U.S. patent application number 15/749725 was filed with the patent office on 2018-08-16 for high-tensile manganese steel containing aluminium, method for producing a sheet-steel product from said steel and sheet-steel product produced according to this method.
This patent application is currently assigned to SALZGITTER FLACHSTAHL GMBH. The applicant listed for this patent is SALZGITTER FLACHSTAHL GMBH. Invention is credited to PETER PALZER.
Application Number | 20180230579 15/749725 |
Document ID | / |
Family ID | 56567612 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180230579 |
Kind Code |
A1 |
PALZER; PETER |
August 16, 2018 |
HIGH-TENSILE MANGANESE STEEL CONTAINING ALUMINIUM, METHOD FOR
PRODUCING A SHEET-STEEL PRODUCT FROM SAID STEEL AND SHEET-STEEL
PRODUCT PRODUCED ACCORDING TO THIS METHOD
Abstract
A formable lightweight steel having improved mechanical
properties and a high resistance to delayed hydrogen-induced
cracking formation and hydrogen embrittlement includes the
following elements (in wt. %): C 0.02 to .ltoreq.1.0; Mn 3 to 30;
Si.ltoreq.4; P max. 0.1; S max. 0.1; N max. 0.03; Sb 0.003 to 0.8,
particularly advantageously to 0.5, as well as at least one or more
of the following carbide-forming elements in the specified
proportions (in wt. %): Al.ltoreq.15; Cr>0.1 to 8; Mo 0.05 to 2;
Ti 0.01 to 2; V 0.005 to 1; Nb 0.005 to 1; W 0.005 to 1; Zr 0.001
to 0.3; with the remainder being iron including the usual
steel-accompanying elements, with the optional addition of the
following elements, in wt. %: max. 5 Ni, max. 10 Co, max. 0.005 Ca,
max. 0.01 B and 0.05 to 2 Cu.
Inventors: |
PALZER; PETER; (Liebenburg,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SALZGITTER FLACHSTAHL GMBH |
38239 Salzgitter |
|
DE |
|
|
Assignee: |
SALZGITTER FLACHSTAHL GMBH
38239 Salzgitter
DE
|
Family ID: |
56567612 |
Appl. No.: |
15/749725 |
Filed: |
August 3, 2016 |
PCT Filed: |
August 3, 2016 |
PCT NO: |
PCT/EP2016/068564 |
371 Date: |
February 1, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/20 20130101;
C22C 38/30 20130101; C21D 8/0226 20130101; C21D 8/0236 20130101;
C21D 8/0263 20130101; C22C 38/02 20130101; C21D 2211/001 20130101;
C22C 38/00 20130101; C22C 38/12 20130101; C22C 38/18 20130101; C21D
6/008 20130101; C22C 38/04 20130101; C22C 38/001 20130101; C22C
38/22 20130101; C22C 38/24 20130101; C22C 38/06 20130101; C21D 9/46
20130101; C21D 6/002 20130101; C22C 38/28 20130101; C22C 38/38
20130101; C22C 38/26 20130101; C21D 6/005 20130101; C21D 2211/008
20130101 |
International
Class: |
C22C 38/38 20060101
C22C038/38; C22C 38/22 20060101 C22C038/22; C22C 38/06 20060101
C22C038/06; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 6/00 20060101 C21D006/00; C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2015 |
DE |
10 2015 112 886.1 |
Claims
1.-15. (canceled)
16. A high-strength, aluminium-containing manganese steel
comprising a following chemical composition in wt. %: C: 0.01 to
<0.3 Mn: 4 to <10 Al: >1 to 4 Si: 0.01 to 1 Cr: 0.1 to 4
Mo: 0.02 to 1 P: <0.1 S: <0.1 N: <0.3 with the remainder
being iron with unavoidable steel-associated elements.
17. The steel of claim 16, further comprising at least one alloying
element, in wt. %, selected from the group consisting of: V: 0.01
to 1 Nb: 0.01 to 1 Ti: 0.01 to 1 Sn: 0 to 0.5 Cu: 0.005 to 3 W:
0.03 to 3 Co: 0.05 to 3 Zr: 0.03 to 0.5 Ca: 0.0005 to 0.1.
18. The steel of claim 16, wherein a content of Si is 0.01 to
<1.
19. The steel of claim 16, wherein a content of V is 0.02 to 1.
20. The steel of claim 16, wherein a content of Nb is 0.02 to
1.
21. The steel of claim 16, wherein a content of Ti is 0.02 to
1.
22. The steel of claim 16, wherein a content of Sn is 0.005 to
0.5.
23. The steel of claim 16, wherein a content of Cu is 0.5 to 3.
24. The steel of claim 16, wherein a content of W is 0.05 to 3.
25. The steel of claim 16, wherein a content of Co is 0.08 to
3.
26. The steel of claim 16, wherein a content of Zr is 0.05 to
0.5.
27. The steel of claim 16, wherein the steel has a tensile strength
Rm>800 to 1700 MPa and an elongation at fracture A50 of 6 to
45%.
28. The steel of claim 16, wherein the steel has an elongation at
fracture A50 of >8 to 45%.
29. A method for producing a flat steel product, comprising:
smelting a steel melt having a chemical composition comprising in
wt. %: C: 0.01 to <0.3 Mn: 4 to <10 Al: >1 to 4 Si: 0.01
to 1 Cr: 0.1 to 4 Mo: 0.02 to 1 P: <0.1 S: <0.1 N: <0.3
with the remainder being iron with unavoidable steel-associated
elements; casting the steel melt to form a pre-strip by a
horizontal or vertical strip casting process approximating a final
dimension or casting the steel melt to form a slab or thin slab by
a horizontal or vertical slab or thin slab casting process;
re-heating the slab or thin slab to 1050.degree. C. to 1250.degree.
C. and then hot rolling the slab or thin slab to form a hot strip
or thick plate, or re-heating the pre-strip which approximates the
final dimension to 1000.degree. C. to 1200.degree. C. and then hot
rolling the pre-strip to form a hot strip or thick plate, or hot
rolling the pre-strip without re-heating from the casting heat to
form a hot strip or thick plate with optional intermediate heating
between individual rolling passes of the hot rolling; reeling the
hot strip and optionally the thick plate at a reeling temperature
between 780.degree. C. and room temperature; optionally annealing
the hot strip or thick plate at an annealing temperature of 610 to
780.degree. C. and annealing duration of 1 minute to 48 hours;
optionally cold rolling the hot strip or produced pre-strip which
approximates the final dimensions, with a thickness of less than or
equal to 3 mm to form a cold strip; optionally annealing the cold
strip at an annealing temperature of 610 to 780.degree. C. and
annealing duration of 1 minute to 48 hours.
30. The method of claim 29, wherein the steel melt contains at
least one alloying element in wt. % selected from the group
consisting of: V:0.01 to 1 Nb: 0.01 to 1 Ti: 0.01 to 1 Sn: 0 to 0.5
Cu: 0.005 to 3 W: 0.03 to 3 Co: 0.05 to 3 Zr: 0.03 to 0.5 Ca:
0.0005 to 0.1.
31. The method of claim 29, wherein the produced pre-strip has a
thickness of greater than 3 mm.
32. The method of claim 29, wherein the slab is hot-rolled to form
a hot strip having a thickness of 70 mm to 1.5 mm or the pre-strip
is hot rolled to form a hot strip having a thickness of 8 mm to 1
mm.
33. A flat steel product produced by a method as set forth in claim
29, said steel product comprising a tensile strength Rm of >800
to 1700 MPa, and an elongation at fracture A50 of 6 to 45%.
34. The steel product of claim 33, wherein the steel product has an
elongation at fracture A50 of >8 to 45%.
35. The steel product of claim 33, wherein the steel product is
galvanised by hot-dipping or electrolytically or is coated
metallically, inorganically or organically.
Description
[0001] The invention relates to a high-strength,
aluminium-containing manganese steel, to a method for producing a
flat steel product from this steel, and to a flat steel product
produced by this method.
[0002] European patent application EP 2 383 353 A2 discloses a
high-strength, manganese-containing steel, a flat steel product
formed from this steel and a method for producing this flat steel
product. The steel consists of the elements (contents are in weight
percent and relate to the steel melt): C: to 0.5; Mn: 4 to 12.0;
Si: up to 1.0; Al: up to 3.0; Cr: 0.1 to 4.0; Cu: up to 4.0; Ni: up
to 2.0; N: up to 0.05; P: up to 0.05; S: up to 0.01; with the
remainder being iron and unavoidable impurities. Optionally, one or
more elements from the group "V, Nb, Ti" are provided, wherein the
sum of the contents of these elements is at most equal to 0.5. This
steel is said to be characterised in that it can be produced in a
more cost-effective manner than steels containing a high content of
manganese and at the same time has high elongation at fracture
values and, associated therewith, a considerably improved
deformability. A method for producing a flat steel product from the
high-strength, manganese-containing steel described above comprises
the following working steps: --smelting the above-described steel
melt,--producing a starting product for subsequent hot rolling, in
that the steel melt is cast into a string from which at least one
slab or thin slab is separated off as a starting product for the
hot rolling, or into a cast strip which is supplied to the hot
rolling process as a starting product,--heat-treating the starting
product in order to bring the starting product to a hot rolling
starting temperature of 1150 to 1000.degree. C.,--hot rolling the
starting product to form a hot strip having a thickness of at most
2.5 mm, wherein the hot rolling is terminated at a hot rolling end
temperature of 1050 to 800.degree. C.,--reeling the hot strip to
form a coil at a reeling temperature of .ltoreq.700.degree. C.
[0003] Proceeding therefrom, the object of the present invention is
to provide a high-strength, aluminium-containing manganese steel
having good deformation properties and an increased resistance to
delayed crack formation and hydrogen embrittlement, a method for
producing a flat steel product from this steel and a flat steel
product produced by this method, which offer a good combination of
strength and deformation properties in relation to the steel.
[0004] This object is achieved by a high-strength,
aluminium-containing manganese steel having the features of claim
1, a method for producing a flat steel product, in particular using
the aforementioned steel, having the features of claim 12, and a
flat steel product produced by this method as claimed in claim 14.
Advantageous embodiments of the invention are described in the
dependent claims.
[0005] In accordance with the invention, a high-strength,
aluminium-containing manganese steel having the following chemical
composition (in wt. %): C: 0.01 to <0.3; Mn: 4 to <10; Al:
>1 to 4; Si: 0.01 to 1; Cr: 0.1 to 4; Mo; 0.02 to 1; P: <0.1;
S: <0.1; N: <0.3; with the remainder being iron with
unavoidable steel-associated elements, with optional alloying of
one or more of the following elements (in wt. %): V: 0.01 to 1; Nb:
0.01 to 1; Ti: 0.01 to 1; Sn: 0 to 0.5; Cu: 0.005 to 3; W: 0.03 to
3; Co: 0.05 to 3; Zr: 0.03 to 0.5 and Ca: 0.0005 to 0.1 offers a
good combination of strength, strain and deformation properties.
Moreover, the production of this manganese steel in accordance with
the invention having a medium manganese content (medium manganese
steel) on the basis of the alloy elements C, Mn, Cr, Al, Si and Mo
is relatively cost-effective. Owing to the increased Al content,
the steel has a lower specific density compared with other
manganese steels alloyed with a small amount of Al and having
medium manganese contents. The manganese steel in accordance with
the invention is also characterised by an increased resistance to
delayed crack formation (delayed fracture) and to hydrogen
embrittlement. This is achieved by a precipitation of molybdenum
carbide which acts as a hydrogen trap.
[0006] The steel in accordance with the invention has a multi-phase
microstructure, consisting of ferrite and/or martensite and/or
bainite and residual austenite and a TRIP and/or TWIP effect. The
residual austenite content is 5% to 65%. The residual austenite is
partially or completely converted into martensite by the TRIP
effect upon applying high mechanical stresses. Owing to the TRIP
effect, the elongation at fracture, in particular uniform
elongation, and tensile strength increase considerably.
[0007] The use of the term "to" in the definition of the content
ranges, such as e.g. 0.01 to 1 wt. %, means that the limit
values--0.01 and 1 in the example--are also included.
[0008] The steel in accordance with the invention is suitable in
particular for producing higher-strength thick plates, hot and cold
strips which can be provided with a metallic or non-metallic
coating. Applications are feasible inter alia in the automotive
industry, shipbuilding, plant design, infrastructure, the aerospace
industry and in household appliances.
[0009] Advantageously, the steel has a tensile strength Rm of
>800 to 1700 MPa and an elongation at fracture A50 of 6 to 45%,
preferably >8 to 45%. A test piece body A50 was used for the
elongation at fracture tests as per DIN 50 125.
[0010] Alloy elements are generally added to the steel in order to
influence specific properties in a targeted manner. An alloy
element can thereby influence different properties in different
steels. The effect and interaction generally depend greatly upon
the quantity, presence of further alloy elements and the solution
state in the material. The correlations are varied and complex. The
effect of the alloy elements in the alloy in accordance with the
invention will be discussed in greater detail hereinafter. The
positive effects of the alloy elements used in accordance with the
invention will be described hereinafter:
[0011] Carbon C: is required to form carbides, stabilises the
austenite and increases the strength. Higher contents of C impair
the welding properties and result in the impairment of the strain
and toughness properties, for which reason a maximum content of
less than 0.3 wt. % is set. In order to achieve a sufficient
strength for the material, a minimum addition of 0.01 wt. % is
required.
[0012] Manganese Mn: stabilises the austenite, increases the
strength and the toughness and permits a deformation-induced
martensite formation and/or twinning in the alloy in accordance
with the invention. Contents of less than 4 wt. % are not
sufficient to stabilise the austenite and thus impair the strain
properties whereas with contents of 10 wt. % and more the austenite
is stabilised too much and as a result the strength properties, in
particular the yield strength, are reduced. For the manganese steel
in accordance with the invention having medium manganese contents,
a range of 4 to <10 wt. % is preferred.
[0013] Aluminium Al: an Al content of greater than 1 wt. % improves
the strength and strain properties, decreases the specific density
and influences the conversion behaviour of the alloy in accordance
with the invention. Contents of Al of more than 4 wt. % impair the
strain properties. Higher Al contents also considerably impair the
casting behaviour in the continuous casting process. This produces
increased outlay when casting. At less than 4 wt. %, Al delays the
precipitation of carbides. Therefore, a maximum content of 4 wt. %
and a minimum content of >1 wt. % are set.
[0014] Silicon Si: impedes the diffusion of carbon, reduces the
specific density and increases the strength and strain properties
and toughness properties. Furthermore, an improvement in the
cold-rollability could be seen by alloying Si. Contents of more
than 1 wt. % result in embrittlement of the material and negatively
influence the hot- and cold-rollability and the coatability e.g. by
galvanising. Therefore, a maximum content of 1 wt. % and a minimum
content of 0.01 wt. % are set. Preferably, a maximum content of
less than 1 wt. % is set.
[0015] Chromium Cr: improves the strength and reduces the rate of
corrosion, delays the formation of ferrite and perlite and forms
carbides. The maximum content is set to less than 4 wt. % since
higher contents result in an impairment of the strain properties. A
minimum Cr content is set to 0.1 wt. %.
[0016] Molybdenum Mo: acts as a carbide forming agent, increases
the strength and increases the resistance to delayed crack
formation and hydrogen embrittlement. Contents of Mo of more than 1
wt. % impair the strain properties, for which reason a maximum
content of 1 wt. % and a minimum content of 0.02 wt. % are set.
[0017] Phosphorus P: is a trace element from the iron ore and is
dissolved in the iron lattice as a substitution atom. Phosphorous
increases the hardness and improves the hardenability by means of
mixed crystal solidification. However, attempts are generally made
to lower the phosphorous content as much as possible because inter
alia it exhibits a strong tendency towards segregation owing to its
low diffusion rate and greatly reduces the level of toughness. The
attachment of phosphorous to the grain boundaries can cause cracks
along the grain boundaries during hot rolling. Moreover,
phosphorous increases the transition temperature from tough to
brittle behaviour by up to 300.degree. C. For the aforementioned
reasons, the phosphorus content is limited to less than 0.1 wt.
%.
[0018] Sulphur S: like phosphorous, is bound as a trace element in
the iron ore. It is generally not desirable in steel because it
exhibits a strong tendency towards segregation and has a greatly
embrittling effect, whereby the strain and toughness properties are
impaired. An attempt is therefore made to achieve amounts of
sulphur in the melt which are as low as possible (e.g. by deep
vacuum treatment). For the aforementioned reasons, the sulphur
content is limited to less than 0.1 wt. %.
[0019] Nitrogen N: N is likewise an associated element from steel
production. In the dissolved state, it improves the strength and
toughness properties in steels containing a high content of
manganese of greater than or equal to 4 wt. % Mn. Lower Mn-alloyed
steels of <4 wt. % with free nitrogen tend to have a strong
ageing effect. The nitrogen even diffuses at low temperatures to
dislocations and blocks the same. It thus produces an increase in
strength associated with a rapid loss of toughness. Binding of the
nitrogen in the form of nitrides is possible e.g. by alloying
aluminium, vanadium, niobium or titanium. For the aforementioned
reasons, the nitrogen content is limited to less than 0.3 wt.
%.
[0020] Microalloy elements are generally added only in very small
amounts (<0.1 wt. % per element). In contrast to the alloy
elements, they mainly act by precipitation formation but can also
influence the properties in the dissolved state. Despite the small
amounts added, microalloy elements greatly influence the production
conditions and the processing properties and final properties.
[0021] Typical microalloy elements are vanadium, niobium and
titanium. These elements can be dissolved in the iron lattice and
form carbides, nitrides and carbonitrides with carbon and
nitrogen.
[0022] Vanadium V and niobium Nb: these act in a grain-refining
manner in particular by forming carbides, whereby at the same time
the strength, toughness and strain properties are improved.
Contents of more than 1 wt. % do not provide any further
advantages. For vanadium and niobium, a minimum content of greater
than or equal to 0.02 wt. % and a maximum content of less than or
equal to 1 wt. % are optionally preferred.
[0023] Titanium Ti: acts in a grain-refining manner as a carbide
forming agent, whereby at the same time the strength, toughness and
strain properties are improved and the inter-crystalline corrosion
is reduced. Contents of Ti of more than 1 wt. % impair the strain
properties, for which reason a maximum content of 1 wt. % is
optionally set. Minimum contents of 0.02 wt. % may be
preferred.
[0024] Tin Sn: tin increases the strength but, similar to copper,
accumulates beneath the scale layer and at the grain boundaries at
higher temperatures. This results, owing to the penetration into
the grain boundaries, in the formation of low-melting phases and,
associated therewith, to cracks in the microstructure and to solder
brittleness, for which reason a maximum content of less than or
equal to 0.5 wt. % and a minimum content of 0.005 wt. % are
optionally provided.
[0025] Copper Cu: reduces the rate of corrosion and increases the
strength. Contents of above 3 wt. % impair the producibility by
forming low-melting phases during casting and hot rolling, for
which reason a maximum content of 3 wt. % and a minimum content of
0.005 wt. % are optionally set. A minimum content of 0.5 wt. % is
preferred.
[0026] Tungsten W: acts as a carbide forming agent and increases
the strength and heat resistance. Contents of W of more than 3 wt.
% impair the strain properties, for which reason a maximum content
of 3 wt. % and a minimum content of 0.03 wt. % are optionally set.
A minimum content of 0.05 wt. % is preferred.
[0027] Cobalt Co: increases the strength of the steel, stabilises
the austenite and improves the heat resistance. Contents of more
than 3 wt. % impair the strain properties, for which reason a
maximum content of less than or equal to 3 wt. % and a minimum
content of 0.05 wt. % are optionally set. A minimum content of 0.08
wt. % is preferred.
[0028] Zirconium Zr: acts as a carbide forming agent and improves
the strength. Contents of Zr of more than 0.5 wt. % impair the
strain properties, for which reason a maximum content of 0.5 wt. %
and a minimum content of 0.03 wt. % are optionally set. A minimum
content of 0.05 wt. % is preferred.
[0029] Calcium Ca: Calcium is used for modifying non-metallic
oxidic inclusions which could otherwise result in the undesired
failure of the alloy as a result of inclusions in the
microstructure which act as stress concentration points and weaken
the metal composite. Furthermore, Ca improves the homogeneity of
the alloy in accordance with the invention. In order to achieve a
corresponding effect, a minimum content of 0.0005 wt. % is
optionally necessary. Contents of above 0.1 wt. % Ca do not provide
any further advantage in the modification of inclusions, impair
producibility and should be avoided by reason of the high vapour
pressure of Ca in steel melts. Therefore, a maximum content of 0.1
wt. % is provided.
[0030] In accordance with the invention, a method for producing a
flat steel product, in particular from the steel described above,
comprising the steps of: [0031] smelting a steel melt containing
(in wt. %): C: 0.01 to <0.3; Mn: 4 to <10; Al: >1 to 4;
Si: 0.01 to 1; Cr: 0.1 to 4; Mo: 0.02 to 1; P: <0.1; S: <0.1;
N: <0.3; with the remainder being iron including unavoidable
steel-associated elements, with optional alloying of one or more of
the following elements (in wt. %): V: 0.01 to 1; Nb: 0.01 to 1; Ti:
0.01 to 1; Sn: 0 to 0.5; Cu: 0.005 to 3; W: 0.03 to 3; Co: 0.05 to
3; Zr: 0.03 to 0.5 and Ca: 0.0005 to 0.1; [0032] casting the steel
melt to form a pre-strip by means of a horizontal or vertical strip
casting process approximating the final dimensions or casting the
steel melt to form a slab or thin slab by means of a horizontal or
vertical slab or thin slab casting process, [0033] re-heating the
slab or thin slab to 1050.degree. C. to 1250.degree. C. and then
hot rolling the slab or thin slab to form a hot strip or thick
plate, or re-heating the produced pre-strip which approximates the
final dimensions, in particular with a thickness of greater than 3
mm, to 1000.degree. C. to 1200.degree. C. and then hot rolling the
pre-strip to form a hot strip or thick plate, or hot rolling the
pre-strip without re-heating from the casting heat to form a hot
strip or thick plate with optional intermediate heating between
individual rolling passes of the hot rolling, [0034] reeling the
hot strip and optionally the thick plate at a reeling temperature
between 780.degree. C. and room temperature, [0035] optionally
annealing the hot strip or thick plate with the following
parameters: annealing temperature: 610 to 780.degree. C., annealing
duration: 1 minute to 48 hours, [0036] optionally cold rolling the
hot strip or produced pre-strip which approximates the final
dimensions, with a thickness of less than or equal to 3 mm to form
a cold strip, [0037] optionally annealing the cold strip with the
following parameters: annealing temperature: 610 to 780.degree. C.,
annealing duration: 1 minute to 48 hours, provides a flat steel
product having a good combination of strength, strain and
deformation properties, and an increased resistance to delayed
crack formation and hydrogen embrittlement and has a TRIP and/or
TWIP effect during mechanical loading owing to its residual
austenite content in the microstructure.
[0038] In relation to other advantages, reference is made to the
above statements relating to the steel in accordance with the
invention. The method results in a steel product in the form of a
thick plate, hot strip or cold strip. Provision is made that the
hot strip is wound at a temperature of at most 780.degree. C. Room
temperature is provided as the lower limit because the winding
temperature has only a small influence on subsequent processing
properties. In the context of the present invention, strips having
thicknesses of over 3 mm are defined as the thick plate, wherein
these strips can certainly still be wound e.g. at a thickness of 5
mm. A thick plate having a greater thickness, e.g. 50 mm, is made
into a sheet after hot rolling to form sheet material because it
can no longer be wound. The hot strip or cold strip can also be
made into a sheet as required.
[0039] Typically, the hot rolling end temperature is between
950.degree. C. and A.sub.c1+50 K.
[0040] Typically, thickness ranges for the pre-strip are 1 mm to 35
mm and for slabs and thin slabs they are 35 mm to 450 mm. Provision
is preferably made that the slab or thin slab is hot rolled to form
a hot strip or thick plate having a thickness of 70 mm to 1.5 mm or
the cast pre-strip approximating the final dimensions is hot rolled
to form a hot strip having a thickness of 8 mm to 1 mm. The cold
strip in accordance with the invention has a thickness of e.g.
greater than 0.15 mm.
[0041] In the context of the above method in accordance with the
invention, a pre-strip produced with the two-roller casting process
and approximating the final dimensions and having a thickness of
less than or equal to 3 mm, preferably 1 mm to 3 mm is already
understood to be a hot strip. The pre-strip thus produced as a hot
strip does not have a 100% cast structure owing to the introduced
deformation of the two rollers running in opposite directions. Hot
rolling thus already takes place in-line during the two-roller
casting process which means that separate hot rolling is not
necessary.
[0042] Re-heating temperatures in the range of 720.degree. C. to
1200.degree. C. are provided for hot rolling of the pre-strip from
the casting heat to form a hot strip with optional intermediate
heating between individual rolling passes of the hot rolling
process. If only a few rolling passes are necessary, the re-heating
temperature can be selected at the lower end of the range.
[0043] The hot strip, like the thick plate, can optionally be
subjected to a heat treatment in the temperature range between 610
and 780.degree. C. for 1 minute to 48 hours, wherein higher
temperatures are associated with shorter treatment times and vice
versa. Annealing can take place both in a batch-type annealing
process (longer annealing times) and e.g. in a continuous annealing
process (shorter annealing times). The heat treatment can likewise
be omitted if the hot strip or thick plate already has the finished
usage properties.
[0044] After the annealing process, the annealed hot strip can
optionally be cold-rolled with the aim of setting the thicknesses
of greater than or equal to 0.15 mm as required for the end use.
Subsequent thereto, a further annealing process can be performed,
if necessary coupled with a coating process and finally a
temper-rolling process, by means of which the surface structure
required for the end use is set.
[0045] Preferably, the flat steel product is galvanised by
hot-dipping or electrolytically or is coated metallically,
inorganically or organically.
[0046] A flat steel product produced by the method in accordance
with the invention in the form of a thick plate, hot strip or cold
strip has a tensile strength Rm>800 to 1700 MPa and an
elongation at fracture A50 of 6 to 45%, preferably >8 to 45%. In
this case, high strengths tend to be associated with, lower
elongations at fracture and vice versa.
* * * * *