MULTI-PHASE STEEL, PRODUCTION OF ROLLED PRODUCTS AND USE OF THE STEEL

ACIER POLYPHASE, PRODUCTION DE PRODUITS LAMINES ET UTILISATION DUDIT ACIER

English Abstract

The invention concerns a multiphase steel, a process for producing rolled
products from this steel with an up to 70 vol % polygonal-ferritic structure,
and its use. The steel should have high strength, good cold-working properties
and improved surface quality after the final hot-working stage.

French Abstract

L'invention concerne un acier polyphasé, un procédé de production de produits laminés à partir dudit acier comportant jusqu'à 70 % en volume de structure polygonale-ferritique, ainsi que son utilisation. L'acier doit être très résistant, présenter une bonne aptitude au façonnage à froid et une meilleure qualité de surface après la dernière étape de déformation à chaud.

Claims

CLAIMS
1. A multi-phase steel, comprising (in % by mass)
0.12 to 0.3% carbon
1.2 to 3.5% manganese
1.1 to 2.2% aluminium
less than 0.2% silicon
the remainder being iron, including unavoidable
impurities, including phosphorus and sulphur,
with a perlite-free structure comprising up to 70% by
volume of soft polygonal ferrite and the remainder
being bainitic ferrite and more than 4% by volume of
carbon-enriched residual austenite as well as if applicable,
smaller percentages of carbon-enriched martensite which
contains aluminium in a quantity in% by mass of
Al < 7.6 ~ Cequ.- 0.36
with a carbon equivalent (Caqu.) of
0.2 ~ Cequ. =% C+1/20% Mn+ 1/20% Cr + 1/15% Mo ~ 0.325.
2. A multi-phase steel according to claim 1,
characterised by a residual austenite content of up to
20% by volume.
3. A process for producing rolled product from a
multi-phase steel composed according to claim 1 or 2, of
high strength, good tenacity, good surface quality in
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hot-rolled condition and good ability to be cold-rolled,
with a perlite-free structure comprising up to 70% by
volume polygonal ferrite with the remainder being
bainitic ferrite and more than 4% by volume of
carbon-enriched residual austenite as well as if applicable, in
addition, smaller percentages of carbon-enriched
martensite; the said steel being continuous-cast and
hot-rolled at a hot roll initial temperature exceeding 1000°C
and a hot-roll end-temperature (ET) in the range of
Ar3 - 50°C < ET < Ar3 + 100°C,
and subsequently cooled down from the hot-roll end
temperature (ET) at a rate of 15 to 70 K/s to a reeling
temperature in the range from 200 to 500°C and reeled up.
4. A process according to claim 3,
characterised in that one or several of the following are
added to the steel by alloying (in % by mass):
up to 0.05% titanium
up to 0.8% chromium
up to 0.5% molybdenum
up to 0.5% copper
up to 0.8% nickel.
5. A process according to claim 3,
characterised in that in the temperature range between
Ar3 to Ar3 - 200°C, a cooling pause of 2 to 30 s is
observed, during which the cooling rate is below 15 K/s.
6. The use of a steel with a composition according to
claim 1, 2 or 4 as a material for the production of
cold-reduced structural elements for motor vehicles, such as
floor stiffener elements, transverse links, or for wheel
disks.
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Description

CA 02224813 1997-12-16
WE/wa 96043
15 February 1996
Multi-pha5~ steel, production Or rolled products and ~se
o~ the 8tel3l
The invention concerns a ~ulti-phase steel, a process for
producing rolled products from this steel with up to ~0~
by volume of polygonal-ferritic structure, as ~ell as the
use of the said s~eel. The steel should be of high
strength and have a good ability to be cold-reduced a~
well as have an improved surface quality after hot-
forming in the last production stage.
Dual-phase steels are known which have a structure, e.g.
of up to 80~ by volume of polygonal, relatively soft,
ferrite with the remainder being carbon-rich martensite.
The second phase, of lesser quantity, which is rich in
carbon, is embedded in the proeutectic ferritic phase, in
the shape of an island. Such a steel has good mechanical
properties and a good ability to be cold-reduced.
Known steels with predominantly polygonal ferrite in the
struct~re as well as martensite embedded therein,
comprise (in% by mass) 0.03 to 0.12~ C, up to 0.8~ Si and
0.8 to 1.7% Mn (De 29 24 340 C2) or 0.02 to 0.2~ C, 0.05
to 2.0~ Si, 0.5 to 2~ Mn, 0.3 to 1.5~ Cr as well as up to
1~ Cu, Ni a~d Mo (~P 0 072 867 sl). Both steels are
killed by aluminium and have soluble residual con~ents of
less than 0.1~ Al. Silicon in these steels promotes
ferrite tran~formation. In combination with manganese and
if applicable chromium, perlite formation is suppressed
In this way adequate enrichment of carbon in the second
phase is ensured and the formation of polygonal ferrite

CA 02224813 1997-12-16
in predomln~nt relation to the second phase is attained.
However, these known alloys have the disadvantage that
during hot rolling an inhomogenous surface structure
forms which becomes evident from patterns of red scaLe.
After pickling, surface unevenne~s remains. For many
applications, such material i~ not saleable. So far
improvement of the surface quality o~ these hot-rolled
steels has not been successful. Therefore these steels
are not useable for certain purposes such as cold-reduced
wheel disks for motor vehicles or other products made by
cold-reduction such as cold-reduced construction profile~
and similar. In addition, steels of this type with a
predominant quantity of relatively soft polygonal ferrite
in the structure only attain tensile strengths up to 700
N/mm2. Thus weight reduction which maintains a linear
relationship to strength, is severely limited.
It is thus the object of the invention to develop a steel
which at least maintains the outstanding spectrum of the
mechanical properties of known steels, has greater
streng~h ~han the known dual-phase steels, can be cold-
reduced as well as these, but after production by hot-
forming provides a better surface structure than these
steels.
To meet this objective, a multi-phase steel is proposed,
comprising (in% by mass)
0.12 to 0.3~ carbon
1.2 to 3.5~ manganese
1.1 to 2.2~ aluminium
less than 0.2% silicon
the remainder being iron, including unavoidable
impurities such as phosphorus and sulphur

CA 02224813 1997-12-16
~ith a perlite-free structure of le~s than 70~ by volume
of soft polygonal ferrite and the remainder being
bainitic ferrite and more than 4~ by volu~e, preferably
up to 20~ by volume, of carbon-enriched residual
austenite as well as if applicable, smaller percentages
of carbon-enriched martensite which contains aluminium in
a quantity in% by mass of
Al ~ 7.6 Cequ. - 0.36
with a carbon equivalent (Ce~.) of
0.2 ~ Ce~" -% C ~ 1/20% Mn + 1/20~6 Cr ~ l/1596 Mo ~ 0.325.
Such a steel surpasses the product Rm . A5 of known
silicon-alloyed dual-phase steels and after completion of
hot-forming, its surface quality is improved, as is for
example required for wheel disks for motor ~ehicles,
which disks are produced by cold-reducing the hot-rolled
steel. In addition, the following elements up to the
percentages indicated ~in% by mass) can be added to the
steel by alloying:
up to 0.05~ titanium
up to 0.8~ chromium
up to 0.5% molybdenum
up to 0.8~ copper
Up to O.5~s nickel.
Such a steel, all~yed with aluminium instead of with
silicon, attains a ductile yield Rm ~ As > 18~000 N/mm2 %,
i.e. a ductile yield of A5 > 18,000/ Rm in ~ at a value o~
tensile strength Rm up to 900 N/mm2.

CA 02224813 1997-12-16
The steel according to the invention is characterised by
an aluminium content of 1.1 - 2.2% which is significan~ly
higher than that of known steels. Instead, according to
the invention, the-silicon content is limited to le~s
than 0.2~.
By contrast, known steels of this type usually require
silicon contents in excess of 0.5%. The steel alloyed
with aluminium, according to the invention, compri~es the
multi-phase mlcrostructure with residual austenite, as
described, and has excellent mechanical strength
characteristics. Above all, the surface quality of the
hot-formed product after the last hot-forming stage is
significantly improved when compared to hitherto-known
silicon-alloy steels. There is a more pronounced delay in
perlite formation, when compared with known steels:
perlite formation can be safely avoided by observing the
claimed process parameters.
At 0.12 to 0.3~, the carbon content is in the usual range
for steels of this type.
In order to a~oid perlite ~ormation, manganese is added
in percentages of 1.2 to 3.5~. ~anganese has a solid
solution hardening effect and increases the level of
strength. In view of perlite avoidance and the effect on
ferrite formation, carbon content and manganese content
are exchangeable within the limits of the carbon
equivalent. ~he carbon equivalent i5 determin~d ;~35
follows:
O . 2 ~ Ce~ =% C + 1/20~- Mn ~ 1~20~ Cr + 1/15~ Mo ' O . 325 .
According to the invention, the intersection of ~he
caxbon-equivalent value and the matching aluminium value

CA 02224813 1997-12-16
is to be within the shaded area shown in Fig. 1 in order
to ensure a ferrite content below 70~ by volume and a
residual austenite content exceeding 4% by volume.
Addition of titanium up to 0.05% ensures nitrogen setting
and avoids the formation of elongated manganese
sulphides.
Up to 0.8~ by mass of chromium can be added to improve
martensite tempering properties and to avoid perlite
formation.
Molybdenum, up to 0.5~ by mass increases the range of
successful cooling rates.
Copper and nickel, up to 0.5~ by mass each, can
contribute to lowering the transformation temperature and
to avoiding perlite.
In order to influence coalescence o~ sulphides, treatment
of the molten bath with calcium-silicon is advantageous.
The hot-rolling end-temperature ~T should be in the range
of
Ar3 - 50~C < ET < Ar3 + 100~C.
The Ar3 temperature which should be in the range of 750
to 950~C is calculated as follows
750~C ' Ar3 = 900 ~ 100~ Al - 60~ Mn - 300~ c ~ 950~C
Cooling down from hot-roll end-temperature to reeling
temperature which i~ bet~een 200 and 500~C takes place in

CA 02224813 1997-12-16
an accelerated way at a cooling-down speed of 15 to 70
E~/s .
Whe~ cooling down from the hot-roll end-temperature, in
the process according to the invention, it is possible in
the range of A~3 to Ar3 - 200~C, to further improve the
formation of polygonal ferrite by observing a cooling
pause of 2 to 30 s, during which the cooling rate is
~elow 15 K/s.
Fig. 2 shows a diagrammatic representation of hot-strip
production coupled with the cooling progression of the
steel according to the invention, at and after hot-
rolling.
It shows that any undesired entering of the perlite
region can safely be avoided if the conditions stated for
the hot-roll end-temperature, the cooling-down speed and
the reeling temperature, are observed.
A steel A according to the invention, with a ccmposi~ion
according to Table 1 was hot rolled to a final strip
thickness of 3.7 mm at a hot-roll end-temperature of
855~C.
Cooling from this temperature was at 30 K/s to the reel
tempera~ure (RT) of 415~C. The characteristics of this
steel A according to the in~ention were determined
according to DIN EN 10002 on flat-drawn specimens.
Table 2 shows the values for the apparent yield point,
tensile strength, elongation and the ratio of yield point
to tensile strength for the layers along and across the
~ire~t.ion of ro.l.llng.

CA 02224813 1997-12-16
By way o~ comparison, Table 2 also shows the respective
strength properties of a steel B, known from EP O 586 704
Al, with a composition as shown in Table 1.
Due to its spectrum of characteristics, the steel
according to the invention is particularly suitable for
the production of cold-reduced structural elements for
motor ~ehicles, such as floor stiffener elements,
transverse links, or for wheel disks.

Table 1
C~ Mn~ Si~ Cu-~O Ni9~ Cr~ P~ S% N%
A 0, 21G 1,3~ 1, B3 0, 06 0, 5 1 0,27 0,52 0,010 cO, 001 0,OOZ4
0,21 1,50 0,057 1,'16 <0,01 0,01 0,02 ~0,005 0,004 0,006 r
'~ * ~ccording to EP O 586 704 A1
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~ CA 02224813 1997-12-16
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