U.S. patent application number 10/380792 was filed with the patent office on 2004-02-12 for method for manufacturing a steel strip or sheet consisting mainly of mn-austenite.
Invention is credited to Bruckner, Gabriele, Krautschick, Hans-Joachim, Schlump, Wolfgang.
Application Number | 20040025979 10/380792 |
Document ID | / |
Family ID | 7656678 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040025979 |
Kind Code |
A1 |
Bruckner, Gabriele ; et
al. |
February 12, 2004 |
Method for manufacturing a steel strip or sheet consisting mainly
of mn-austenite
Abstract
The method according to the invention can be used for the
economic manufacture of a steel strip (W) or sheet consisting
mainly of Mn-austenite which possesses enhanced strength compared
with the prior art. For this purpose a steel is melted which
contains at least the following alloying components (in wt. %),
15.00-24.00% Cr, 5.00-12.00% Mn, 0.10-0.60% N, 0.01-0.2% C, max.
3.00% Al and/or Si, max. 0.07% P, max. 0.05% S, max. 0.5% Nb, max.
0.5% V, max. 3.0% Ni, max. 5.0% Mo, max. 2.0% Cu as well as iron
and unavoidable impurities as the remainder. This steel is cast
into a thin strip (D) having a maximum thickness of 10 mm in a
casting gap formed between two rotating rollers (2, 3) or rolls.
The rollers (2, 3) or rolls are cooled so intensively that the thin
strip (D) in the casting gap (4) is cooled at a cooling rate of at
least 200 K/s.
Inventors: |
Bruckner, Gabriele; (Essen,
DE) ; Schlump, Wolfgang; (Essen, DE) ;
Krautschick, Hans-Joachim; (Solingen, DE) |
Correspondence
Address: |
Charles Guttman
Proskauer Rose
1585 Broadway
New York
NY
10036
US
|
Family ID: |
7656678 |
Appl. No.: |
10/380792 |
Filed: |
September 16, 2003 |
PCT Filed: |
September 14, 2001 |
PCT NO: |
PCT/EP01/10645 |
Current U.S.
Class: |
148/542 ;
420/59 |
Current CPC
Class: |
C22C 38/001 20130101;
B22D 11/0622 20130101; C22C 38/06 20130101; C21D 8/0215 20130101;
C21D 8/0205 20130101; C22C 38/38 20130101 |
Class at
Publication: |
148/542 ;
420/59 |
International
Class: |
C22C 038/38 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 19, 2000 |
DE |
100 46 181.6 |
Claims
1. Method for manufacturing a steel strip (W) or sheet consisting
mainly of Mn-austenite, in which a steel is melted which contains
the following alloying constituents (in wt. %):
4 15.00-24.00% Cr, 5.00-12.00% Mn, 0.10-0.60% N, 0.01-0.2% C, max.
3.00% Al and/or Si, max. 0.07% P, max. 0.05% S, max. 0.5% Nb, max.
0.5% V, max. 3.0% Ni, max. 5.0% Mo, max. 2.0% Cu, and iron and
unavoidable impurities as the remainder,
and in which the steel is cast in a casting gap formed between two
rotating rollers (2, 3) or rolls into a thin strip (D) having a
maximum thickness of 10 mm, whereby the rollers (2, 3) or rolls are
cooled so intensively that the thin strip (D) in the casting gap
(4) is cooled at a cooling rate of at least 200 K/s.
2. Method according to claim 1, characterised in that the thickness
of the thin strip (D) is 1 to 5 mm.
3. Method according to one of the preceding claims, characterised
in that the steel contains 17.00-21.00 wt. % Cr.
4. Method according to one of the preceding claims, characterised
in that the steel contains 8.00-12.00 wt. % Mn.
5. Method according to one of the preceding claims, characterised
in that the steel contains 0.40-0.60 wt. % N.
6. Method according to one of the preceding claims, characterised
in that the steel additionally contains Ni, Mo and/or Cu.
7. Method according to one of the preceding claims, characterised
in that the casting of the thin strip (D) takes place in a
protective gas atmosphere.
8. Method according to one of the preceding claims, characterised
in that following the casting, the thin strip (D) is continuously
hot-rolled to give a hot strip (W).
9. Method according to claim 8, characterised in that before hot
rolling the thin strip (D) is heated to an initial rolling
temperature.
10. Method according to claim 9, characterised in that the heating
takes place under protective gas.
11. Method according to one of claims 8 to 10, characterised in
that after hot rolling the hot strip (W) is subjected to a heat
treatment.
12. Use of a steel strip manufactured according to one of claims 1
to 11 as material for automobile-body sheet-metal parts.
13. Use of a steel strip manufactured according to one of claims 1
to 11 as material for stiffening structural components.
14. Use of a steel strip manufactured according to one of claims 1
to 11 as material for landing-gear or chassis parts.
15. Use of a steel strip manufactured according to one of claims 1
to 11 as material for vehicle wheels.
16. Use of a steel strip manufactured according to one of claims 1
to 11 as material for fuel tanks.
Description
[0001] The invention relates to a method of manufacturing a steel
strip or sheet consisting mainly of Mn-austenite. Steels suitable
for manufacturing these products are assigned to AISI 200 and bear
the designation S20100 to S24000. Steel materials of this type are
distinguished by a high strength which is conserved after welding
even in the region of the weld seam.
[0002] These good strength properties are achieved by interstitial
and substitutional mixed crystal hardening. Carbon and nitrogen are
particularly effective in this respect. Higher carbon contents are
avoided however because of the undesirable carbide formation. Thus,
nitrogen is preferentially used for interstitial mixed crystal
hardening in steels of the type in question. However, the
production of steels having an elevated nitrogen content is
expensive in relation to the alloying constituents or the apparatus
required for the production.
[0003] In a known method for producing steels having higher
nitrogen contents the melt is molten under the application of a
compressive load. The pressure acting on the melt in this case is
so far above the nitrogen partial pressure that the nitrogen in the
appropriate steel goes into solution. The advantage of this
procedure is that steels having higher nitrogen contents can be
produced without adding particular quantities of other alloying
elements. A disadvantage however is the high expenditure on
apparatus required for this.
[0004] An alternative method for dissolving the nitrogen by
applying a compressive load during melting involves increasing the
solubility of the melt itself. This can be achieved by high
contents of chromium and manganese. A description of the properties
of steels having corresponding compositions compiled by M. du Toit
can currently be found on the internet at
"www.tecnet.co.za/mags/steel/feature1.htm". The known steels can be
melted and cast conventionally without applying any compressive
load, but not in continuous casting. Casting of known steels thus
incurs high costs.
[0005] A further increase in the strength of conventionally
castable steels of the type described previously can be achieved by
alloying with aluminium and/or silicon. These two elements support
the mixed crystal hardening and thus lead to a further increase in
strength. Furthermore, the addition of aluminium and silicon can
influence the stacking fault energy which again influences the
deformation processes.
[0006] Thus, the addition of aluminium leads to an increase in the
stacking fault energy and favours deformation by twinning. Silicon
however, reduces the stacking fault energy but favours deformation
by martensite formation. As a result of the combined addition of
silicon and aluminium the strengthening of the material during
deformation can thereby be specifically influenced. The formation
of martensite leads to high strengthening whereas the strengthening
is reduced by twinning.
[0007] The advantages of adding amounts of aluminium and silicon to
steels of the type in question are offset by the disadvantage that
they are ferrite formers and favour primary ferritic
solidification. The resulting ferrite only has a low solubility for
nitrogen.
[0008] Consequently the nitrogen is eliminated in the form of gas
bubbles during the solidification. In order to achieve a
high-strength austenitic steel whilst retaining the increased
nitrogen content however, the austenite must thus be stabilised. In
addition to increasing the raw material costs, the further
increased manganese contents required for this give rise to
appreciable problems in the production of such high-manganese
steels in steelworks.
[0009] The problem for the invention is thus to provide a method of
manufacturing a steel consisting mainly of Mn-austenite which can
be manufactured economically and at the same time exhibits
increased strength compared with the prior art.
[0010] The problem is solved by a method for manufacturing a steel
strip or sheet consisting mainly of Mn-austenite in which a steel
is melted which contains the following alloying constituents (in
wt. %):
1 15.00-24.00% Cr, 5.00-12.00% Mn, 0.10-0.60% N, 0.01-0.2% C, max.
3.00% Al and/or Si, max. 0.07% P, max. 0.05% S, max. 0.5% Nb, max.
0.5% V, max. 3.0% Ni, max. 5.0% Mo, max. 2.0% Cu, and iron and
unavoidable impurities as the remainder,
[0011] and in which the steel is cast into a thin strip having a
maximum thickness of 10 mm in a casting gap formed between two
rotating rollers or rolls, whereby the rollers or rolls are cooled
so intensively that the thin strip in the casting gap is cooled at
a cooling rate of at least 200 K/s. The thickness of the thin strip
is preferably between 1 and 5 mm. Naturally, the details of the
steel composition used according to the invention also include such
alloys for which the content of these alloying elements is zero for
which only a maximum permissible upper limit of the content is
given.
[0012] According to further refinements of the invention, the
chromium content of the steel can be limited to 17.00-21.00 wt. %
Cr, the manganese content can be limited to 8.00-12.00 wt. % Mn
and/or the nitrogen content can be limited to 0.40-0.60 wt. % N. In
addition, contents of Ni, Mo and/or Cu can be present in the
steel.
[0013] The contents of the alloying elements contained in the steel
composition used according to the invention are optimised in each
case in terms of the action of these elements. Thus, Cr, Mn, Mo, V,
Nb and Al increase the nitrogen solubility in the melt whereas Ni
and Cu, being austenite formers, and Si reduce the nitrogen
solubility. As mentioned, Si also acts as a mixed crystal hardener.
In addition, it is also used for grain refinement and lowers the
stacking fault energy. Aluminium on the other hand increases the
stacking fault energy. Molybdenum also acts as a mixed crystal
hardener and improves the corrosion behaviour. Vanadium also has a
grain-refining action and enhances the strength. The addition of Nb
leads to an increase in strength by precipitation hardening.
[0014] The invention makes use of the fundamentally known technique
of a strip casting plant where the steel is cast in the casting gap
formed between the rollers or rolls of, for example, a
double-roller casting apparatus, and is cooled so intensively that
there is a shift from primary ferritic towards primary austenitic
solidification. This makes it possible to transfer the nitrogen
dissolved in the melt into the steel since the austenite possesses
a high solubility for nitrogen. Such intensive cooling is only made
possible by casting a thin strip in a casting gap whose walls
formed by the casting rolls or rollers move essentially at the same
speed as the cast strip so that a continuous intensive heat
exchange is ensured between the walls (casting roll/roller) and the
cast steel in the casting gap.
[0015] The intensive cooling taking place at a high cooling rate
ensures that nitrogen gas bubbles possibly forming in the
solidifying melt remain small and the pressure directed towards
them is high. This prevents any nitrogen outgassing in the course
of the solidification. In addition, such an escape of nitrogen is
also suppressed by the high ferrostatic pressure which occurs as a
result of the large height of the melt pool in the casting gap. In
this way it is ensured that the pressure P.sub.N in any forming
nitrogen gas bubbles is always lower than the sum of the ambient
pressure P.sub.A, the ferrostatic pressure P.sub.F and twice the
surface tension a of the gas bubbles relative to the bubble radius
r (i.e. P.sub.N<P.sub.A+P.sub.F+2.sigma./r).
[0016] The rapid solidification of the cast strip during strip
casting thus offers great freedom in terms of the choice of steel
composition especially in connection with steels of the type used
according to the invention. As explained, as a result of the rapid
solidification larger quantities of nitrogen can be dissolved.
Alloying elements which improve the material properties can thus be
added in larger quantities than in the conventional method of
manufacture without regard to their possible negative influence on
the nitrogen solubility. For example, if the steel contains higher
quantities of Si, the risk of nitrogen outgassing present in
conventional manufacture as a result of the slow solidification and
the associated increased ferrite formation is eliminated in the
method according to the invention. Also in the case of increased Al
contents the formation of AlN which occurs during slower cooling is
avoided by the rapid cooling provided according to the invention.
Thus, without regard to the harmful influences caused by slow
cooling, the invention allows the deformation mechanism of each
alloy used to be specifically adjusted by a suitable choice of Al
and Si content so that an end product having optimised properties
is obtained.
[0017] The cost advantage achieved by the invention in the
processing of steels of the type used according to the invention
which are inherently difficult to deform is quite considerable.
This applies both to those steels containing up to 7.5 wt. % Mn
which can be cast by conventional continuous casting and also to
those containing more than 7.5 wt. % Mn which conventionally can
only be cast by block casting and then rolled to the desired end
thickness by several passes with reheating if necessary.
[0018] At the present time hot strip made of continuously castable
alloy can only be manufactured with minimum thicknesses of 3.5 mm
in a conventional hot wide-strip mill. The production of cold strip
having target thicknesses of 0.8-1.2 mm is only feasible by
intermediate annealing. In the method involving strip casting
according to the invention intermediate annealing is no longer
necessary however because of the smaller thickness of the hot strip
obtained. Since a thin strip having final thicknesses between 1 and
3 mm can be produced by the strip casting provided by the
invention, in many cases it is also possible to adjust the final
thickness of the strip produced so that cold rolling can be
dispensed with completely. In this way the problems caused by the
low deformability of Mn-austenites in the conventional method of
manufacture can be avoided.
[0019] The method according to the invention can be used to produce
steel strip and sheet having particularly high nitrogen contents of
0.4 to 0.6 wt. % and alloyed with up to 3% aluminium and/or silicon
without the steel production needing to take place under excess
pressure or particularly high manganese contents being required.
The steel products thus produced possess a fine-grained isotropic
structure with slight macro-segregation or a small number of coarse
inclusions. As a result of their Al and/or Si content, these
products also exhibit an enhanced strength and ductility compared
with the prior art. For a steel strip or sheet produced according
to the invention the strengthening and thus the energy absorption
during deformation can be specifically adjusted by the choice of
alloy.
[0020] Casting of the thin strip preferably takes place in a
protective gas atmosphere. As a result of casting in a protective
gas atmosphere it is easy to produce a thin strip having a modified
surface whose degree of oxidation can be specifically influenced.
In this way scale formation can be avoided.
[0021] The strip thus produced can then be hot-rolled "in-line" in
a roll stand without the risk of the rollers sticking. It is
particularly advantageous in this respect if the thin strip is
heated to an initial rolling temperature before hot rolling. As a
result of this increase in temperature, higher degrees of
deformation can be achieved during hot rolling.
[0022] By subjecting the hot strip to heat treatment after the hot
rolling its structure can be specifically optimised. The heat
treatment can comprise annealing followed by controlled
cooling.
[0023] As a result of its spectrum of properties, steel sheet
produced according to the invention is especially suitable for the
manufacture of automobile-body sheet metal parts, stiffening
structural components used particularly in general vehicle building
and especially in automobile building, landing-gear or chassis
parts, vehicle wheels and fuel tanks. In all these applications the
especially good strength properties of the steel sheet produced by
the method according to the invention have an advantageous effect.
In addition, the good corrosion resistance of the steel sheet and
strip according to the invention is advantageous in such
applications where they come in contact with aggressive media, such
as fuels for example.
[0024] The invention is subsequently explained in greater detail
with reference to a drawing showing an example of embodiment.
[0025] The FIGURE shows a schematic diagram of a strip casting
plant 1. In this plant for example, a steel is processed which in
addition to the usual unavoidable impurities contains (in wt. %)
0.08% C, 0.5% Si, 10% Mn, 19% Cr, 0.5% N, 0.3% Al and the remainder
is iron.
[0026] The strip casting plant 1 comprises a double-roller casting
apparatus called a "double roller" of which the rollers 2, 3 each
rotating in opposite directions about an axis of rotation are shown
in the FIGURE. Between the rollers 2, 3 there is formed a casting
gap 4 which is continuously filled with melt so that a melt pool S
forms above the casting gap 4.
[0027] The rollers 2, 3 are intensively cooled during the casting
process by cooling devices not shown so that the melt entering the
casting gap 4 solidifies primarily austenitically at cooling rates
higher than 200 K/s and leaves the casting gap 4 as a thin strip D
having a thickness of 1 to 5 mm. The thin strip D thus produced
then passes through a furnace 5 in which it is heated to an initial
rolling temperature.
[0028] Both the double-roller casting device with the rollers 2, 3
and the furnace 5 are accommodated in a housing 6 which contains a
protective gas atmosphere. As a result of casting the thin strip D
and re-heating it in the furnace 5 in a protective gas atmosphere
the formation of scale on the surface of the thin strip D is
largely avoided.
[0029] The thin strip D heated to the initial rolling temperature
enter a roll mill 7 in which it is hot-rolled to a final size. As a
result of the high initial rolling temperature high degrees of
deformation are possible. The hot strip W rolled from the thin
strip D entering the roll mill essentially scale-free exhibits a
particularly high-quality surface after the hot rolling.
[0030] After the hot rolling in the roll mill 7 the hot strip W is
annealed in a continuous annealing furnace 8 and then cooled in a
controlled fashion under a cooling device 9 in order to
specifically improve its structure. The hot strip W thus
heat-treated is then coiled to form a coil 10.
[0031] Steel strip produced in the manner described previously
exhibits particularly high strength accompanied by good
deformability and equally good energy absorption capacity compared
with steel strips having the convention composition and produced by
conventional methods as a result of the high nitrogen content
achieved by the rapid cooling between the rollers 2, 3 of the
double-roller casting apparatus.
[0032] The following table compares the superior strength values of
the hot strip W produced in the casting roller plant 1 according to
the invention with the strength values of Mn austenite steels
produced conventionally by continuous casting.
2 R.sub.P0.2 Rm A80 [Mpa] [MPa] [%] Invention 550-650 850-900 35-45
Conventional 420 750-800 50
[0033]
3 SYMBOLS 1 Casting roller plant 2, 3 Rollers 4 Casting gap 5
Furnace 6 Housing 7 Roll mill 8 Continuous annealing furnace 9
Cooling device 10 Reel D Thin strip W Hot strip S Melt pool
* * * * *