U.S. patent application number 15/746317 was filed with the patent office on 2018-08-02 for formable lightweight steel having improved mechanical properties and method for producing semi-finished products from said steel.
This patent application is currently assigned to Salzgitter Flachstahl GmbH. The applicant listed for this patent is Salzgitter Flachstahl GmbH. Invention is credited to ZACHARIAS GEORGEOU, Peter PALZER.
Application Number | 20180216207 15/746317 |
Document ID | / |
Family ID | 56618124 |
Filed Date | 2018-08-02 |
United States Patent
Application |
20180216207 |
Kind Code |
A1 |
PALZER; Peter ; et
al. |
August 2, 2018 |
FORMABLE LIGHTWEIGHT STEEL HAVING IMPROVED MECHANICAL PROPERTIES
AND METHOD FOR PRODUCING SEMI-FINISHED PRODUCTS FROM SAID STEEL
Abstract
The invention relates to a formable lightweight steel having
improved mechanical properties and a high resistance to delayed
hydrogen-induced cracking formation and hydrogen embrittlement
comprising 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 consisting
of 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. The
invention also relates to a method for producing the said
lightweight steel.
Inventors: |
PALZER; Peter; (Liebenburg,
DE) ; GEORGEOU; ZACHARIAS; (Braunschweig,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Salzgitter Flachstahl GmbH |
38239 Salzgitter |
|
DE |
|
|
Assignee: |
Salzgitter Flachstahl GmbH
38239 Salzgitter
DE
|
Family ID: |
56618124 |
Appl. No.: |
15/746317 |
Filed: |
July 20, 2016 |
PCT Filed: |
July 20, 2016 |
PCT NO: |
PCT/EP2016/067347 |
371 Date: |
January 19, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/008 20130101;
C21D 6/002 20130101; C21D 6/005 20130101; C21D 8/0226 20130101;
C21D 9/52 20130101; C22C 38/24 20130101; C21D 8/0205 20130101; C22C
38/26 20130101; C22C 38/38 20130101; C21D 8/0263 20130101; C21D
8/0273 20130101; C22C 38/06 20130101; C21D 9/46 20130101; C22C
38/02 20130101; C21D 8/0231 20130101; C21D 1/26 20130101; C21D
8/0236 20130101 |
International
Class: |
C21D 9/52 20060101
C21D009/52; C22C 38/38 20060101 C22C038/38; C22C 38/26 20060101
C22C038/26; C22C 38/24 20060101 C22C038/24; C22C 38/06 20060101
C22C038/06; C22C 38/02 20060101 C22C038/02; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00; C21D 1/26 20060101
C21D001/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2015 |
DE |
10 2015 111 866.1 |
Claims
1.-8. (canceled)
9. A formable lightweight steel having improved mechanical property
and high resistance to delayed hydrogen-induced crack formation and
hydrogen embrittlement, the formable lightweight steel comprising
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, and
at least one carbide-forming element selected from the group
consisting of, 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 typical steel-associated
elements.
10. The formable lightweight steel of claim 9, wherein a proportion
of Sb is 0.003 to 0.5.
11. The formable lightweight steel of claim 9, further comprising
at least one element, in wt-%: Ni max. 5; Co max. 10; Ca max.
0.005; B max. 0.01; and Cu 0.05 to 2.
12. The formable lightweight steel of claim 9, wherein a ratio Sb/C
is less than or equal to 1.5.
13. The formable lightweight steel of claim 9, wherein the elements
have the following proportions, in wt-%: C is 03 to 0.5 Mn 3 to 10
Al 0.1 to 4 Si 0.1 to 3 Sb 0.005 to 0.3 Cr>0.1 to 5 V 0.005 to
1, wherein the steel has a product of tensile strength and
elongation at facture of at least 20,000 MPa % and a tensile
strength of at least 800 MPa.
14. The formable lightweight steel of claim 9, wherein the elements
have the following proportions, in wt-%: C 0.1 to 0.35 Mn 5 to 9 Al
1 to 3.5 Si 0.1 to 1 Sb 0.01 to 0.1 Cr 0.5 to 4 V 0.02 to 0.1,
wherein the steel has a product of tensile strength and elongation
at facture of at least 20,000 MPa % and a tensile strength of at
least 800 MPa.
15. The formable lightweight steel of claim 9, wherein the elements
have the following proportions, in wt.-%: C 0.4 to 0.9 Mn 12 to 18
Al 0.5 to 4 Si 0.5 to 3 Sb 0.005 to 0.4, said at least one
carbide-forming element being selected in the following proportions
(in wt.-%): Cr>0.1 to 4 Mo 0.05 to 1 Ti 0.01 to 0.1 V 0.005 to
0.3 Nb 0.005 to 0.3 W 0.005 to 0.5 Zr 0.001 to 0.3, with the
remainder being iron, including typical steel-associated elements,
wherein the steel has a product of tensile strength and elongation
at facture of at least 30,000 MPa % and a tensile strength of at
least 800 MPa.
16. The formable lightweight steel of claim 9, wherein the elements
have the following proportions, in wt.-%: C 0.6 to 1.4 Mn 10 to 30
Al>4 to 15 Si 0.05 to 0.5 Sb 0.005 to 0.5, said at least one
carbide-forming element being selected in the following proportions
(in wt.-%): Cr>0.1 to 4 Mo 0.05 to 1 Ti 0.01 to 0.1 V 0.005 to
0.3 Nb 0.005 to 0.3 W 0.005 to 0.5 Zr 0.001 to 0.3, with the
remainder being iron, including typical steel-associated elements,
wherein the steel has finely distributed kappa-carbide
precipitations and a product of tensile strength and elongation at
facture of at least 30,000 MPa % and a yield strength of at least
700 MPa and a tensile strength of at least 800 MPa.
17. A method for producing a formable lightweight steel having
improved mechanical properties and a high resistance to delayed
hydrogen-induced crack formation and hydrogen embrittlement, the
formable lightweight steel comprising 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; and at least one
carbide-forming element selected from the group consisting of, 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 typical steel-associated elements,
the method comprising: casting the lightweight steel in a
continuous casting process, a thin-slab casting process, to form a
cast strip or a cast slab with a thickness of more than 5 mm, or a
horizontal or vertical strip casting process approximating the
final dimensions to form a cast strip with a thickness of at most 5
mm; hot rolling the cast slab or cast strip with a thickness of
more than 5 mm to a uniform thickness, or flexibly hot rolling the
cast slab or cast strip to different thicknesses.
18. The method of claim 17, further comprising: after hot-rolling,
cold rolling the hot-rolled strip having the uniform thickness or
cold rolling the cast strip, which has a thickness of at most 5 mm
and is produced by a casting process approximating the final
dimensions, to a uniform thickness or flexibly cold-rolling to
different thicknesses.
19. The method of claim 17, further comprising: after hot-rolling,
annealing the hot-rolled strip or cold-rolled strip at an annealing
temperature of 480 to 770.degree. C. and an annealing duration of 1
minute to 48 hours.
20. The method of claim 18, further comprising: after hot-rolling,
annealing the hot-rolled strip or cold-rolled strip at an annealing
temperature of 480 to 770.degree. C. and an annealing duration of 1
minute to 48 hours.
21. The method of claim 17, further comprising: after hot-rolling,
cold-rolling the cast strip, which has a thickness of at most 5 mm
and is produced by a casting process approximating the final
dimensions, to a uniform thickness or flexibly cold-rolling the
cast strip to different thicknesses and then annealing the cold
strip with the following parameters: annealing temperature: 480 to
770.degree. C., annealing duration: 1 minute to 48 hours.
22. The method of claim 17, wherein, when the steel has a
proportion, in wt-%, of Al>1, the annealing temperature is 670
to 770.degree. C. and the annealing duration is 1 minute to 12
hours.
Description
[0001] The invention relates to a formable lightweight steel having
improved mechanical properties and a high resistance to delayed,
hydrogen-induced crack formation, according to claim 1. The
invention further relates to a method for producing semi-finished
products from this steel.
[0002] "Semi-finished product" is understood hereinafter to mean a
hot or cold strip produced from this steel or an intermediate or
final product produced therefrom, such as tubes for example.
[0003] In recent years, there have been many advances in the field
of so-called lightweight steels which are characterised by a low
specific weight whilst maintaining high strength and toughness, and
have a high ductility and are thus of great interest for vehicle
construction.
[0004] In these steels which are austenitic in the initial state, a
reduction in weight, which is advantageous for the automotive
industry, is achieved whilst maintaining the previous mode of
construction owing to the high content of alloy components (Si, Al)
having a specific weight far below the specific weight of iron.
[0005] The deformable lightweight steel known from the laid open
document DE 10 2004 061 284 A1 has e.g. the following alloy
composition (in wt. %): C 0.04 to .ltoreq.1.0, Al 0.05 to <4.0,
Si 0.05 to .ltoreq.6.0, Mn 9.0 to <18.0, with the remainder
being iron including typical steel-associated elements. Optionally,
Cr, Cu, Ti, Zr, V and Nb can be added as required.
[0006] This lightweight steel has a partly stabilised .gamma. mixed
crystal microstructure having defined stacking fault energy with a
partly multiple TRIP effect which permits the stress-induced or
expansion-induced conversion of a face-centred .gamma. mixed
crystal (austenite) into an .epsilon. martensite (hexagonal densest
sphere packing) and then upon further deformation into a
body-centred .epsilon. martensite and residual austenite.
[0007] The high degree of deformation is achieved by TRIP
(Transformation Induced Plasticity) and TWIP (Twinning Induced
Plasticity) properties of the steel.
[0008] However, in this and comparable steels delayed embrittlement
triggered by hydrogen and, as a result thereof, crack formation can
occur in the presence of residual stresses in the material
depending upon the microstructure and strength.
[0009] To overcome this problem, laid open document DE 10 2004 061
284 A1 already proposed to limit the hydrogen content to <20
ppm, preferably to <5 ppm.
[0010] Although this proposal is helpful, it is not yet sufficient
because the effect of the delayed crack formation can still occur
even when the hydrogen contents are set low. Moreover, in steel
production it is possible that the fixed maximum value of hydrogen
is exceeded for various reasons, which can be tolerated in terms of
the alloy but increases the risk of hydrogen embrittlement.
[0011] An austenitic steel is known from laid open document WO
2011/154153 A1 and is said to have an excellent resistance to
delayed crack formation. In addition to iron and impurities, the
steel contains, in wt. %: 0.5 to 0.8 C, 10 to 17 Mn, at least 1.0
Al, at most 0.5 Si, at most 0.020 S, at most 0.050 P, 50 to 200 ppm
N and 0.050 to 0.25 V.
[0012] A steel alloy for a high-strength, cold-rolled steel sheet
is known from laid open document WO 2009/142362 A1 and is likewise
said to have an improved resistance to delayed crack formation. In
addition to iron and impurities, the steel contains, in wt. %: 0.05
to 0.3 C, 0.3 to 1.6 Si, 4.0 to 7.0 Mn, 0.5 to 2.0 Al, 0.01 to 0.1
Cr, 0.02 to 0.1 Ni, 0.005 to 0.03 Ti, 5 to 30 ppm B, 0.01 to 0.03
Sb and 0.008 or less S.
[0013] Furthermore, a lightweight steel having an improved
expansion is known from laid open document EP 2 128 293 A1 and
comprises, in addition to iron and impurities, in wt. %: 0.2 to 0.8
C, 2 to 10 Mn, 0.2 or less P, at most 0.015 S, 3.0 to 15 Al, at
most 0.01 N and a ratio Mn/Al of 0.4 to 1.0.
[0014] Furthermore, a method for continuously heat-treating a steel
strip is described in laid open document US 2009/0050622 A1, the
strip thickness thereof varying along its length. This steel strip
with varying thickness is produced continuously by so-called
flexible rolling. In this regard, a nip of a roller system is
varied in a targeted manner during the production of the steel
strip.
[0015] The object of the invention is to provide a lightweight
steel of the generic type which does not have the effect of a
delayed crack formation or hydrogen embrittlement whilst
maintaining very good mechanical properties (ductility,
strength).
[0016] This object is achieved, proceeding from the preamble, in
conjunction with the characterising features of claim 1 and, in
relation to a method, by the features of claim 6. Advantageous
developments are described in the dependent claims.
[0017] According to the teaching of the invention, the formable
lightweight steel having TRIP and TWIP properties comprises 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
[0018] Sb 0.003 to 0.8, advantageously to max. 0.5 and at least one
or more of the following carbide-forming elements in the stated
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
[0019] with the remainder being iron including typical
steel-associated 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.
[0020] Surprisingly, it has been found during extensive testing
that by alloying antimony (Sb) at the stated limits the diffusion
of elements, in particular C, N and O is impeded and as a result
the material behaviour can be advantageously influenced in
conjunction with a targeted heat treatment.
[0021] The addition of antimony results in the carbides growing
more slowly and thus being distributed more finely and being
precipitated to a smaller size. As a result, alloy elements are
used more effectively which results in more cost-favourable alloy
concepts with improved mechanical properties and a clear
improvement in terms of avoiding delayed hydrogen-induced crack
formation (delayed fracture) and hydrogen embrittlement.
[0022] It has proved to be favourable if the ratio of Sb/C does not
exceed a value of 1.5. Values above 1.5 do not provide any further
advantage in terms of the invention and primarily produce negative
effects such as e.g. a loss of ductility and toughness owing to
precipitation of antimony at the grain boundaries.
[0023] In accordance with the invention, the mechanical properties
are evaluated by determining the tensile strength and elongation at
fracture of the product, which is a measurement for the performance
of the material.
[0024] It has been shown in tests that in the case of the alloys in
accordance with the invention, the tensile strength and elongation
at fracture are considerably higher owing to the addition of
antimony compared with steel alloys to which no antimony is added,
whereby steels can be produced which are more cost-favourable and
are of higher value.
[0025] It has also been demonstrated that the above-described
effect of antimony can be considerably increased by heat-treating
the steel.
[0026] In order to obtain a further improvement in the required
properties, the product or semi-finished product, which is produced
from the alloy in accordance with the invention by deformation and
may be e.g. a hot strip, cold strip, flexibly rolled hot or cold
strip, a tube or a vehicle body component, is thus advantageously
subjected to a heat treatment at 480 to 770.degree. C. for 1 minute
to 48 hours, e.g. in a batch-type annealing process with
predominantly long annealing times or in a continuous annealing
process with predominantly short annealing times.
[0027] However, such annealing can also already take place prior to
the final shaping to form a finished product, e.g. on the cold
strip which will be subsequently further processed. The timing of
the annealing can thus be adapted in a flexible manner to the
production process. Annealing the final product in addition to
earlier annealing of the semi-finished product can result in a
further improvement of the material properties.
[0028] Furthermore, the invention is accomplished by a method for
producing the steel in accordance with the invention with the
following steps: [0029] casting the steel in a continuous casting
process or thin-slab casting process or a horizontal or vertical
strip casting process approximating the final dimensions, [0030]
hot rolling the cast slab or the cast strip with a thickness of
more than 5 mm to a unitary thickness or flexibly hot rolling the
cast slab or the cast strip with a thickness of more than 5 mm to
different thicknesses, [0031] optionally cold rolling the hot strip
rolled to a unitary thickness or the cast strip--which is produced
by means of a casting process approximating the final dimensions
and is at most 5 mm thick--to a unitary thickness or optionally
flexibly cold rolling the hot strip rolled to a unitary thickness
or the cast strip--which is produced by means of a casting process
approximating the final dimensions and is at most 5 mm thick--to
different thicknesses, [0032] optionally annealing the hot strip or
cold strip with the following parameters: annealing temperature:
480 to 770.degree. C., annealing duration: 1 minute to 48
hours.
[0033] In relation to the cast strip which is produced by means of
a casting process approximating the final dimensions and is at most
5 mm thick, it is particularly advantageous if this is cold-rolled
to a unitary thickness or is flexibly cold-rolled to different
thicknesses and then if the cold strip is annealed with the
following parameters: annealing temperature: 480 to 770.degree. C.,
annealing duration: 1 minute to 48 hours.
[0034] In alloys with Al contents of >1 wt. %, the annealing
treatment is preferably carried out at temperatures of 670 to
770.degree. C. at annealing times of 1 minute to 12 hours, because
lower temperatures and longer annealing times result is a lower
tensile strength and elongation at fracture.
[0035] For the annealing itself, for a hot strip, cold strip and
flexibly rolled strips a continuous annealing process is preferably
used for short annealing times and a batch-type annealing process
is preferably used for long annealing times. For other products and
semi-finished products, other annealing devices having the
predetermined parameters, such as e.g. a muffle furnace, can be
used.
[0036] By means of the invention, the production of cost-favourable
Sb-alloyed steels having a higher content of manganese is possible,
said steels having improved tensile strength and elongation at
facture compared with non-Sb-alloyed steels having a higher content
of manganese with the same chemical composition.
[0037] Moreover, by adding antimony the behaviour with respect to
hydrogen (delayed crack formation and hydrogen embrittlement) is
also considerably improved.
[0038] The improvement in the material properties is caused by the
antimony impeding the diffusion of carbon and aluminium.
Furthermore, antimony decreases the interfacial energy which
results in the carbides being distributed more finely. The reduced
carbon diffusion thus delays the local enrichment of carbon at the
grain boundaries and in the microstructure and in conjunction with
aluminium the forming of kappa-carbides or in particular with V,
Nb, Mo, Cr, W, Zr and Ti the forming of local larger carbides. The
homogeneity of the material is thus improved with the described
positive effects on the mechanical properties and the resistance to
delayed crack formation and hydrogen embrittlement. The
precipitation of finely distributed carbides results in grain
refinement in the microstructure which is associated with an
improvement in the behaviour with respect to hydrogen-induced
negative effects (delayed crack formation, hydrogen embrittlement)
and an increase in the strength and improvement in the toughness
and expansion properties.
[0039] Owing to the addition of small amounts, up to max. 0.8 wt.
%, of antimony in accordance with the invention, the behaviour of
the material with respect to hydrogen-induced influences is thus
considerably improved.
[0040] In contrast, the addition of excessive amounts of antimony
causes an undesirably strong precipitation of antimony at the grain
boundaries and thus reduces the toughness and expansion properties.
In order for antimony to be able to be effective, proportions of at
least 30 ppm are required. However, antimony contents of over 0.8
wt. % embrittle the material and are thus to be avoided. Optimally,
the maximum content of antimony is 0.5 wt. %.
[0041] The small carbides which are precipitated in a much more
finely distributed manner compared with the prior art
(predominantly Cr-, Mo-, Ti-, Nb-, V-, W-, Zr- and kappa-carbides)
improve the efficiency of the corresponding alloy elements which
potentially allows a reduction in the amount added. Furthermore,
the reduced carbon diffusion and the reduced grain growth owing to
the alloying of antimony increase the process window for the heat
treatments required in accordance with the invention, i.e. the
steel reacts in a less sensitive manner to process fluctuations
(temperature, time) in relation to the resulting mechanical
properties.
[0042] The positive effects of the alloy elements used in
accordance with the invention will be described hereinafter:
[0043] Al: improves the strength and expansion properties,
decreases the specific density and influences the conversion
behaviour of the alloys in accordance with the invention. Contents
of Al of more than 15 wt. % impair the expansion properties for
which reason a maximum content of 15 wt. % is set. High Al contents
of greater than or equal to 4 wt. % act in conjunction with high C
contents of greater than or equal to 0.6 wt. % as carbide forming
agents for kappa-carbides. At less than 4 wt. %, Al delays the
precipitation of carbides.
[0044] B: Improves the strength and stabilises the austenite.
Contents of more than 0.01 wt. % result in embrittlement of the
material for which reason a maximum content of 0.01 wt. % is
set.
[0045] C: is required to form carbides, stabilises the austenite
and increases the strength. Contents of more than 1 wt. % C impair
the welding properties and result in the precipitation of
undesirably large carbides and thus in the impairment of expansion
and toughness properties for which reason a maximum content of 1
wt. % is set. In order to achieve a sufficient strength for the
material, a minimum addition of 0.01 wt. % is required.
[0046] Ca: used to modify non-metallic oxidic inclusions which can
lead to inhomogeneities and an undesired material failure. Owing to
its high vapour pressure in liquid steel, the content is limited to
at most 0.005 wt. %.
[0047] Co: increases the strength of the steel, stabilises the
austenite and improves the heat resistance. Contents of more than
10 wt. % impair the expansion properties for which reason a maximum
content of 10 wt. % is set.
[0048] Cr: improves the strength and reduces the rate of corrosion,
delays the formation of ferrite and perdite and forms carbides. The
maximum content is set to 8 wt. % since higher contents result in
an impairment of the expansion properties.
[0049] Cu: reduces the rate of corrosion and increases the
strength. Contents of above 2 wt. % impair the producibility by
forming low melting point phases during casting and hot rolling for
which reason a maximum content of 2 wt. % is set.
[0050] Mn: stabilises the austenite, increases the strength and the
toughness and permits a deformation-induced martensite formation
and/or twinning in the alloys in accordance with the invention.
Contents of less than 3 wt. % are not sufficient to stabilise the
austenite and thus impair the expansion properties whilst no
further advantages are to be expected for contents of greater than
30 wt. % and the production is made more difficult owing to the low
Mn vapour pressure.
[0051] Mo: acts as a strong carbide forming agent and increases the
strength. Contents of Mo of more than 2 wt. % impair the expansion
properties for which reason a maximum content of 2 wt. % is
set.
[0052] Nb+V: act in a grain-refining manner in particular by
forming carbides, whereby at the same time the strength, toughness
and expansion properties are improved. Contents of more than 1 wt.
% do not provide any further advantages.
[0053] Ni: stabilises the austenite and improves expansion
properties in particular at low application temperatures. An
addition of more than 5 wt. % of Ni does not provide any further
advantages.
[0054] Si: Impedes the diffusion of carbon, reduces the specific
density and increases the strength and expansion properties and
toughness properties. Furthermore, an improvement in the
cold-rollability can be seen by alloying Si. Contents of more than
4 wt. % result in embrittlement of the material and negatively
influence the hot- and cold-rollability for which reason a maximum
content of 4 wt. % is set.
[0055] Ti: acts in a grain-refining manner as a carbide forming
agent, whereby at the same time the strength, toughness and
expansion properties are improved and the inter-crystalline
corrosion is reduced. Contents of Ti of more than 2 wt. % impair
the expansion properties for which reason a maximum content of 2
wt. % is set.
[0056] W: acts as a carbide forming agent and increases the
strength and heat resistance. Contents of W of more than 1 wt. %
impair the expansion properties for which reason a maximum content
of 1 wt. % is set.
[0057] Zr: acts as a carbide forming agent and improves the
strength. Contents of Zr of more than 0.3 wt. % impair the
expansion properties for which reason a maximum content of 0.3 wt.
% is set.
[0058] Advantageous alloy combinations are shown in claims 3 to
5.
[0059] An alloy as claimed in claim 3 has, using optimised heat
treatment parameters (see tables 1 to 4), a product of tensile
strength and elongation at fracture of at least 20,000 MPa % and a
tensile strength of at least 800 MPa. The product of tensile
strength and elongation at fracture is a measurement for the
performance of the material upon deformation.
[0060] Although the heat treatment at 680.degree. C. for 10 min in
table 2 still does not provide optimum values for the product of
tensile strength and elongation at fracture of at least 20,000 MPa
%, the positive effect of alloying antimony can already be seen
here.
[0061] An alloy as claimed in claim 4 has a product of tensile
strength and elongation at facture of at least 30,000 MPa % and a
tensile strength of at least 800 MPa.
[0062] An alloy as claimed in claim 5 has finely distributed
kappa-carbide precipitations and a product of tensile strength and
elongation at facture of at least 30,000 MPa % and a yield strength
of at least 700 MPa and a tensile strength of at least 800 MPa.
[0063] The examined alloy compositions are provided in table 1. The
content of Sb and additions of Nb are varied, with the remaining
chemical composition being approximately identical.
[0064] Hot strips with a thickness of 2 mm were produced from these
steels and then cooled in air after hot rolling. Test pieces were
removed from these hot strips and the tensile strength and
elongation at fracture were determined thereon.
[0065] The results of the product of tensile strength and
elongation at fracture are shown in tables 2 to 4, wherein the heat
treatment having the highest product of tensile strength and
elongation at fracture is considered to be the most favourable for
the respective alloy. It is clear that the steels alloyed with Sb
in accordance with the invention always have a higher product of
tensile strength and elongation at facture than the comparative
alloys.
TABLE-US-00001 TABLE 1 Alloy composition Alloy C Mn Al Si Cr V Sb
other L1 0.19 7.1 2 0.55 1 0.05 0 L2 0.19 7.1 2 0.55 1 0.05 0.012
L3 0.19 7.1 2 0.55 1 0.05 0.027 L4 0.19 7.1 2 0.55 1 0.05 0.041 L5
0.21 6.3 2 0.2 1 0.06 0 Nb 0.05 L6 0.21 6.3 2 0.2 1 0.05 0.039 Nb
0.05 L7 0.25 7.9 1 0.5 0.9 0.08 0 L8 0.25 7.9 1 0.5 0.9 0.08
0.04
TABLE-US-00002 TABLE 2 determined products of tensile strength and
elongation at fracture L1 to L4 Heat TS*El treatment L1 L2 L3 L4
650.degree. C., 24 h 22453 23195 23772 22633 680.degree. C., 10 min
15263 16695 16830 16111 680.degree. C., 5 h 27162 27997 28258 29000
680.degree. C., 24 h 26660 28985 30546 29720
TABLE-US-00003 TABLE 3 determined products of tensile strength and
elongation at fracture L5 and L6 Heat TS*El treatment L5 L6
690.degree. C., 3 h 19368 21449 750.degree. C., 10 min 22751 25502
500.degree. C., 10 min 23525 26737
TABLE-US-00004 TABLE 4 determined product of tensile strength and
elongation at fracture L7 and L8 Heat TS*El treatment L7 L8
650.degree. C., 24 h 18378 20457
* * * * *