably maximum 150 ppm otherwise too
much AlN and/or TiN precipitates can form which are detrimental to
formability.
[0079] Ti: maximum 1000 ppm, preferably below 200 ppm for products
produced according to the present invention having a tensile
strength below 980 MPa. Titanium can be added in order to increase
the tensile strength of the steel by grain refinement and
precipitation strengthening. However for tensile strengths below
980 MPa, even without adding Ti, using the appropriate processing
parameters, will result in the targeted mechanical properties per
carbon sub-range and thus avoid an increase in analysis cost or
extra processing difficulties (e.g. rolling forces).
[0080] Nb: maximum 1000 ppm, preferably below 100 ppm for products
produced according to the present invention having a tensile
strength below 980 MPa. Niobium can be added in order to increase
the tensile strength of the steel by grain refinement and
precipitation strengthening. However for tensile strengths below
980 MPa, even without adding Nb, using the appropriate processing
parameters, will result in the targeted mechanical properties per
carbon sub-range and thus avoid an increase in analysis cost or
extra processing difficulties (e.g. rolling forces).
[0081] V: maximum 1000 ppm, preferably below 100 ppm for products
produced according to the present invention having a tensile
strength below 980 MPa. Vanadium can be added in order to increase
the tensile strength of the steel by grain refinement and
precipitation strengthening. However for tensile strengths below
980 MPa, even without adding V, using the appropriate processing
parameters, will result in the targeted mechanical properties per
carbon sub-range and thus avoid an increase in analysis cost.
[0082] B: maximum 10 ppm, preferably maximum 5 ppm. Boron is
avoided because of its detrimental influence on ferrite
nucleation.
[0083] The present invention is equally related to the process for
producing said steel product. This process comprises the steps of:
[0084] preparing a steel slab having a composition according to the
invention, such as defined above, [0085] if necessary, reheating
said slab to a temperature higher than 1000.degree. C., preferably
above 1200.degree. C., [0086] hot rolling the slab, wherein the
finishing rolling temperature FT at the last stand of hot rolling
is higher than the Ar3 temperature, [0087] cooling to coiling
temperature CT, preferably by continuous cooling to the CT,
typically at 40-50.degree. C./s. Stepwise cooling may be used as
well, [0088] hot rolling mill coiling of said substrate at a
coiling temperature CT comprised between 500.degree. C. and
680.degree. C., preferably between 600.degree. C. and 680.degree.
C. This temperature range is chosen so as to create a hot band
which is as soft as possible in order to facilitate cold rolling,
[0089] pickling the substrate to remove the oxides, [0090] cold
rolling to obtain a reduction of thickness. The cold rolling
reduction is preferably higher than 40%.
[0091] According to a first embodiment of the invention, these
steps are followed by an annealing treatment in a continuous
annealing line, comprising the following steps: [0092] soaking said
pickled cold rolled substrate in a temperature range between 760
and 850.degree. C. in order to create a microstructure that
consists of ferrite and austenite. If the soaking temperature is
chosen above 850.degree. C., the amount of austenite formed will be
too large, which leads to a less stable retained austenite in the
end product. Due to the reduced austenite stability, a substantial
part of it could also transform to martensite during final cooling
to room temperature which deteriorates elongation properties. If,
on the other hand, the soaking temperature would be chosen too low,
insufficient austenite would be formed during soaking. This could
lead to over-stabilisation of the retained austenite, which again
deteriorates mechanical properties. [0093] Cooling said substrate
with a cooling rate higher than 2.degree. C./s to a holding
temperature in the range 360.degree. C. to 450.degree. C. The
holding time in said temperature range is less than 700 seconds.
When the holding temperature would be chosen below 360.degree. C.,
a substantial part of the retained austenite will transform to
martensite, leading to a DP-like behaviour (high initial n-value
that decreases as a function of increasing strain) of the final
product. Holding temperatures above 450.degree. C., will on the
other hand lead to a decomposition of the retained austenite by
carbon precipitation. This will again deteriorate elongation
properties. [0094] Final cooling of said substrate to a temperature
below 150.degree. C. with a cooling rate higher than 1.degree. C./.
[0095] Finally said substrate can be subjected to a skinpass
reduction which is preferably in the range 0.3% to 1.5%.
[0096] A second preferred embodiment comprises the same processing
steps mentioned above, but additionally also comprises an
electrolytic zinc coating step.
[0097] According to a third embodiment of the invention, the cold
rolling step is followed by an annealing treatment in a continuous
hot dip galvanising line, comprising the following steps: [0098]
soaking said pickled cold rolled substrate in a temperature range
between 760 and 850.degree. C. in order to create a microstructure
that consists of ferrite and austenite. If the soaking temperature
is chosen above 850.degree. C., the amount of austenite formed will
be too large, which leads to a less stable retained austenite in
the end product. Due to the reduced austenite stability, a
substantial part of it could also transform to martensite during
final cooling to room temperature which deteriorates elongation
properties. If, on the other hand, the soaking temperature would be
chosen too low, insufficient austenite would be formed during
soaking. This could lead to over-stabilisation of the retained
austenite, which again deteriorates mechanical properties. [0099]
cooling said substrate with a cooling rate higher than 2.degree.
C./s to the temperature of the Zn-bath, [0100] holding said
substrate in the temperature range between 490.degree. C. and
460.degree. C. for less than 200 seconds and preferably between 5
seconds and 80 seconds. [0101] hot dip galvanising said substrate
in said Zn-bath, [0102] final cooling to room temperature at a
cooling rate higher than 2.degree. C./s. [0103] Finally said
substrate can be subjected to a skinpass reduction which is
preferably in the range 0.3% to 1.5%.
[0104] The thickness of the steel substrates of the invention after
cold rolling can be lower than 1 mm according to the initial hot
rolled sheet thickness and the capability of the cold rolling mill
to perform the cold rolling at a sufficiently high level. Thus,
thicknesses between 0.3 and 2.5 mm are feasible.
[0105] The resulting cold rolled product has a multiphase structure
with 30-75% ferrite, 10-40% bainite, 0-20% retained austenite and
possibly amounts of martensite (0-10%) present at room temperature.
The amount of martensite at room temperature should however be
limited in order to maintain an n-value behaviour (constant or
increasing with strain) and mechanical properties that are
characteristic for TRIP-steels. Specific mechanical properties as a
function of processing parameter values are given in the
examples.
[0106] The cold rolled non-temper rolled product showed in all
cases a yield point elongation, which is typical for TRIP-steels
and indicates that no or only very small amounts of martensite are
present in the microstructure. This yield point elongation can be
suppressed by temper rolling the final product. Small temper
rolling reductions are sufficient to avoid the occurrence of a
yield point elongation and temper rolling reductions above 1.5%
should be avoided in order to prevent a too large yield strength
increase.
[0107] The final cold rolled product furthermore preferably
exhibits a constant or increasing n-value with increasing strain.
This behaviour implies that the retained austenite is gradually
transformed into martensite as the tensile test progresses thereby
postponing the occurrence of necking, leading to an excellent
combination of tensile strength and total elongation.
[0108] The robustness of TRIP steel products produced according to
this invention is ensured by the minimum Al-content specified in
the preferred Al-range: 8000-14000 ppm and most preferably in the
range 9000-13000 ppm. Adding less Al will render the retained
austenite less stable. This will increase the risk of loss of
mechanical properties by austenite decomposition through carbon
precipitation and on the other hand the less stable retained
austenite will more easily transform into martensite during
straining, limiting the formability of the material. Adding less Al
will also retard the bainite transformation kinetics. As a
consequence the mechanical properties will become more dependent on
processing conditions such as line speed and overageing temperature
as well on the actual line lay-out (short or long overageing
section). Using an Al-content within the preferred range, avoids
such line dependency and loss of robustness.
[0109] Concerning weldability of the obtained cold rolled product,
the addition of phosphorus allows to decrease the carbon content as
compared to P-free Al--Si TRIP steels of the same tensile strength
level. As in the investigated carbon range, weldability is improved
by lowering the carbon content, such a carbon content reduction by
P-addition can be considered as the main advantage of the current
invention.
[0110] The different drawbacks described above as to the
compositions described in state of the art publications are not
encountered when the composition of the present invention is
applied: [0111] Si is limited in order to ensure the hot dip
galvanisability. The surface appearance of the hot dip galvanised
cold rolled steel of the present invention is sufficient for
automotive unexposed applications whereas substrates with higher
Si-contents in general lead to insufficient surface appearance for
automotive applications, with moreover a much higher risk on the
presence of bare spots on the surface. [0112] Si is furthermore
limited in order to avoid the need of hot charging the slabs to
prevent crack formation. [0113] The presence of red scale on the
hot rolled substrate's surface is also avoided by limiting the
Si-content. [0114] A small amount of Si, which is compatible with
hot dip galvanising, is added to more easily reach the targeted
tensile strength levels. In comparison with Si-free Al-TRIP steels,
this allows to use a lower C-content or lower Al-content. [0115]
The maximum Al-content is limited to avoid the use of an adapted
very fine casting powder that could cause health problems.
Furthermore in the case of high Al-contents (>1.5%) weldability
could deteriorate due to the presence of Al-oxides in the welded
area. A minimum Al-content however ensures a high process
robustness and less sensitivity to changes in line speed,
overageing temperature and lay-out of the continuous annealing or
hot dip galvanising line. [0116] For a given tensile strength
level, the C content is limited as compared to other Al--Si TRIP
steels in order to improve weldability, by the addition of P and
without the need of micro-alloying. [0117] No Ni, Cu or Cr are
added to the steel products of the invention. This avoids logistic
problems with scrap recycling and reduces the cost of the
analysis.
EXAMPLES
1. Example Compositions
[0118] Table 1 shows examples of compositions of laboratory
castings of the P-alloyed Al--Si TRIP steel product according to
the present invention (codes A, E and F), and of reference
compositions (B, C and D) having either a C-content which is higher
than the claimed range and/or no intentionally added phosphor.
Laboratory thermal cycle simulations and tensile tests were
performed to obtain the mechanical properties of the test specimens
of these example compositions. It is to be noted that in what
follows, all mentioned tensile test mechanical properties are
measured according to the standard EN10002-1.
1.1 Cold Rolled and Continuously Annealed Product
[0119] The processing steps were: [0120] Casting, [0121] Reheating
at 1250.degree. C. for 1 hour, [0122] Hot rolling in the austenite
region to a final thickness of 3.5 mm, [0123] Water cooling to the
coiling temperature of 600.degree. C., [0124] Cold rolling to a
final thickness of 1 mm,
[0125] After the previous processing steps, tensile specimens with
80 mm gauge length parallel to the rolling direction were machined
out of the sheets. These specimens were heat treated in two salt
baths following a two-step thermal cycle. After annealing in the
intercritical region (IAT) for a given time (IAt), the samples were
quenched in a salt bath at a lower temperature and isothermally
held (BHT) for a given time (BHt). The mechanical properties are
listed in table 3 and are conform the specifications for a TRIP700
grade. No temper rolling was applied. When comparing the mechanical
properties mentioned in table 3 (example composition A), with those
mentioned in table 5 (in particular reference composition C), it
becomes clear that the addition of P in the steel of the present
invention clearly allows to reduce the carbon content with 500 ppm
while maintaining the tensile strength level. This C-reduction will
seriously improve weldability (cfr. Table 2: difference between
0.25 C and 0.20 C). Table 7 contains the mechanical properties
obtained after applying several continuous annealing simulations on
steel samples of compositions E and F. Looking at the data in table
5 and 7 (in particular E compared to B), it is clear that the
tensile strength is even higher for the composition of the
invention, as compared to the reference composition which has 600
ppm more carbon and no intentionally added phosphor.
1.2 Cold Rolled and Hot Dip Galvanised Product
[0126] The processing steps were: [0127] Casting, [0128] Reheating
at 1250.degree. C. for 1 hour, [0129] Hot rolling in the austenite
region to a final thickness of 3.5 mm, [0130] Water cooling to the
coiling temperature of 600.degree. C., [0131] Cold rolling to a
final thickness of 1 mm.
[0132] After the previous processing steps, tensile specimens with
80 mm gauge length parallel to the rolling direction were machined
out of the sheets. These specimens were heat treated in two salt
baths following a two-step thermal cycle. After annealing in the
intercritical region, the samples were quenched in a salt bath at a
lower temperature and isothermally held. The temperatures and
holding times were adapted in order to simulate closely the process
steps, including a hot dip galvanizing step. The mechanical
properties are listed in table 4 and clearly illustrate that the
current invention leads to a very robust hot dip galvanized TRIP700
product: a broad variation of process parameters (IAT, Iat, BHT,
BHt) all result in products with comparable mechanical properties.
No temper rolling was applied. When comparing the mechanical
properties mentioned in table 4 (example composition A), with those
mentioned in table 6 (in particular reference composition C), it
becomes clear that the addition of P in the steel of the present
invention clearly allows to reduce the carbon content with 500 ppm
while maintaining the tensile strength level. This C-reduction will
seriously improve weldability (cfr. Table 2).
[0133] Table 8 contains the mechanical properties obtained after
applying several hot dip galvanising simulations on steel samples
of compositions E and F. Looking at the data in table 6 and 8 (in
particular E compared to B), it is clear that the tensile strength
is even higher for the composition of the invention, as compared to
the reference composition which has 600 ppm more carbon and no
intentionally added phosphor.
TABLE-US-00001 TABLE 1 Compositions (ppm) of Al--Si TRIP steels.
Compositions A, E, F according to the invention, B, C and D are
reference compositions. Code C Mn Al Si P S N Ti Nb V A 2000 16200
9000 3000 850 20 69 20 20 30 B 3000 15000 12000 3000 173 60 43 47
30 43 C 2500 15000 12000 3000 167 55 32 37 26 45 D 2000 15000 10000
3000 223 32 48 61 38 55 E 2400 15100 11200 2700 820 10 100 50 F
2200 15000 13400 4400 730 10 50 50
TABLE-US-00002 TABLE 2 Average of max. force measured at Imax in
cross-tensile test on spot welds according to AFNOR-A87001. Code
Fmax at Imax (N) B 6906 C 7489 D 9608
TABLE-US-00003 TABLE 3 Mechanical properties of the cold rolled and
continuously annealed P-alloyed Al--Si TRIP steel, composition A,
according to the present invention. No temper rolling applied.
Thickness 1 mm. IAT BHT Rp0.2 Rm Au A80 Code (.degree. C.) IAt (s)
(.degree. C.) BHt (s) (MPa) (MPa) (%) (%) A 770 60 400 120 486 708
24.7 30.9
TABLE-US-00004 TABLE 4 Mechanical properties of the cold rolled and
hot dip galvanised P-alloyed Al--Si TRIP steel, composition A,
according to the present invention. No temper rolling applied.
Thickness 1 mm. IAT IAt BHT BHt Rp0.2 Rm Au A80 (.degree. C.) (s)
(.degree. C.) (s) (MPa) (MPa) (%) (%) Re/Rm RmxA80 770 73 460 20
411 738 23.6 28.6 0.557 21107 490 408 756 22.6 28.0 0.540 21168 800
460 436 756 24.2 29.6 0.577 22378 490 428 775 22.5 28.1 0.552 21778
830 460 427 746 24.6 30.2 0.572 22529 490 434 761 22.4 28.1 0.570
21384 770 52 460 14 419 734 24.3 29.4 0.571 21580 490 419 754 22.5
27.2 0.556 20509 800 460 432 755 23.7 28.8 0.572 21744 490 427 760
23.1 28.3 0.562 21508 830 460 427 748 22.7 27.8 0.571 20794 490 428
771 21.7 27.0 0.555 20817 770 36 460 10 423 701 24.7 30.3 0.603
21240 490 419 726 23.8 28.7 0.577 20836 800 460 433 755 23.9 28.9
0.574 21820 490 426 743 22.0 26.8 0.573 19912 830 460 441 768 23.6
28.9 0.574 22195 490 449 765 23.0 27.9 0.587 21344 (BH = bainitic
holding = holding before passing through the zinc bath)
TABLE-US-00005 TABLE 5 Mechanical properties of the cold rolled and
continuously annealed reference Al--Si TRIP steels. Thickness SKP
Re (MPa) Rm (MPa) A80 (%) Code (mm) (%) RD TD RD TD RD TD B 1 0.50
508 499 737 726 34.0 32.0 1.5 0.75 508 500 732 746 31.0 27.0 1.6
1.00 524 515 737 745 31.0 29.0 C 0.7 0.50 502 499 713 717 34.3 32.1
1.45 0.80 507 496 692 700 34.2 31.5 D 0.85 0.70 483 463 664 668
27.8 29.2 1.22 0.70 451 451 647 657 30.1 29.9 1.47 0.75 466 457 650
658 28.5 27.7 (RD: rolling direction; TD: transverse direction)
TABLE-US-00006 TABLE 6 Mechanical properties of the hot dip
galvanised reference Al--Si TRIP steels. Thickness SKP Re (MPa) Rm
(MPa) A80 (%) Code (mm) (%) RD TD RD TD RD TD B 1.00 0.5 490 482
800 803 32.1 30.1 1.2 0.7 483 485 802 810 32.7 30.7 C 1.00 0.6 458
449 727 732 34.6 35.4 1.45 0.9 472 462 732 727 33.2 31.4 D 1.49 1.0
446 438 652 647 33.8 30.9 1.23 1.1 451 456 673 675 32.8 33.0 (RD:
rolling direction; TD: transverse direction)
TABLE-US-00007 TABLE 7 Mechanical properties of the cold rolled and
continuously annealed P-alloyed Al--Si TRIP steel, compositions E
and F, according to the present invention. No temper rolling
applied. Thickness 1 mm. IAT IAt BHT Rp0.2 Rm A80 Code (.degree.
C.) (s) (.degree. C.) BHt (MPa) (MPa) (%) Re/Rm RmxA80 E 800 209
375 448 595 820 26.1 0.726 21402 425 593 795 33.2 0.746 26394 125
375 280 562 834 26.9 0.674 22434 425 589 798 27.3 0.738 21785 F 800
209 375 448 597 837 27.5 0.713 23017 425 582 819 32 0.710 26208 125
375 280 569 855 27.3 0.665 23341 425 589 820 32.4 0.718 26568
TABLE-US-00008 TABLE 8 Mechanical properties of the cold rolled and
hot dip galvanised P-alloyed Al--Si TRIP steel, compositions E and
F, according to the present invention. No temper rolling applied.
Thickness 1 mm. IAT IAt BHT BHt Rp0.2 Rm A80 Code (.degree. C.) (s)
(.degree. C.) (s) (MPa) (MPa) (%) Re/Rm RmxA80 E 800 73 460 20 496
862 26.9 0.575 23188 490 481 901 24.6 0.534 22165 45 460 65 544 858
28.3 0.634 24281 490 528 872 24.3 0.606 21190 36 460 10 555 799
24.4 0.695 19496 490 526 881 27.9 0.597 24580 22 460 32 490 872
23.6 0.562 20579 490 549 850 27.8 0.646 23630 F 800 73 460 20 487
924 25.1 0.527 23192 490 437 971 22.1 0.450 21459 45 460 65 551 830
29.9 0.664 24817 490 509 938 25.3 0.543 23731 36 460 10 552 855
27.7 0.646 23684 490 480 922 24.7 0.521 22773 22 460 32 480 920
24.9 0.522 22908 490 532 881 25.8 0.604 22730 (BH = bainitic
holding = holding before passing through the zinc
* * * * *