Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02989710 2017-12-15
(Prilitg.0:- 8405/29.171 [O. ESc P IY Guibi uta v
tE F.! 201.6064. 7041
HIGH STRENGTH HOT DIP GALVANISED STEEL STRIP
The invention relates to a high strength hot dip galvanised steel strip having
improved formability, such as used in the automotive industry. The invention
also
relates to a method for producing such steel strip.
Such steel types are known and have been developed under the name of dual
phase steel types. These steel types do not provide the formability as
required in many
=
applications for the automotive industry. For this reason, TRIP assisted dual
phase steel
types have been developed.
Formability, however, is not the only requirement for a TRIP assisted dual
phase
steel strip. The alloying elements should be low in amount to make the cost of
the steel
as low as possible, and it should be as easy as possible to produce the steel
strip at a
broader width both in the hot rolling mill and in the cold rolling mill.
Moreover, the
steel strip should be easy to coat with a zinc based coating, the steel strip
has to have
high strength and a good weldability, and should also exhibit a good surface
,quality.
These requirements are especially important for industrially produced TRIP
assisted
dual phase steel types, which have to be formed into for instance automotive
parts that
will be spot welded into a body in white.
It is thus an object of the invention to find a composition of a high
strefigth hot
dip galvanised steel strip that strikes a balance between the formability, the
homogeneity and the processability of the strip.
It is a further object of the invention to provide a high strength hot dip
galvanised
steel strip that has a good coatability during the hot dip galvanising
process.
It is a still further object of the invention to provide a high strength hot
dip
galvanised steel strip that has a good weldability.
It is another object of the invention to provide a high strength hot dip
galvanised
steel strip that has a good surface quality.
It is still another object of the invention to provide a high strength hot dip
galvanised steel strip having a cost price that is as low as possible.
AMENDED SHEET
117102/2017i
CA 02989710 2017-12-15
LP rjr77.feT:1:: W05/2917)ibc..S.c.PAM-01 FL! /t1" Ll11.0/ U134
teP20i 606 47.04;
= - 2 -
One or more of these objects are met according to the invention by providing a
high strength hot dip galvanised steel strip consisting, in mass percent, of
the following
elements:
0.10¨ 0.21 %C
1.75 ¨ 2.50 % Mn
0.04 ¨ 0.60 % Si
0.20 ¨ 1.40 % Al
0.001 ¨ 0.025 % P
0.0005 - 0.0050 % B =
max 0.50 % Cr
max Ø20 % Ti
max 0.004 % Ca
max 0.015 %N
the balance being Fe and inevitable impurities.
The inventors have found that by a careful selection of the amounts of the
main
constituting elements of the steel, being carbon, manganese, silicon,
aluminium,
chromium and boron, a high strength hot dip galvanised steel strip can be
produced that
has the required formability, homogeneity, processability, strength and
elongation,
while at the same time providing a sufficient weldability, coatability and
surface
quality.
The inventors have especially found that it is advantageous to add boron to
the
composition of the steel. By adding boron, the hot rolled steel can be. cooled
fast
enough on the run-out table to get a coiling temperature that provides a
suitable
microstructure for further processing. Moreover, due to the addition of boron,
the
inventors have found that the properties of the end product have a high degree
of
homogeneity. Thus, the steel strip can be produced in a width that is
commercially
interesting.
Boron suppresses ferrite formation during austenite cooling. This minimises
local
carbon enrichment in the strip. Therefore boron is normally avoided if TRIP
type of
1 30 steels are manufactured. The inventors have found that good TRIP
assisted steel grades
can be made by intercritically annealing of the cold rolled strip so that
ferrite nucleation
AMENDED SHEET
11.7A212917)
CA 02989710 2017-12-15
(Erinted::08/0M.017j ft.K5C EMI I /tr Ll).110/ 004
iE.P2Q1 60 6 4:7_04)
- 3 -
is not required.
Moreover, adding boron improves the hardenability of the steel, resulting in
the
possibility to use less of the other alloying elements. This results in an
improved
dimensional window for the steel strip, meaning a higher width to thickness
ratio while
the mechanical properties of the steel over the width of the strip remain
suitable.
Boron segregates to the grain boundaries and replaces phosphorus at the grain
boundaries, which makes it possible to have a higher P amount in the steel,
while still
achieving a good weldability.
The reason for the amounts of the main constituting elements is as follows.
C: 0.10 ¨ 0.21 mass %. Carbon has to be present in an amount that is high
enough to
ensure hardenability and the formation of martensite at the cooling rates
available in a
conventional annealing/galvanising line. Martensite is required to deliver
adequate
strength. Free carbon also enables stabilisation of austenite which delivers
improved
work hardening potential and good formability for the= resulting strength
level. A lower
limit of 0.10 mass % is needed for these reasons. A maximum level of 0.21 mass
% has
been found to be essential to ensure good weldability.
Mn: 1.75 ¨ 2.50 mass %. Manganese is added to increase hardenability thus
making the formation of hard phases like martensite or bainite easier within
the cooling
rate capability of a conventional continuous annealing/galvanising line.
Manganese also
contributes to the solid solution strengthening which increases the tensile
strength and
strengthens the ferrite phase, and also helps to stabilise retained austenite.
Manganese
lowers the transformation temperature range of the dual phase steel, thus
lowering the
required annealing temperature to levels that can be readily attained in a
conventional
= continuous annealing/galvanising line. A lower limit of 1.75 mass % is
needed for the
above reasons. This lower limit is possible in view of the addition of other
elements,
such as boron. A maximum level of 2.50 mass % is imposed to ensure acceptable
rolling forces in the hot mill and to ensure acceptable rolling forces in the
cold mill by
ensuring sufficient transformation of the dual phase steel to soft
transformation
products (ferrite and pearlite). This maximum level is also given in view of
the stronger
segregation during casting and the forming of a band of martensite in the
strip at higher
values. Preferably, the amount of manganese is between 1.9 and 2.3 mass %,
more
AMENDED SHEET
[1:7/92/2017]
CA 02989710 2017-12-15
tiinted: 08/05/201.71 (15---E- o¨P A M rk..1 Lk/1b/UW.1
tEP20160647,04,
- 4 -
= preferably between 2.0 and 2.2 mass %
Si: 0.04 ¨ 0.60 mass %. Silicon provides solid solution strengthening thus
enabling the attainment of high strength, and the stabilisation of austenite
via
strengthening of the ferrite matrix. Silicon very effectively retards the
formation of
carbides during overaging, thus keeping carbon in solution for stabilisation
of austenite.
For these reasons a lower limit of 0.04 mass % is needed. A maximum level of
0.60
mass % is imposed in view of the coatability of the steel strip, since high
levels of
silicon lead to unacceptable coating quality due to reduced adherence.
Al: 0.20 ¨ 1.40 mass %. Aluminium is added to liquid steel for the purpose of
de-
i 0 oxidation. In the right quantity it also provides an acceleration of
the bainite
transformation, thus enabling bainite formation within the time constraints
imposed by
the annealing section of a conventional continuous annealing/galvanising line.
Aluminium also retards the formation of carbides thus keeping carbon in
solution, thus
causing partitioning to austenite during overaging, and promoting the
.stabilisation of
austenite. A lower level of 0.20 mass % is required for the above reasons. A
maximum
level of 1.40 mass % is imposed for castability, since high aluminium contents
lead to
poisoning of the casting mould slag and consequently an increase in mould slag
viscosity, leading to incorrect heat transfer and lubrication during casting.
Cr: max 0.50 mass %. Chrome is added to increase hardenability. Chrome
promotes formation of ferrite. A maximum level of 0.50 mass % is imposed to
ensure
that not too much martensite forms at the cost or retained austenite. It is
also possible to
add no chrome. Preferably, the amount of Cr is between 0.01 and 0.40 mass%,
more
preferably between 0.02 and 0.25 mass %.
Ti: max 0.20 %. Titanium is mainly added to strengthen the steel. A maximum
level of 0.20 % is imposed to limit the cost of the steel. It is also possible
to add no Ti.
Ca: max 0.004 mass %. The addition of calcium modifies the morphology of
manganese sulphide inclusions. When calcium is added the inclusions get a
globular
rather than an elongated shape. Elongated inclusions, also called stringers,
may act as
planes of weakness along which lamellar tearing and delamination fracture can
occur.
The avoidance of stringers is beneficial for forming processes of steel sheets
which
entail the expansion of holes or the stretching of flanges and promotes
isotropic forming
, : AMENDED SHEET
t17/02/201,7]
CA 02989710 2017-12-15
1115010: 81.014 1711 (CIESCPAA Di FL_ 1 /LI' ZU1b/U04
/
F_P201.60,047_041
- 5.,
behaviour. Calcium treatment also prevents the formation of hard, angular,
abrasive
alumina inclusions in aluminium deoxidised steel types, forming instead
calcium
aluminate inclusions which are softer and globular at rolling temperatures,
thereby
improving the material's processing characteristics. In continuous casting
machines,
some inclusions occurring in molten steel have a tendency to block the nozzle,
resulting
in lost output and increased costs. Calcium treatment reduces the propensity
for
blockage by promoting the formation of low melting point species which will
not clog
the caster nozzles. It is also possible to add no calcium when the sulphur
content is very
low. Preferably, the amount of Ca is between 0.0005 and 0.003 mass%.
P: 0.001 - 0.025 mass%. Phosphorus interferes with the formation of carbides,
and therefore some phosphorus in the steel is advantageous. However,
phosphorus can
make steel brittle upon welding,, so the amount . of phosphorus should be
carefully =
controlled during steelmaking, especially in combination with other
embrittling
elements such as sulphur and nitrogen. On the other hand, in view of the
addition of
boron it is possible to have more phosphorus in the steel then usual.
The content of Nitrogen is limited to max 0.015 wt% as is typical for
continuous
casting plants. Usually, the amount of N is between 0.001 and 0.010 wt%.
In addition the reasons given above, the ranges for aluminium, boron, silicon,
chromium and manganese are chosen such that a correct balance is found to
deliver a
transformation that is as homogeneous as possible on the run-out table and
during coil
cooling, to ensure a steel strip that can be cold rolled, and to provide a
starting structure
enabling rapid dissolution of carbon in the annealing line to promote
hardenability and
correct ferritic/bainitic transformation behaviour. Moreover, because
aluminium
accelerates and chromium decelerates the bainitic transformation, the right
balance
between aluminium and chromium has to be present to produce the right quantity
of
bainite within the timescales permitted by a conventional hot dip
galvanising.line with a
restricted overage section. In practice, this means that the content of
aluminium should
be higher than the content of chromium.
According to a preferred embodiment, the amounts of Al and Si are chosen such
1
that 0.60 % < Al + Si < 1.40 %
According to another preferred embodiment, the amounts of Mn and Cr are
1
AM EN DED--H EET
07/02/20,11j
CA 02989710 2017-12-15
5?.rjrit@ft.SP,05/.20171 ibESCPAMD) /tP ZU1b/Ubli
EP201,60647041
- 6 ,
chosen such that Mn + Cr > 2.00 %.
Preferably, the amounts of AI and Si are chosen such that Si < Al.
Apart from the absolute contents of the elements as given above, also the
relative
amounts of certain elements are of importance.
Aluminium and silicon together should be maintained between 0.60 and 1.40
mass% to ensure suppression of carbides in the end product and stabilisation
of a
sufficient amount of austenite, with the correct composition, to provide a
desirable
extension of formability.
Manganese and chromium together should be above 2.00 mass% to ensure
to sufficient
hardenability for formation of martensite and/or bainite and thus achievement
of strength in a conventional continuous annealing line and hot dip
galvanising line. In
addition, Mn aids to stabilise retained austenite. Preferably, Mn + Cr should
be above
2.10 mass%, especially when the amount of Si is low.
Al should preferably be present in an amount equal to or higher then Si in
view of
a good zinc coatability.
Preferably element C is present in an amount of 0.13 ¨ 0.18=%. In this range
the
hardenability of the steel is optimal while the weldability of the steel is
enhanced, also
by the presence of boron. More preferably element C is present in an amount of
0.14 ¨
0.17 %. This amount of C has been found to work well in practice.
Preferably element Si is present in an amount of 0.05 ¨ 0.50 %, more
preferably
0.05 - 0.40 %. A amount of silicon lower then 0.50 % improvet the coatability
of the
steel strip, even more so when the amount of silicon is below 0.40 %.
According to a preferred embodiment element Al is present in an amount of 0.30
- 1.20 %, preferably an amount of 0.40 ¨ 1.00 %. A raised lower level of
aluminium has
the same effect as a higher amount of silicon, but hardly increases the
strength of the
steel. A lower upper limit of aluminium improves the castability of the steel.
The amount of element B is preferably between 0.0011 and 0.0040 %, more
preferably between 0.0013 and 0.0030 %. to provide the desired hardenability
and
hence bring sufficient strength.
The amount of Ti is preferably max 0.10 % so as to limit the cost of the steel
and
keep the dimensional window as large as possible. More preferably, the amount
of Ti is
:
=
AMENDED SHEET 07102/20173
= ==-
CA 02989710 2017-12-15
1E0610 0:SW95/2011j [DESCPAIVIP) PCT/EP 101b/0b4
FP20.1,6064704) '
- 7 -
between 0.005 and 0.05 %.
Preferably the hot dip galvanised steel strip has an ultimate tensile strength
Rm
above 750 MPa and/or a 0.2 % proof strength Rp of 430 - 700 MPa, preferably
the
difference between the middle and the edges of the steel strip being less then
75 MPa
for both Rp and/or Rm, more preferably this difference being less then 60 MPa.
These
strength levels can be achieved with the composition according to the
invention.
According to a preferred embodiment the hot dip galvanised steel strip has a
microstructure, consisting of 20-50 volume % ferrite, 10-25 volume % retained
austenite + martensite, of which 5-12% retained austenite, the remainder being
io tempered martensite, bainite and cementite.
According to a second aspect of the invention there is provided a method for
producing a high strength hot dip galvanised steel strip as defined above,
wherein the
cast steel is hot rolled to a thickness of 2.0 - 4.0 mm and coiled at a
Coiling
Temperature CT below Bs-20 C temperature and above Ms+60 C temperature, the
= 15 strip is cold rolled with a reduction of 40 % or more, after
'which the strip is
intercritically annealed at a temperature between Acl and Ac3 temperature, and
the
strip. is overaged at a temperature below Bs temperature to form bainite
and/or tempered
martensite, after which the strip is hot dip galvanised.
Due to the coiling at a Coiling Temperature below Bs-20 C temperature and
20 above Ms+60 C temperature, a well-defined microstructure is achieved,
that can be
cold rolled with the right reduction and annealed at the right teMperatures
and
galvanised afterward, to get a galvanised steel strip with the right strength
and the right
properties.
Preferably the hot rolled coil has a microstructure consisting of 50 - 70
volume
25 % ferrite, 20 - 50 volume % pearlite and/or bainite, and less then 10 %
cementite. With
such a microstructure the coil has the right properties for further
processing, especially
for the annealing step, and can be cold rolled in a wide dimensional window.
According to a preferred embodiment the hot dip galvanised strip is tension
rolled
with a reduction of 0.2 - 0.8 %. This percentage of tension rolling can
provide the right
30 mechanical properties to the strip, such as the right strength level,
while the other
properties remain inside the desired= window. =
2 AMENDED SHEET
[17/02/201.7)
CA 02989710 2017-12-15
[Priri.10:,01.0501:4 IQESCPW)) I /tr zulb/u
- 8 -
According to a further aspect of the invention there is provided a method for
producing a high strength hot dip galvanised complex phase steel strip
according to the
first .aspect of the invention, wherein the cast steel is hot rolled to a
thickness of 2.0 -
4.0 mm and coiled at a Coiling Temperature CT below Bs-20 C temperature and
above
Ms+60 C temperature, the strip is cold rolled with a reduction of 40% or
more, after
which the strip is annealed at a temperature above Acl temperature plus 50 C,
and the
strip is overaged at a temperature below Bs temperature to form bainite and/or
tempered
martensite, after which the strip is hot dip galvanised. This complex phase
steel strip
can be Made due to the precise Coiling Temperature and the prescribed
annealing and
overaging temperatures.
Preferably, this hot dip galvanised complex phase steel strip is tension
rolled with
a reduction of 0.4 - 2.0 %, preferably With a redtiction of 0.4 - 1.2 %. This
percentage of
tension rolling can provide the right mechanical properties to the strip, such
as the right
strength level, while the other properties remain inside the desired window.
The invention will be elucidated hereafter.
Figure 1 shows measurement of the ultimate tensile strength Rm and 0.2% proof
strength Rp after annealing.
Line trials were performed, as shown in Table 1, indicated in milli-wt% unless
indicated otherwise.
=
Table I: Compositions of line trialled materials. Unless indicated different,
the compositions are defined in mill-wt%. Bs and Ms values were calculated
from [1],.
B
Al_ Bs
Cast C Mn P S Cr PPm zo PPm Ms
H
Ti N Al+Si Mn+Cr -20 C +60 C:
i
= , =
1 151...2101 12) 58 . , 20 : 682__22; 43
740 2200 581 477
_
2 161 2061' 101 120 206 20 660 21: 40 : 780 : 2267 573
472
3 147 2046 110 130 204 20 610 20. 46 740 2250 ; 578
479
,
4 149 2057 _10 392, 26 24 602 6 30 994 2083 583
476
5 146 2071. 12i 100 :184 19 690 21 48 790 : 2255 578
478
6 153 2093 9 102:: 204 19 685 22 47 787 2297 573
475
, _______________________________
I S.M.C. van Bohemen, Bainite and martensite Start Temperature calculated with
exponential carbon
dependence, Materials Science and Technology 28, 4 (2012) 487-495.
71020171
. .
AMENDED SHEET
CA 02989710 2017-12-15
[Print0; El/95/291A QPiNM PCT/ EP 20 lb /0h4
taP20.1 6064704,
- 9 -
Casts number 1, 2, 3, 5 and 6 were hot rolled with a hot roll finishing
temperature of
approximately 875 C. Cast number 4 was hot rolled with a hot roll finishing
temperature of approximately 950 C.
Table2: typical martensite + austenite distribution over different positions
in the coil.
Cast 6 Average martenSite + austenite
=
Head M 14.3
,
Head R 19.6
. . =
. Middle M 18.
Middle It - 18.9 '
,
Tail M 17.7
Tail R 18.3
Typically the material was hot and subsequently cold rolled to a typical gauge
in
the range 0.8-2.0 mm.
For casts 5 and 6 microstructure and phase fractions defined is provided in
Figure
to 2. For cast 6 the microstructure over different sampling positions over
.the coil is
provided in Table 2. The microstructure is given for the head, middle and tail
of a coil.
M indicates the middle of the ship, R the right hand side.
Using casts 1 to 4 and 6, a Dual Phase steel strip was produced. The hot roll
finishing temperature was approximately 875 C for all casts but for cast 4,
as indicated
above. The coiling temperature was between 500-520 C, well between Bs-20 C
and
Ms + 60 C. Subsequently, the material was cold rolled and intercritically
annealed at
around 800 C, and the overage temperature was 400 'V. After hot dip
galvanising, the
strip was temper rolled with a reduction of around 0.3%.
Using cast 5, a Complex Phase steel strip was produced. The hot roll finishing
temperature was approximately 875 C, the coiling temperature was 550 C. The
material was cold rolled and intercritically annealed at around 840 C, and
the overage
temperature was 400 C. After hot dip galvanising, the strip was temper rolled
with a
reduction of 1.0%
For cast 1 to 6, the mechanical properties were measured, depending on the
AMENDED SHEET
tl 7 1(02011:1
CA 02989710 2017-12-15
printpc1;_08/950171 EDES.C.P../MAD) K. I MI' ZU1b/Ub4
/
tEP?pleo 6g zog)
- o -
coiling temperature. These mechanical properties are shown in Table 3.
Table 3: Tensile properties at mid coil and HEC measurements.
.-, .0
g
0
=-= g
u 0.0
00 IiJ
a- g
1 520 450 782 14.5 21.7 0.15 0.76 28-32* 137 *
1 '498 490 817 14.0 21.0 0.14 0.54
2 506 505 808 13.5 20.4 0.13 0.69 18-30* 100-110 *
3 506 497 811 13.2 22.1 0.13 0.72
4 500 494 815 14.5 21.1 0.15 0.74
550 622 869 = 9.5 13.6 <0.1 0.70 33-36*
I 6 520 475 837 13.1 18.6
*: typical values
5
Of the casts shown in Table 1, several slabs were produced. These slabs were
hot
rolled to strips having a thickness of 2.5-3 mm and thereafter the strips were
coiled at
different coiling temperatures (CT) between 500 and 590 C. These coils were
cold
rolled to a thickness of 1.3 mm, continuous annealed and hot dip galvanised.
Measurement of the ultimate tensile strength Rm and 0.2% proof strength Rp
after annealing show that Rp and Rm increase when the CT is lower. This is
shown in
Figure 1. The measurements also show that the elongation becomes slightly
lower when
=
CT is lower, but the elongation remains satisfactory at low CT.
Figure 2 shows typical microstructures (Nital etched) obtained at the middle
of
the hot rolled strip product, after coiling and cooling down. Used is the
composition of
= cast 1. For the left-hand picture, the coiling temperature CT was 500 C.
The right-hand
picture shows the same material but after a Coiling temperature of 550 C. The
dark
phase is perlite/bainite and the light phase is ferrite; black dots are
cementite. In the left-
hand picture, perlite/bainite is present in 25-35%, ferrite 60-70% and
cementite less
than 10%. In the right-hand picture, perlite/bainite is present in 20-30%,
ferrite 65-75%
and cementite less than 10%.
Measurements have also shown that at low coiling temperatures the tensile
- = 117/..02/20,1
. AMENDED SHEET
CA 02989710 2017-12-15
Print.Pd: 0/05/20J IDESCe FL 1 / U 10 / UCILI
/
EP20-160647.04i
- 11 -
properties over the width of strip are improved, meaning the difference
between the
middle of the strip and the edges of the strip are small. The difference is
now at most
approximately 50 MPa for Rp and Rm, whereas it used to be approximately 100
MPa.
Figure 3 shows the variation in Rm and Rp over the width of the strip. Figure
3a
shows this variation for a strip composition that is not according to the
inyention,
having a composition of 0.15 C, 2.05 Mn, 0.2 Cr, 0.7 Al, 0.07 Si, 0.015 Nb and
0.004 N
(in wt%). The difference in Rm between middle and edge of the strip is
approximately
100 MPa, the difference in Rp is approximately 50 MPa.
Figure 3b shows the variation in Rm and Rp for a strip with the composition of
cast 1. This figure shows that it is possible to get a variation between the
middle and the
edge of the strip that is less than 20 MPa for both Rm and Rp. Figure 3c shows
in fact
the same for a strip with the composition of cast 3. The strip shown in
figures 3b and 3c
has been manufactured in accordance with the method of the invention.
Figure 4 shows three different ways of graphically indicating the
microstructures
of the casts after using the method of the invention. These are the well known
Picral,
Nital and LePera representations. In the Picral graphs black represents
bainite or
tempered martensite. In the Nital graph the white spots indicate ferrite. In
contrast, in
the LePera graph white indicates (tempered) martensite + retained austenite.
The
differences between DP800 on the left-hand side and CP800 on the right-hand
side are
clearly visible.
The length indications in Figures 2 and 4 all indicate a length of 10 p.m.
AMENDCD=SHEET
0.7./.02/29.17i
