U.S. patent application number 13/978969 was filed with the patent office on 2014-01-09 for method for producing a hot-rolled flat steel product.
This patent application is currently assigned to THYSSENKRUPP STEEL EUROPE AG. The applicant listed for this patent is Evgeny Balichev, Jian Bian, Harald Hofmann. Invention is credited to Evgeny Balichev, Jian Bian, Harald Hofmann.
Application Number | 20140007992 13/978969 |
Document ID | / |
Family ID | 45315832 |
Filed Date | 2014-01-09 |
United States Patent
Application |
20140007992 |
Kind Code |
A1 |
Balichev; Evgeny ; et
al. |
January 9, 2014 |
Method for Producing a Hot-Rolled Flat Steel Product
Abstract
A method for generating a flat steel product comprising the
steps of: melting a steel melt, comprising, in addition to Fe and
unavoidable impurities (in wt %) C: 0.5-1.3%, Mn: 18-26%, Al:
5.9-11.5%, Si: <1%, Cr: <8%, Ni: <3%, Mo: <2%, N:
<0.1%, B: <0.1%, Cu: <5%, Nb: <1%, Ti: <1%, V:
<1%, Ca: <0.05%, Zr: <0.1%, P: <0.04%, S: <0.04%;
casting the steel melt into a cast strip; heating the cast strip to
an initial hot-rolling temperature of 1100-1300.degree. C. at a
heating rate of at least 20 K/s; hot rolling the cast strip into a
hot strip; cooling the hot strip within 10 seconds after the hot
rolling at a cooling rate of at least 100 K/s to <400.degree.
C.; and winding the cooled hot strip into a coil at a coiling
temperature of up to 400.degree. C.
Inventors: |
Balichev; Evgeny;
(Dusseldorf, DE) ; Bian; Jian; (Koeln, DE)
; Hofmann; Harald; (Dortmund, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Balichev; Evgeny
Bian; Jian
Hofmann; Harald |
Dusseldorf
Koeln
Dortmund |
|
DE
DE
DE |
|
|
Assignee: |
THYSSENKRUPP STEEL EUROPE
AG
Duisburg
DE
|
Family ID: |
45315832 |
Appl. No.: |
13/978969 |
Filed: |
December 14, 2011 |
PCT Filed: |
December 14, 2011 |
PCT NO: |
PCT/EP2011/072671 |
371 Date: |
September 24, 2013 |
Current U.S.
Class: |
148/546 |
Current CPC
Class: |
B22D 11/0622 20130101;
C21D 8/02 20130101; C22C 38/04 20130101; C21D 8/0226 20130101; B22D
11/1213 20130101; C22C 38/00 20130101; C22C 38/06 20130101; C21D
2211/001 20130101; C21D 8/005 20130101 |
Class at
Publication: |
148/546 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2011 |
DE |
10 2011 000 089.5 |
Claims
1. A method for producing a hot-rolled flat steel product
comprising the following working steps: melting a steel melt,
comprising, in addition to iron and unavoidable impurities (in wt
%) C: 0.5-1.3%, Mn: 18-26%, Al: 5.9-11.5%, Si: less than 1%, Cr:
less than 8%, Ni: less than 3%, Mo: less than 2%, N: less than
0.1%, B: less than 0.1%, Cu: less than 5%, Nb: less than 1%, Ti:
less than 1%, V: less than 1%, Ca: less than 0.05%, Zr: less than
0.1%, P: less than 0.04%, S: less than 0.04%, casting the steel
melt into a cast strip, heating the cast strip to an initial
hot-rolling temperature of 1100-1300.degree. C. at a heating rate
of at least 20 K/s, hot rolling the cast strip heated to the
initial hot-rolling temperature into a hot strip, cooling of the
hot strip, the cooling starting within 10 seconds after the hot
rolling at a cooling rate of at least 100 K/s to <400.degree.
C., winding the cooled hot strip into a coil at a coiling
temperature of up to 400.degree. C.
2. The method according to claim 1, wherein the steel melt contains
(in wt. %) 0.1-0.4% Si, <3.0% Cr, <1.0% Ni, <0.5% Mo,
0.005-0.04% N, <0.0050% B, <1% Cu, <0.2% Nb, <0.3% Ti,
<0.3% V, <0.005% Ca, <0.005% Zr, 0.01-0.03% P or
0.005-0.02% S.
3. The method according to claim 1, wherein the casting of the
steel melt into a cast strip is performed in a two roller casting
machine.
4. The method according to claim 1, wherein the thickness of the
cast strip is a maximum of 5 mm.
5. The method according to claim 1, wherein the heating to the
initial hot rolling temperature is performed by means of an
inductively operating heating device.
6. The method according to claim 1, wherein the initial hot rolling
temperature, to which the cast strip is heated, is at least
1150.degree. C.
7. The method according to claim 1, wherein a degree of deformation
achieved in the course of the hot rolling is at least 10%.
8. The method according to claim 1, wherein a final hot rolling
temperature of the hot rolling is 1000-1050.degree. C.
9. The method according to claim 1, wherein the hot rolling takes
place in a single pass.
10. The method according to claim 1, wherein the cooling of the hot
strip begins within 10 seconds of the end of hot rolling.
11. The method according to claim 1, wherein the working steps
performed prior to hot rolling are carried out under a protective
atmosphere.
12. The method according to claim 1, further comprising hot strip
annealing the hot strip obtained at an annealing temperature of
900-1150.degree. C.
13. The method according to claim 12, wherein the Al content of the
cast strip is at least 10 wt. %.
14. The method according to claim 1, further comprising
cold-rolling the hot strip into a cold strip.
15. The method according to claim 1, wherein a degree of
deformation achieved in the course of the hot rolling is 10-20%.
Description
[0001] The invention relates to a method for producing a hot-rolled
flat steel product from a high-strength, highly ductile manganese
steel, which in addition to a high Mn content has an Al content of
5.9-11.5 wt. %.
[0002] A steel of this kind and a method for the production thereof
are known from, for example, DE-AS 1 262 613. According to the
method described in that publication blocks with a low diameter are
cast from molten steel with a suitable composition, which are then
hot-rolled to form bar stock. Through heat treatment at
800-1250.degree. C. the elongation and the notched impact strength
of the material obtained in this way can be improved. From the
stock obtained in this way, components for aircraft, floors,
turbines, gears, valves and so on are to be made.
[0003] More recent developments have shown that the steels of the
kind indicated above, because of a very good combination of
characteristics of high strength, high deformability, a
significantly reduced density and an associated minimised weight
are particularly suited to flat products, thus to steel strips or
sheets, in particular to the manufacture of components for motor
vehicle manufacture, in particular the construction of car bodies
or chassis parts.
[0004] The problem here, however, is that the steels concerned,
because of their alloying state generated via conventional routes,
as normally applied to steels with a high carbon content, are
difficult to process. Thus the known steels have a high tendency
towards core segregations of Mn and Al during casting and
solidification. Furthermore with these there is an increased danger
that surface cracks will result during continuous casting and the
strand bending back while removing it from the casting mould.
Furthermore, because of their low thermal conductivity as a rule
long preheating times are necessary in order to bring the slabs
cast from the steels in question up to the temperature necessary
for hot rolling. The long oven dwell times of the slabs are
associated with a pronounced tendency towards surface
decarburisation. At the same time the low thermal conductivity
brings with it the problem that during preheating, bloom and
hot-rolling cracks form as a result of the recrystallisation
inertia of the cool strip edges. Finally, the steels offer
extremely high resistances, to heat and cold during hot- and
cold-rolling which are considerably higher than with other
high-alloy steels, such as for example RSH steels or conventional
high-alloy Mn steels.
[0005] From U.S. Pat. No. 7,794,552 B2 a method is known for
generating a flat steel product from such a conventionally
composed, austenitic, high manganese content hot-rolled steel,
which apart from iron and unavoidable impurities (in wt. %)
contains 0.85%-1.05% C; 16%-19% Mn; up to 2% Si; up to 0.050% Al;
up to 0.030% S; up to 0.050% P; up to 0.1% N, and, optionally, one
or a plurality of elements chosen from "Cr, Mo, Ni, Cu, Nb, V",
provided that the Cr content is up to 1%; the Mo content is up to
1.5%; the Ni content is up to 1%; the Cu content is up to 5%; the
Ti content is up to 0.50%; the Nb content is up to 0.50%; the V
content is up to 0.50%. The recrystallised surface fraction of the
steel strip or sheet being equal to 100% here, while the
recrystallised surface fraction of precipitated carbides being
equal to 0%. At the same time the average grain size of the steel
will be .ltoreq.10 .mu.m. The strength of the known steel created
in this way is larger than 1200 MPa and the product of the strength
and the elongation at break is larger than 65 000 MPa.
[0006] In order to achieve this, according to the known method a
correspondingly composed steel melt is cast to form a semi-finished
product, which can be a slab, thin slab or cast strip. The
semi-finished product is heated to a temperature of
1100-1300.degree. C. and at an end-of-rolling temperature of at
least 900.degree. C. rolled into a hot sheet. If necessary then a
holding time is observed in order to achieve the desired complete
recrystallisation of the strip surface. The hot strip obtained is
then cooled at a cooling rate of at least 20.degree. C./s to a
maximum coiling temperature of 400.degree. C. and wound into a
coil. The hot strip obtained in this way can then, with
intermediate annealing as necessary, be rolled into a cold
strip.
[0007] The method known from U.S. Pat. No. 7,794,552 B2 is intended
for steels where although during smelting Al can be used for
deoxidation, the Al content is restricted to a maximum of 0.05 wt.
%, in order to avoid the precipitation of AlN. The presence of AlN
precipitations will accordingly pose a danger of the formation of
cracks during deformation of the steel strip generated in the known
manner.
[0008] Against the background of the prior art mentioned above, the
object of the invention was to indicate an economical and reliably
operating method for generating a flat steel product from a steel
comprising a high Al content in addition to a high Mn content.
[0009] According to the invention this object is achieved by the
method indicated in claim 1. Advantageous configurations of the
method according to the invention are indicated in the dependent
claims.
[0010] According to the invention for the production of a
hot-rolled flat steel product initially a steel is melted
comprising, in addition to iron and unavoidable impurities (in wt.
%) C: 0.5-1.3%, Mn: 18-26%, Al: 5.9-11.5%, Si: less than 1%, Cr:
less than 8%, Ni: less than 3%, Mo: less than 2%, N: less than
0.1%, B: less than 0.1%, Cu: less than 5%, Nb: less than 1%, Ti:
less than 1%, V: less than 1%, Ca: less than 0.05%, Zr: less than
0.1%, P: less than 0.04%, and S: less than 0.04%.
[0011] In practical configurations of the invention here the
contents of the alloying elements Si, Cr, Ni, Mo, N, B, Cu, Nb, Ti,
V, Ca, Zr, P and S individually or in combination with one another
are set according to the following (in wt. %): 0.1-0.4% Si,
<3.0% Cr, <1.0% Ni, .sub.<0.5% Mo, 0.005-0.04% N,
<0.0050% B, <1% Cu, <0.2% Nb, <0.3% Ti, <0.3% V,
<0.005% Ca, <0.005% Zr, 0.01-0.03% P or 0.005-0.02% S.
[0012] A steel melt with the abovementioned composition is then for
example cast in a conventional two roller casting machine in a
manner known per se to form a cast strip.
[0013] The advantage of casting the melt into a cast strip is known
to be that with strip casting as a result of the rapid hardening
less segregations occur. In high-alloy steels of the kind processed
according to the invention this is particularly advantageous,
because through a more even distribution of the alloying elements
homogenous strip characteristics and optimum quality of the product
obtained are achieved.
[0014] If for generating the cast strip a conventional two roller
casting machine is used, in which the cast strip emerges in a
vertical direction and by means of a strand guidance device is
diverted in an arc into a horizontal direction of conveyance, then
the cast strip cools on its way from the casting machine to the
heating device typically at a cooling rate of 10-20 K/s to an
intermediate temperature of as a rule not less than 700.degree. C.
According to the invention this temperature loss is kept as low as
possible, so that the intrinsic casting heat of the cast strip upon
leaving the casting machine is retained to the greatest possible
extent as far as the heating device. In this way the amount of
energy needed in the heating device for the increase in temperature
to the initial hot rolling temperature carried out there can be
minimised.
[0015] The heating of the cast strip to the respective initial hot
rolling temperature in the range 1100-1300.degree. C. takes place
according to the invention at a heating rate of at least 20
K/s.
[0016] The cast strip which is in this way rapidly heated to the
initial hot rolling temperature is then hot rolled in one or more
passes to a hot strip.
[0017] Within 10 seconds of the end of hot rolling according to the
invention cooling then commences, during which the hot strip
obtained is cooled with a cooling rate of at least 100 K/s to
<400.degree. C. Through this rapid cooling the formation of
components with an embrittling effect, such as carbides or
intermetallic phases, is suppressed.
[0018] Finally, the cooled hot strip is wound at a coiling
temperature of up to 400.degree. C. to form a coil.
[0019] The individual work stages of the method according to the
invention are performed in a continuous, uninterrupted
sequence.
[0020] The invention is based on the knowledge that the production
of a flat steel product free of edge or surface cracks from a steel
having a high content of C, Mn and Al, is successful if from a melt
with a corresponding composition a thin, maximum 5 mm, in
particular 3-5 mm, thick, strip is cast. The thickness of the cast
strip accordingly is already in the range of the thickness that the
hot rolled flat product generated is ultimately to have.
[0021] The possibility used by the method according to the
invention of casting a steel with a high content of C, Al and Mn,
in strip casting and the associated rapid hardening of the steel
after casting reduces the frequency of core segregations in the
cast strip. Transversal cracks and crazing do not occur at all
during casting of the cast strip and longitudinal cracks only to a
very limited extent. When casting the strip in a two roller casting
machine the occurrence of core segregations through variation in
the casting roller force can be controlled.
[0022] The thin cast strip, according to the invention only a
maximum of 5 mm, in particular 3-5 mm, thick, when leaving the
roller gap already has a favourable cross-section with low bending
stresses. Accordingly, the cast strip can be bent without problems
from a vertical to a horizontal direction of conveyance, in which
it passes through the further stations for its processing.
[0023] At the same time, through use of the strip casting the
surface decarburisation is significantly reduced, since arduous
slab heating is no longer necessary. The danger of crack formation
during hot rolling is minimised due to the homogenised temperature
distribution which is achieved during the rapid heating carried out
according to the invention prior to hot rolling.
[0024] The cast strip according to the invention is characterised
by a three-layer cast structure with dendritic marginal zones and a
globular core.
[0025] The cast strip is heated using to the greatest possible
extent the intrinsic casting heat upon leaving the casting machine
to the required initial hot rolling temperature of
1100-1300.degree. C. Here the heating takes place as quickly as
possible, in particular at a heating rate of at least 20 K/s.
[0026] With the heating performed according to the invention the
temperature increase achieved in the cast strip is typically up to
250.degree. C., wherein the minimum increase in temperature is
typically 50.degree. C. Apart from avoiding the occurrence of
undesired precipitations through the rapid heating of the strip
performed according to the invention the temperature distribution
across the width of the strip can be specifically set. Thus on the
one hand it is possible, through the rapid heating to homogenise
the temperature distribution. In order to achieve a certain
deformation behaviour of the cast strip during the hot rolling
process, on the other hand the heating can also be carried out in
such a way that across the width of the cast strip a defined
temperature profile occurs. In this way unevenness in the strip,
deviations from directional stability and other geometric defects
in the strip can be achieved, without the need for expensive
additional measures.
[0027] For the accelerated heating to the initial hot rolling
temperature an inductive heating device is in particular suitable,
such as the one described in DE 10323796 B3. The advantage of using
an induction furnace for rapid heating or soaking of the product to
be rolled is that the rolled material after a short heating time
can be heated to a precisely definable temperature.
[0028] The initial hot rolling temperature reached in the course of
the rapid heating is selected in such a way that the rolling
resistances, working against the cast strip during hot rolling, are
minimised. This is in particular the case if the initial hot
rolling temperature is at least 1050.degree. C. The final hot
rolling temperature of the hot rolling performed according to the
invention is typically in the range 1000-1050.degree. C. here. This
stipulation is based on the knowledge that the steel to be
processed according to the invention, because of its high aluminium
content must be processed in a narrow temperature window.
[0029] The hot rolling of the cast band performed in-line with the
strip casting reduces the process- and material-related core
porosity of the cast strip, promotes homogeneity of the
microstructure and thus improves the strip characteristics
overall.
[0030] The hot rolling of the cast strip which is difficult to roll
per se is also made easier by the fact that the cast strip prior to
hot rolling already has a thickness close to the final dimension,
so that in the course of the hot rolling only comparably low
degrees of deformation need to be achieved. These are typically at
least 10%, in particular 10-20%. Such low degrees of deformation
can be achieved in a single pass, which further contributes towards
optimising the economic efficiency of the method according to the
invention.
[0031] The rapid cooling performed subsequent to the hot rolling
with a cooling rate of at least 100 K/s ensures that in the hot
strip obtained after leaving the final hot rolling mill no grain
growth takes place. Furthermore, in this way at this point in the
method according to the invention the precipitation of carbides,
nitrides and carbonitrides is prevented. Typically the cooling
rates achieved during the cooling following hot rolling are in the
range 100-250 K/s.
[0032] In order to reliably prevent the start of grain growth, the
cooling should as far as possible commence within the shortest
possible timeframe from the end of hot rolling, but at the outside
within 10 s.
[0033] In order to avoid oxidation of the melt and the cast strip
on its way to the hot rolling device, in the method according to
the invention the work steps performed prior to the hot rolling can
be performed under a protective gas atmosphere. Inertisation
carried out in the respective strip casting device of the meniscus
region of the steel melt awaiting casting there reduces the
formation of oxide coatings of the surfaces.
[0034] The hot strip obtained according to the invention has an
austenitic-ferritic structure with a ferritic content of typically
5-50%.
[0035] Carbon can be present in a steel according to the invention
in contents of 0.5-1.2 wt. %, wherein here in particular steels are
considered whose C content is above 0.5 wt. %. The C content is
important for the austenite formation and for the strength grade
due to solid solution hardening, an increase in the stacking fault
energy and the formation of carbides. Where the hot strip generated
according to the invention is cold-rolled into a cold strip, in
order to improve the yield strength of the cold strip by means of a
specific over-ageing treatment following a final recrystallisation
annealing on the cold strip an extremely fine carbide can be
precipitated. At C contents of above 1.2 wt. % there is a danger of
carbide occurring in quantities having an embrittling effect.
[0036] Manganese is present in a steel processed according to the
invention in contents of 18-26 wt. %. Manganese is important for
austenite formation and increases the stacking fault energy, which
has a favourable effect on the processability and
deformability.
[0037] A steel processed according to the invention has 5.9-11.5
wt. %, in particular >6-11.5 wt. % of Al. Aluminium reduces the
density, has a solid solution hardening effect and increases the
stacking fault energy. Aluminium also has a passivating effect and
increases the resistance to corrosion. As a result of the very high
stacking fault energy, the high contents of Al lead to the
manifestation of the so-called "shear bend plasticity" as the
dominating deformation mechanism with a particularly good
combination of strengths and deformability. Excessively high
aluminium contents, however, can bring about a highly embrittling
DO.sub.3 order structure in the ferrite or excessive contents of
Al-containing k-carbides ((Fe, MN).sub.3AlC) with an embrittling
effect.
[0038] Si can be present in a steel processed according to the
invention in contents of less than 1 wt. %, in particular 0.1-0.4
wt. %, in order to bring about solid solution hardening. Contents
of Si in excess of 1 wt. % make welding and painting of the steel
processed according to the invention more difficult however.
[0039] Cr, Ni and Mo likewise have a solid solution hardening
effect and improve the oxidation and corrosion resistance of the
steel processed according to the invention. At excessively high
contents, however, Cr leads to the formation of special carbides,
which can have a highly embrittling effect. Of optimum use are the
positive effects of Cr, Ni and Mo, if, as specified by the
invention, in a steel processed according to the invention the Cr
content is restricted to less than 8 wt %, in particular less than
wt. %, the Ni content to less than 3 wt. %, in particular less than
1 wt. %, and the No content to less than 2 wt. %, in particular to
less than 0.5 wt. %.
[0040] Together with aluminium nitrogen forms nitrides and has a
strength-increasing effect. Excessive contents of N, however, lead
to coarse AlN, which can have a negative effect on the
processability, the surface quality and the deformability of a
steel processed according to the invention. Therefore the N content
of a steel according to the invention is restricted to N<0.1 wt.
%, in particular 0.005-0.04 wt. %.
[0041] The B content of a steel according to the invention is
restricted to <0.1 wt. %, in particular less than 0.0050 wt. %.
B has a strength-increasing effect and forms boron nitrides and
carbides, which act as nucleation points for the occurrence of
other carbides. Due to grain boundary precipitations, excessive B
contents have an embrittling effect.
[0042] In steel processed according to the invention Cu has a solid
solution hardening effect and increases the corrosion resistance.
With excessively high Cu contents, however, there is a risk of hot
cracking during hot rolling or hot joining. Therefore the Cu
content of a steel processed according to the invention is
restricted to less than 5 wt. %, in particular less than 1 wt.
%.
[0043] The micro-alloying elements Nb, Ti and V lead to
precipitations and grain refinement and thus contribute to the
increase in strength. In addition, through the grain refinement
effect, these elements reduce the tendency of the steel to develop
weld cracks during hot joining.
[0044] Optimum use can be made of these effects if a steel
processed according to the invention has Nb, Ti or V each in
contents of less than 1.0 wt. %, and the Nb content is restricted
in particular to <0.2 wt. %, the Ti content in particular to
<0.3 wt. %, and the V content in particular to <0.3 wt.
%.
[0045] Ca in contents of less than 0.05 wt. %, in particular
<0.005 wt. %, spheroidises non-metallic materials such as
Al.sub.2O.sub.3 and FeS in steel processed according to the
invention and improves the deformability. The formation of Ca
aluminates converts alumina into slag and improves the purity.
[0046] In contents of less than 0.1 wt. %, in particular <0.005
wt. %, in steel processed according to the invention Zr has a solid
solution hardening effect. Since, however, due to grain boundary
segregations, Zr also has an embrittling effect, the content of
this element in a steel processed according to the invention is
restricted.
[0047] P and S segregate in a steel processed according to the
invention at the grain boundaries and have an embrittling effect.
As a result their content should be as low as possible, in
particular lower than 0.04 wt. %. wherein the P content is
advantageously 0.01-0.03 wt. % and the S content advantageously
0.005-0.02 wt. %.
[0048] In order to guarantee an optimum deformability of the hot
strip obtained according to the invention, after winding and before
further processing hot strip annealing is performed during which
the hot strip obtained according to the invention is annealed at an
annealing temperature of 1100-1200.degree. C. If the hot strip
annealing takes place in a continuous annealing furnace, annealing
times of 60-300 s are required for this. Such hot strip annealing
is expedient in particular if the Al content of the steel processed
according to the invention is at least 10 wt %. In the case of such
high Al contents it is also expedient, in order to avoid the
formation of brittle phases, to have the cooling take place after
hot rolling as quickly as possible, in particular at a cooling rate
of at least 40 K/s.
[0049] The hot strip obtained according to the invention can
optionally be pickled in the normal manner following coiling and
used in the coated or uncoated state. It is similarly possible to
coat the hot strip generated according to the invention following
an optionally performed pickling in a manner known per se with a
metallic protective coating, e.g. a corrosion-proofing coating. It
is furthermore conceivable to provide the hot-rolled flat product
generated according to the invention with coatings by which the
deformation of the hot strip is simplified.
[0050] With the procedure according to the invention there is the
possibility of cold-rolling the hot strips obtained according to
the invention to form cold strip products, which can then undergo a
recrystallisation annealing, over-ageing annealing (precipitation
hardening by fine carbides) and various forms of surface refinement
(Z, ZE, ZN, FAL). Here, for example, cold rolling and a subsequent
recrystallisation annealing condenses and homogenises the
microstructure in the core area.
[0051] If flat steel products with even lower thicknesses are
required, then the hot strip generated according to the invention
accordingly allows in a manner known per se a cold strip to be
processed in one or more passes. This can if necessary in turn be
surface coated if necessary to protect it against environmental
influences.
[0052] Because the strip has already been cast at close to the
final dimension and because of the only minor associated
deformations that are necessary during hot and cold rolling, the
intrinsically high resistance to hot rolling and cold rolling of
the steel processed according to the invention has only an
insignificant effect. This allows flat products of low thickness to
be generated even from steels of the kind processed according to
the invention which are problematical in terms of rolling
treatment.
[0053] In the following the invention is explained in more detail
using embodiments.
[0054] The Figure is a schematic representation of a production
line 1 for production of a hot strip W.
[0055] The production line 1 which is set up for a continuous
production sequence comprises a conventional two roller casting
machine 1, in which a melt S is cast in the gap delimited by two
rollers 2, 3 rotating in opposite directions into a cast strip G,
the thickness of which is typically 3-5 mm. The cast strip G
emerging in the vertical direction is, in a manner, likewise known
per se, diverted via a strand guidance device in a horizontal
direction of conveyance F, in which it is driven forward by means
of a conveyor device 4 arranged at the end of the strand
guidance.
[0056] The cast strip G aligned in this way and moving in the
direction of conveyance F enters a heating device 5. On its way to
the heating device 5 the cast strip G cools at a cooling rate of
10-20 K/s to an intermediate temperature.
[0057] In the heating device 5 the cast strip G entering there with
the intermediate temperature is inductively heated by means of
inductors 6 aligned transversally to the direction of conveyance F
to an initial hot rolling temperature which is typically in the
range 1100-1300.degree. C., in particular at least 1150.degree.
C.
[0058] The increase in temperature of the cast strip G achieved by
passing through the heating device as a result of the
electromagnetic field generated by the inductors 6 is up to
300.degree. C., typically 50-150.degree. C. Here the inductors 6,
as for example described in DE 103 23 796 B3, can be adjustable and
controllable, such that on the one hand the cast strip G over its
entire width heats evenly and on the other a defined temperature
profile can be set in the cast strip G.
[0059] In order to avoid contact of the melt S and the cast strip G
with the ambient atmosphere U, the two roller casting machine 1,
the strand guidance device, the conveyor device 4 and the heating
device 5 are kept under a protective gas atmosphere S.
[0060] After the heating device 5 the cast strip G enters a rolling
mill 9, in which in a single pass it is hot rolled into a hot strip
W with a thickness of typically 2.4-4.5 mm. The final hot rolling
temperature, at which the hot strip W leaves the final rolling mill
9 in the direction of conveyance F, is generally in the range 1
000-1 050.degree. C. here. The degrees of deformation achieved via
the single feed rollers are generally in the range 10-30%.
[0061] Within 10 s of leaving the rolling mill 9 the hot strip W
obtained is cooled in a cooling device 10 at a cooling rate of
typically 100-200 K/s, to a coiling temperature in the range
300-400 at which the hot strip W is then wound by a coiling device
11 into a coil C.
[0062] Hot strip annealing in a heat treatment device not shown
here can follow coiling.
[0063] In the production line 1 in the abovementioned manner four
hot strips are generated from melts S1-S3, the compositions of
which are given in Table 1.
[0064] The strips G cast from each of the melts S1-S3 are cooled on
the way to the heating device 5 at a cooling rate in each case of
approximately 15 K/s and in the heating device 5 heated by a
temperature increase .DELTA.T to the respective initial hot rolling
temperature HAT and in the hot rolling mill 9 in three passes with
a total degree of deformation .phi.g and a final hot rolling
temperature WET in each case hot rolled into a hot strip W with a
thickness dWB. Immediately afterwards the hot strips W are in each
case cooled at a cooling rate tk to the respective coiling
temperature HAT, at which they are in each case coiled into a coil
C. The parameters indicated in each case for the processing of the
strips G cast from the steels S1-S3 .DELTA.T, HAT, WET, .phi.g, dW,
tK and HAT are shown in Table 2.
[0065] The hot strip generated from the steel S3 following coiling
also undergoes hot strip annealing at 1100.degree. C. for 120 s in
a continuous annealing furnace. In this way, even with the hot
strip generated from this steel S3, despite its high C, Mn and Al
content, surface defects can be reliably prevented.
[0066] Table 3 indicates the structure, together with the
mechanical characteristics of hot strip thickness dWB, density
.rho.WB, yield strength Rp0,2, tensile strength Rm, elongation A80,
n-value and r-value of the hot strips generated from the steels
S1-S3 by the procedure according to the invention explained
here.
[0067] References
[0068] 1 Production line
[0069] 2, 3 Casting rollers
[0070] 4 Conveyance device
[0071] 5 Heating device
[0072] 6 Inductors
[0073] 9 Rolling mill
[0074] 10 Cooling device
[0075] 11 Coiling device
[0076] A Protective gas atmosphere
[0077] C Coil
[0078] F Direction of conveyance
[0079] G Cast strip
[0080] S Melt
[0081] U Ambient atmosphere
[0082] W Hot strip
TABLE-US-00001 TABLE 1 Steel C Mn Al .SIGMA. other S1 0.55 18.0 6.0
.ltoreq.0.14 S2 0.75 24.0 9.0 .ltoreq.0.09 S3 1.25 26.0 11.5
.ltoreq.0.15 Figures expressed as wt. %, remainder iron and
unavoidable impurities
TABLE-US-00002 TABLE 2 .DELTA.T WAT WET .phi.g tk HAT Steel
[.degree. C.] [.degree. C.] [.degree. C.] [%] [K/s] [.degree. C.]
S1 150 1150 1020 17 200 300 S2 120 1150 1030 15 200 300 S3 150 1150
1025 14 200 300
TABLE-US-00003 TABLE 3 dwB .rho.WB Rp0, 2 Rm Ag Steel Structure
[mm] [g/cm.sup.3] [MPa] [MPa] [%] n r S1 Austenitic- 3.1 7.2 519
761 33.1 0.26 0.90 ferritic S2 Austenitic- 3.0 7.0 680 900 32.5
0.23 0.69 ferritic S3 Austenitic- 3.0 6.8 680 920 35 0.26 0.76
ferritic Fine precipitations of k-carbides N.D. = Not
determined
* * * * *