U.S. patent application number 16/308319 was filed with the patent office on 2019-08-22 for method for producing a cold-rolled steel strip having trip-characteristics made of a high-strength mangan-containing 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 Thomas Evertz, PETER PALZER, MARTIN SCHUBERT.
Application Number | 20190256943 16/308319 |
Document ID | / |
Family ID | 59061988 |
Filed Date | 2019-08-22 |
United States Patent
Application |
20190256943 |
Kind Code |
A1 |
PALZER; PETER ; et
al. |
August 22, 2019 |
METHOD FOR PRODUCING A COLD-ROLLED STEEL STRIP HAVING
TRIP-CHARACTERISTICS MADE OF A HIGH-STRENGTH MANGAN-CONTAINING
STEEL
Abstract
The invention relates to a method for producing a cold-rolled
steel strip made of a high-strength mangan-containing steel with
TRIP-characteristics, containing (in wt. %) C: 0.0005 to 0.9, Mn:
more than 3.0 to 12, with the remaining portion being iron
including unavoidable steel-associated elements, with the optional
addition of one or more of the following elements (in wt. %): Al:
up to 10; Si: up to 6; Cr: up to 6; Nb: up to 1.5; V: up to 1.5;
Ti: up to 1.5; Mo: up to 3; Cu: up to 3; Sn: up to 0.5; W: up to 5;
Co: up to 8; Zr: up to 0.5; Ta: up to 0.5; Te: up to 0.5; B: up to
0.15; P: max. 0.1, in particular <0.04; S: max. 0.1, in
particular <0.02; N: max. 0.1, in particular <0.05; Ca: up to
0.1. According to the invention, in order to improve a
corresponding method, the cold-rolling to a required end thickness
occurs at a temperature of over 50.degree. C. to 400.degree. C.
before the first impact.
Inventors: |
PALZER; PETER; (Liebenburg,
DE) ; Evertz; Thomas; (Peine, DE) ; SCHUBERT;
MARTIN; (Wernigerode, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SALZGITTER FLACHSTAHL GMBH |
38239 Salzgitter |
|
DE |
|
|
Assignee: |
SALZGITTER FLACHSTAHL GMBH
38239 Salzgitter
DE
|
Family ID: |
59061988 |
Appl. No.: |
16/308319 |
Filed: |
June 8, 2017 |
PCT Filed: |
June 8, 2017 |
PCT NO: |
PCT/EP2017/063958 |
371 Date: |
December 7, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0226 20130101;
C21D 6/002 20130101; C21D 6/008 20130101; C21D 2211/008 20130101;
C22C 38/06 20130101; C21D 8/105 20130101; C21D 8/0236 20130101;
C22C 38/02 20130101; C22C 38/38 20130101; C22C 38/22 20130101; C21D
8/0205 20130101; C21D 8/0263 20130101; C21D 8/0278 20130101; C21D
9/52 20130101; C21D 6/005 20130101; C21D 2211/001 20130101 |
International
Class: |
C21D 9/52 20060101
C21D009/52; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; C22C 38/38 20060101 C22C038/38; C22C 38/22 20060101
C22C038/22; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 9, 2016 |
DE |
10 2016 110 661.5 |
Claims
1.-25. (canceled)
26. A method, comprising: producing a steel strip from
high-strength, manganese-containing steel with TRIP properties,
said steel comprising (in wt. %): C: 0.0005 to 0.9 Mn: more than
3.0 to 12, with the remainder being iron including unavoidable
steel-associated elements; and cold-rolling the steel strip to a
required end thickness at a temperature prior to a first rolling
pass above 50.degree. C. to 400.degree. C.
27. The method of claim 26, further comprising adding of one or
more of the following alloying elements (in wt. %): Al: to 10 Si:
to 6 Cr: to 6 Nb: to 1.5 V: to 1.5 Ti: to 1.5 Mo: to 3 Cu: to 3 Sn:
to 0.5 W: to 5 Co: to 8 Zr: to 0.5 Ta: to 0.5 Te: to 0.5 B: to 0.15
P: max. 0.1, in particular <0.04 S: max. 0.1, in particular
<0.02 N: max. 0.1, in particular <0.05 Ca: to 0.1.
28. The method of claim 26, wherein the temperature is 70.degree.
C. to 250.degree. C.
29. The method of claim 26, wherein the steel strip is a hot strip
or a pre-strip which is heated to a temperature above 50.degree. C.
to 400.degree. C., preferably from 70.degree. C. to 250.degree. C.,
or a hot strip or a pre-strip at a temperature above 50.degree. C.
to 400.degree. C., preferably from 70.degree. C. to 250.degree. C.,
before undergoing cold-rolling to the required end thickness.
30. The method of claim 26, further comprising cooling the steel
strip to a temperature of 50.degree. C. to 400.degree. C., in
particular to a temperature of 70.degree. C. to 250.degree. C.
between rolling passes of the cold-rolling.
31. The method of claim 26, wherein the C content is 0.05 to
0.42.
32. The method of claim 26, wherein the Mn content is >5 to
<10.
33. The method of claim 27, wherein the Al content is 0.1 to 5, in
particular >0.5 to 3.
34. The method of claim 27, wherein the Si content is 0.05 to 3, in
particular >0.1 to 1.5.
35. The method of claim 27, wherein the Cr content is 0.1 to 4, in
particular >0.5 to 2.5.
36. The method of claim 27, wherein a sum of Al+Si+Cr is
>1.2.
37. The method of claim 27, wherein the Nb content is 0.005 to 0.4,
in particular 0.01 to 0.1.
38. The method of claim 27, wherein the V content is 0.005 to 0.6,
in particular 0.01 to 0.3.
39. The method of claim 27, wherein the Ti content is 0.005 to 0.6,
in particular 0.01 to 0.3.
40. The method of claim 27, wherein the Mo content is 0.005 to 1.5,
in particular 0.01 to 0.6.
41. The method of claim 27, wherein the Sn content is <0.2, in
particular <0.05.
42. The method of claim 27, wherein the Cu content is <0.5, in
particular <0.1.
43. The method of claim 27, wherein the W content is 0.01 to 3, in
particular 0.2 to 1.5.
44. The method of claim 27, wherein the Co content is 0.01 to 5, in
particular 0.3 to 2.
45. The method of claim 27, wherein the Zr content is 0.005 to 0.3,
in particular 0.01 to 0.2.
46. The method of claim 27, wherein the Ta content is 0.005 to 0.3,
in particular 0.01 to 0.1.
47. The method of claim 27, wherein the Te content is 0.005 to 0.3,
in particular 0.01 to 0.1.
48. The method of claim 27, wherein the B content is 0.001 to 0.08,
in particular 0.002 to 0.01.
49. The method of claim 27, wherein the Ca content is 0.005 to
0.1.
50. The method of claim 26, wherein the required end thickness is
less than 10 mm, preferably less than 4 mm.
51. The method of claim 26, wherein the steel strip is produced by:
melting a melt of the high-strength, manganese-containing steel;
casting the melt to form a pre-strip through a horizontal or
vertical strip casting process approximating a final dimension or
casting the steel melt to form a slab or thin slab through a
horizontal or vertical slab or thin slab casting process;
re-heating the slab or thin slab to a temperature in a range of
1050.degree. C. to 1250.degree. C. and then hot-rolling the slab or
thin slab to form a hot strip, or re-heating the pre-strip to a
temperature in a range of 1000.degree. C. to 1200.degree. C. and
then hot-rolling the pre-strip to form a hot strip, or hot-rolling
the pre-strip without re-heating by using heat generated during
casting to form a hot strip with optional intermediate heating
between individual rolling passes of the hot-rolling; and reeling
the hot strip at a reeling temperature between 820.degree. C. and
ambient temperature.
52. The method of claim 51, further comprising annealing the hot
strip at an annealing temperature of 580 to 820.degree. C. and an
annealing duration of 1 minute to 48 hours, after the hot strip has
been reeled.
53. The method of claim 52, further comprising cold-rolling the hot
strip.
54. The method of claim 26, further comprising annealing the steel
strip at an annealing temperature of 580 to 820.degree. C. and an
annealing duration of 1 minute to 48 hours, after the steel strip
has been cold-rolled.
55. The method of claim 26, further comprising acid-cleaning and/or
skin-pass rolling the steel strip, after the steel strip has been
cold-rolled.
56. The method of claim 26, further comprising coating the steel
strip with a metallic, organic or inorganic corrosion protection
coating, after the steel strip has been cold-rolled.
57. The method of claim 26, further comprising using the steel
strip to produce a component by hot-forming, cold-forming or
warm-forming or to produce a pipe with longitudinal or spiral weld
seam or to produce a component for automotive and utility vehicle
industry and for engineering, white goods and construction, or
using the steel strip in a low-temperature range below 0.degree. C.
to -273.degree. C., or using the steel strip as a ballistic steel.
Description
[0001] The invention relates to a method for producing a
cold-rolled steel strip made of a high-strength,
manganese-containing steel. Steel strip is understood to mean
hereinunder in particular steel strips but also steel sheets.
Typical tensile strengths Rm in these steels are about 800 MPa to
2000 MPa. Elongations at fracture A80 have values of about 3% to
40%.
[0002] European patent application EP 2 383 353 A2 discloses a
high-strength, manganese-containing steel, a steel strip made from
this steel and a method for producing this steel strip. The steel
consists of the elements (contents are in % by weight 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. The steel is said to be
characterised in that it can be produced in a more cost-effective
manner than steels with a high content of manganese and at the same
time has high elongation at fracture values and, associated
therewith, a considerably improved deformability.
[0003] A method for producing a steel strip from the high-strength,
manganese-containing steel described above has the following
working steps: [0004] melting the above-described steel melt,
[0005] 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, [0006] heat-treating the
starting product in order to bring the starting product to a
hot-rolling starting temperature of 1150 to 1000.degree. C., [0007]
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., [0008]
reeling the hot strip to form a coil at a reeling temperature of
.ltoreq.700.degree. C., optionally annealing the hot strip and then
cold-rolling it to a thickness of at most 60% of the thickness of
the hot strip.
[0009] Depending on the alloy point, this steel can have a
metastable austenite with the capacity for stress-induced
martensite formation (TRIP effect).
[0010] The international patent application WO 2005/061152 A1 also
describes a method for producing hot strips from a deformable
lightweight steel, which can be successfully cold-deep-drawn, with
a Mn content of 9 to 30 wt. %. In addition to a high level of
tensile strength, the hot strip has TRIP properties. The German
laid-open document DE 197 27 759 A1 discloses an
ultra-high-strength austenitic lightweight steel which can be
successfully deep-drawn with a tensile strength of up to 1100 MPa,
which likewise has TRIP and TWIP properties. The German laid-open
document DE 10 2012 111 959 A1 describes a high manganese-content
steel material with TRIP and TWIP properties, which undergoes an
increase in harness and deformability by virtue of cold-forming
below ambient temperature preferably in the range of +25.degree. C.
to -200.degree. C. The German laid-open document DE 10 2009 030 324
A1 describes a high manganese-content steel with a low tendency
towards hydrogen embrittlement and with high tensile strengths
while at the same time having high elongation at fracture values.
The patent application US 2012/0059196 A1 discloses a method for
producing a hot strip with a horizontal strip casting installation.
The hot strip consists of the main components, Fe, Mn, Si and Al,
has TRIP and/or TWIP properties and is suitable for deep-drawing.
The patent U.S. Pat. No. 6,358,338 B1 also relates to a method for
producing a steel strip from a high manganese-content steel. In
order to increase the tensile strength and extensibility, the steel
strip is subjected to recrystallisation annealing after
cold-rolling. In the patent application US 2009/0074605 A1 a high
manganese-content steel strip with excellent crash behaviour and
with high tensile strength and elongation values is produced in
that the steel strip is cold-rolled after hot-rolling and then
annealed at 600.degree. C.
[0011] Furthermore, German laid-open document DE 10 2012 013 113 A1
describes TRIP steels which have a predominantly ferritic basic
microstructure having incorporated residual austenite. Owing to its
intense cold-hardening, the TRIP steel achieves high values for
uniform elongation and tensile strength.
[0012] A disadvantage with these manganese-containing steels with a
TRIP effect is that, during production of a cold-rolled steel
strip, the achievable degree of deformation is limited owing to the
intense cold-hardening of the material during cold-rolling and the
high loading on the roll stand associated therewith. In order to
achieve high degrees of cold-forming, a plurality of cold-rolling
steps with correspondingly low degrees of deformation are often
required, wherein prior to a renewed cold-rolling step, in each
case recrystallisation annealing must be carried out in order to
loosen the material and therefore render it capable of being
cold-rolled. This procedure with a plurality of cold-rolling steps
with intermediate recrystallising annealing is very time-consuming
and expensive and associated with additional CO.sub.2
emissions.
[0013] On the basis of the above, the object of the present
invention is to provide a method for producing a cold-rolled steel
strip made of a high-strength, manganese-containing steel with TRIP
properties, with which the cold-rolling to the required end
thickness can be effected in a more economical and
ecologically-friendly manner. In addition, a production route from
the melting of the steel to the steel strip cold-rolled to the
required end thickness is to be provided.
[0014] This object is achieved by a method for producing a steel
strip having the features of claim 1. Advantageous embodiments of
the invention are described in the respective dependent claims.
[0015] The method in accordance with the invention for producing a
cold-rolled steel strip from high-strength, manganese-containing
steel with TRIP properties containing (in wt. %):
C: 0.0005 to 0.9
[0016] Mn: more than 3.0 to 12 with the remainder being iron
including unavoidable steel-associated elements, with optional
addition by alloying of one or more of the following elements
(contents in wt. % and in relation to the steel melt):
Al: to 10
Si: to 6
Cr: to 6
Nb: to 1.5
V: to 1.5
Ti: to 1.5
Mo: to 3
Cu: to 3
Sn: to 0.5
W: to 5
Co: to 8
Zr: to 0.5
Ta: to 0.5
Te: to 0.5
B: to 0.15
[0017] P: max. 0.1, in particular <0.04 S: max. 0.1, in
particular <0.02 N: max. 0.1, in particular <0.05
Ca: to 0.1
[0018] is characterised in that in avoiding cold-rolling at ambient
temperature, the rolling to a required end thickness takes place at
a temperature above 50.degree. C. to 400.degree. C.
[0019] In conjunction with the present invention, high-strength
steels are understood to be steels with a tensile strength of 800
MPa to 2000 MPa.
[0020] The cause of the intense cold-hardening of these
high-strength, manganese-containing steels with a TRIP effect is
the proportion of residual austenite contained in the
microstructure in addition to martensite and/or ferrite and/or
bainite and/or perlite. This residual austenite can be converted at
appropriate ambient temperatures into martensite (TRIP effect both
.epsilon. as well as .alpha.' martensite), wherein at ambient
temperature up to about 50.degree. C. a substantial proportion of
martensite formation always takes place owing to the TRIP effect.
This leads to a hardening of the material and, associated
therewith, to an intense increase in the rolling forces during
cold-rolling, even during the first pass and is associated with a
reduction in the maximum degree of deformation. The cold-rolled
strip then has a high level of strength and low residual
deformation capability. In addition, the influence of mechanical
stresses can cause deformation twins (TWIP effect).
[0021] In accordance with the invention, by means of the raising of
the deformation temperature prior to the first pass to above
50.degree. C. to 400.degree. C., the TRIP conversion mechanism from
austenite to martensite is now wholly or partly suppressed and so
substantially higher degrees of deformation are possible during
rolling in only one rolling pass.
[0022] The term "cold-rolling" is conventionally frequently related
to cold-rolling at ambient temperature. In conjunction with the
present invention, the term "cold-rolling" is also used for
cold-rolling at raised temperature. In contrast to hot-rolling,
this raised temperature in the case of cold-rolling in accordance
with the invention is clearly below the AC1 conversion temperature
associated with a microstructure conversion. The cold-rolling in
accordance with the invention also preferably takes place below a
homologous temperature, at precisely which creeping processes still
do not occur in the steel sheet.
[0023] In the single FIGURE enclosed herewith, FIG. 1, the
influence of the deformation temperature during rolling on the
hardening behaviour of the material is illustrated with the aid of
the characteristic values of tensile tests. In comparison to the
deformation at ambient temperature of 20.degree. C., at deformation
temperatures of 100.degree. C. or 200.degree. C., clearly greater
elongation values are achieved while having a clearly lower
increase in tensile strength.
[0024] Provision is preferably made that a hot strip or a pre-strip
is heated to a temperature above 50.degree. C. to 400.degree. C.,
preferably from 70.degree. C. to 250.degree. C., or a hot strip or
a pre-strip is already at a temperature above 50.degree. C. to
400.degree. C., preferably from 70.degree. C. to 250.degree. C.,
and is then cold-rolled to the required end thickness at a
temperature, prior to the first pass, above 50.degree. C. to
400.degree. C., preferably from 70.degree. C. to 250.degree. C.
"Being at a temperature" is understood to mean that the temperature
is the result of a preceding process step or this temperature has
been maintained. The preceding process step can mean a reheating
step, a continuous or discontinuous processing step using the
available heat in the hot strip or pre-strip, in particular a
hot-rolling process, or maintenance of the temperature in a
furnace.
[0025] By heating the hot strip, prior to cold-rolling, to the
temperature above 50.degree. C. to 400.degree. C., preferably
70.degree. C. to 250.degree. C., the conversion of austenite into
martensite by increasing the stacking fault energy in the first
rolling pass is substantially reduced or avoided and so the strip
hardens less intensely during the cold-rolling process and more
deformation twins (TWIP effect) are formed in the austenite. This
results in both lower rolling forces and also a substantially
improved deformation capability for the strip during the roll
process. In order to compensate for the additional heating of the
strip by reason of the deformability during cold-forming and to
keep the strip temperature in the range which is optimal for the
TWIP effect, cooling of the strip, e.g. by compressed air or other
liquid or gaseous media, can optionally take place between the
individual rolling passes.
[0026] Furthermore, after rolling, the steel strip comprises a
considerably residual deformation capability since the deformation
twins formed in the austenite and residual austenite which may be
present can wholly or partially convert into martensite at ambient
temperature owing to the TRIP effect, this is associated with an
increase in the maximum elongation and therefore an improvement in
the deformation capability for the production of components from
the flat product even without additional annealing associated with
the cold-rolling process.
[0027] In addition, the formation of deformation twins brings about
improved behaviour during subsequent deformations with respect to
hydrogen-induced delayed crack formation and hydrogen embrittlement
compared with cold-rolling without prior heating with an optionally
associated annealing process.
[0028] The steel used for the method in accordance with the
invention has a multi-phase microstructure, including ferrite
and/or martensite and/or bainite and/or perlite and residual
austenite/austenite. The proportion of residual austenite/austenite
can be 5% to 80%. The residual austenite/austenite can partially or
completely convert into martensite owing to the TRIP effect when
mechanical stresses are present.
[0029] The alloy forming the basis of the invention has a TRIP
and/or TWIP effect when subjected to the relevant mechanical
stress. Owing to the intense hardening (similar to cold-hardening)
at ambient temperature, induced by the TRIP and/or TWIP effect and
by the increase in the dislocation density, the steel achieves very
high values in terms of elongation at fracture, in particular
uniform elongation, and tensile strength. In an advantageous
manner, this property is achieved owing to the residual austenite
present, only in the case of manganese contents of over 3 wt.
%.
[0030] The use of the word "to" in the definitions of the content
ranges, such as e.g. 0.01 wt. % to 1 wt. %, means that the limit
values, 0.01 and 1 in the example, are also included.
[0031] The steel in accordance with the invention is suitable in
particular for producing a high-strength steel strip which can be
provided with a metallic or non-metallic coating, e.g. a zinc-based
coating. It may feasibly be used inter alia in the automotive
industry, shipbuilding, plant design, infrastructure, the aerospace
industry and in household appliances. The high proportion of
austenite means that the steel produced in accordance with the
invention is suitable for low-temperature stresses.
[0032] Advantageously, the steel has a tensile strength Rm of
>800 to 2000 MPa and an elongation at fracture A80 of 3 to 40%,
preferably >8 to 40%.
[0033] Particularly uniform and homogeneous material properties can
be achieved if the steel has the following alloy composition in wt.
%:
C: 0.05 to 0.42
Mn: >5 to <10
[0034] with the remainder being iron including unavoidable
steel-associated elements, with optional addition by alloying of
one or more of the following elements (in wt. %): Al: 0.1 to 5, in
particular >0.5 to 3 Si: 0.05 to 3, in particular >0.1 to 1.5
Cr: 0.1 to 4, in particular >0.5 to 2.5 Nb: 0.005 to 0.4, in
particular 0.01 to 0.1 B: 0.001 to 0.08, in particular 0.002 to
0.01 Ti: 0.005 to 0.6, in particular 0.01 to 0.3 Mo: 0.005 to 1.5,
in particular 0.01 to 0.6 Sn: <0.2, in particular <0.05 Cu:
<0.5, in particular <0.1 W: 0.01 to 3, in particular 0.2 to
1.5 Co: 0.01 to 5, in particular 0.3 to 2 Zr: 0.005 to 0.3, in
particular 0.01 to 0.2 Ta: 0.005 to 0.3, in particular 0.01 to 0.1
Te: 0.005 to 0.3, in particular 0.01 to 0.1 V: 0.005 to 0.6, in
particular 0.01 to 0.3
Ca: 0.005 to 0.1
[0035] 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:
[0036] 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
elongation and toughness properties, for which reason a maximum
content of 0.9 wt. % is set. The minimum content is set at 0.0005
wt. %. A content of 0.05 to 0.42 wt. % is preferred because in this
range the ratio of residual austenite to other phase proportions
can be set in a particularly advantageous manner.
[0037] Manganese Mn: stabilises the austenite, increases the
strength and the toughness and renders possible a
deformation-induced martensite formation and/or twinning in the
alloy in accordance with the invention. Contents .ltoreq.3 wt. %
are not sufficient to stabilise the austenite and therefore impair
the elongation properties whereas with contents of over 12 wt. %
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 average
manganese contents, a range of over 5 to <10 wt. % is preferred
because in this range the ratio of the phase proportions to each
other and the conversion mechanisms can be advantageously
influenced during rolling to the end thickness.
[0038] Aluminium Al: improves the strength and elongation
properties, decreases the relative density and influences the
conversion behaviour of the alloy in accordance with the invention.
Al contents of more than 10 wt. % impair the elongation properties
and cause predominantly brittle fracture behaviour. For the
manganese steel in accordance with the invention with average
manganese contents, an Al content of 0.1 to 5 wt. % is preferred in
order to increase the strength while having a good degree of
elongation. In particular, contents of >0.5 to 3 wt. % render
possible a particularly high level of strength and elongation at
fracture.
[0039] Silicon Si: impedes the diffusion of carbon, reduces the
relative density and increases the strength and elongation
properties and toughness properties. Contents of more than 6 wt. %
prevent further processing by cold-rolling by reason of
embrittlement of the material. Thus, a maximum content of 6 wt. %
is set. Optionally, a content of 0.05 to 3 wt. % is set because
contents in this range positively influence the deformation
properties. Si contents of >0.1 to 1.5 wt. % have proved to be
particularly advantageous for the deformation and conversion
properties.
[0040] 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 6 wt. % since higher
contents result in an impairment of the elongation properties and
substantially higher costs. For the manganese steel in accordance
with the invention having average manganese contents, a Cr content
of 0.1 to 4 wt. % is preferred in order to reduce the precipitation
of coarse Cr carbides. In particular, contents of >0.5 to 2.5
wt. % have proved to be advantageous for stabilising the austenite
and precipitating fine Cr carbides. In order to achieve the
advantageous properties of an addition of Al and Si in addition to
Cr, the total content of Al+Si+Cr should be more than 1.2 wt.
%.
[0041] Molybdenum Mo: acts as a carbide-forming agent, increases
the strength and increases the resistance to delayed crack
formation and hydrogen embrittlement. Mo contents of more than 3
wt. % impair the elongation properties, for which reason a maximum
content of 3 wt. % is set. For the manganese steel in accordance
with the invention having average manganese contents, a Mo content
of 0.005 to 1.5 wt. % is preferred in order to avoid the
precipitation of excessively large Mo carbides. In particular,
contents of 0.01 wt. % to 0.6 wt. % bring about the precipitation
of desired Mo carbides while incurring reduced alloy costs.
[0042] Phosphorus P: is a trace element from iron ore and is
dissolved in the iron lattice as a substitution atom. Phosphorous
increases the hardness by means of solid solution hardening and
improves the hardenability. 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 phosphorous content is limited to a maximum of 0.1 wt.
%, wherein contents <0.04 wt. % are advantageously sought for
the aforementioned reasons.
[0043] 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 elongation 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 a maximum of 0.1 wt. %. In a particularly
advantageous manner the limit is <0.2 wt. % in order to reduce
the precipitation of MnS.
[0044] Nitrogen N: is likewise an associated element from steel
production. In the dissolved state, it improves the strength and
toughness properties in steels with a high manganese content of
greater than or equal to 4 wt. % Mn. Lower Mn-alloyed steels with
<4 wt. % Mn, which contain free nitrogen, tend to have a strong
aging effect. The nitrogen diffuses even 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 a maximum of 0.1 wt. %,
wherein contents <0.05 wt. % are preferably sought to
substantially avoid the formation of AlN.
[0045] 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 precipitate 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.
[0046] 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.
[0047] 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 elongation properties are improved.
Contents of more than 1.5 wt. % do not provide any further
advantages. Optionally, for vanadium and niobium, a minimum content
of greater than or equal to 0.005 wt. % and a maximum content of
0.6 (V) or 0.4 (Nb) wt. % is preferably provided, with which the
alloy elements advantageously provide grain refinement.
Furthermore, in order to improve the economic feasibility whilst at
the same time achieving optimum grain refinement, the contents of V
can continue to be restricted to 0.01 wt. % to 0.3 wt. % and the
contents of Nb to 0.01 to 0.1 wt. %.
[0048] Tantalum Ta: tantalum acts in a similar manner to niobium as
a carbide-forming agent in a grain-refining manner and thereby
improves the strength, toughness and elongation properties at the
same time. Contents over 0.5 wt. % do not provide any further
improvement in the properties. Thus, a maximum content of 0.5 wt. %
is optionally set. A minimum content of 0.005 wt. % and a maximum
content of 0.3 wt. % are preferred, with which the grain refinement
can advantageously be produced. In order to improve economic
feasibility and to optimise grain refinement, a content of 0.01 wt.
% to 0.1 wt. % is particularly preferably sought.
[0049] Titanium Ti: acts in a grain-refining manner as a
carbide-forming agent, whereby at the same time the strength,
toughness and elongation properties are improved, and reduces the
inter-crystalline corrosion. Ti contents of more than 1.5 wt. %
impair the elongation properties, for which reason a maximum Ti
content of 1.5 wt. % is set. Optionally, a minimum content of 0.005
and a maximum content of 0.6 wt. % are set, with which Ti is
advantageously precipitated. Preferably, a minimum content of 0.01
wt. % and a maximum content of 0.3 wt. % are provided, which
ensures optimum precipitation behaviour with low alloy costs.
[0050] Tin Sn: tin increases the strength but, similarly to copper,
accumulates beneath the scale layer and at the grain boundaries at
higher temperatures. Owing to the penetration into the grain
boundaries it leads to the formation of low melting point phases
and, associated therewith, to cracks in the microstructure and to
solder brittleness, for which reason a maximum content of
.ltoreq.0.5 wt. % is optionally provided. For the aforementioned
reasons, contents of <0.2 wt. % are preferably set. Contents of
<0.05 wt. % are particularly advantageously preferred in order
to avoid low melting point phases and cracks in the
microstructure.
[0051] Copper Cu: reduces the rate of corrosion and increases the
strength. Contents of over 3 wt. % impair producibility by forming
low melting point phases during casting and hot-rolling, for which
reason a maximum content of 3 wt. % is set. Optionally, a maximum
content of <0.5 wt. % is provided, with which the occurrence of
cracks during casting and hot-rolling can be advantageously
prevented. Cu contents of <0.1 wt. % have proved to be
particularly advantageous in avoiding low melting point phases and
in avoiding cracks.
[0052] Tungsten W: acts as a carbide-forming agent and increases
the strength and heat resistance. W contents of more than 5 wt. %
impair the elongation properties, for which reason a maximum
content of 5 wt. % is set. Optionally, a maximum content of 3 wt. %
and a minimum content of 0.01 wt. % is set, with which the
precipitation of carbides advantageously takes place. In
particular, a minimum content of 0.2 wt. % and a maximum content of
1.5 wt. % are preferred, which renders possible optimum
precipitation behaviour with low alloy costs.
[0053] Cobalt Co: increases the strength of the steel, stabilises
the austenite and improves the heat resistance. Contents of more
than 8 wt. % impair the elongation properties, for which reason a
maximum content of 8 wt. % is set. Optionally, a maximum content of
.ltoreq.5 wt. % and a minimum content of 0.01 wt. % is set which
advantageously improve the strength and heat resistance.
Preferably, a minimum content of 0.3 wt. % and a maximum content of
2 wt. % are provided which advantageously influences the austenite
stability along with the strength properties.
[0054] Zirconium Zr: acts as a carbide-forming agent and improves
the strength. Zr contents of more than 0.5 wt. % impair the
elongation properties, for which reason a maximum content of 0.5
wt. % is set. Optionally, a maximum content of 0.3 wt. % and a
minimum content of 0.005 wt. % is set, since in this range carbides
are advantageously precipitated. Preferably, a minimum content of
0.01 wt. % and a maximum content of 0.2 wt. % is provided which
advantageously render possible optimum carbide precipitation with
low alloy costs.
[0055] Boron B: delays the austenite conversion, improves the
hot-forming properties of steels and increases the strength at
ambient temperature. It achieves its effect even with very low
alloy contents. Contents above 0.15 wt. % greatly impair the
elongation and toughness properties, for which reason the maximum
content is set to 0.15 wt. %. Optionally, a minimum content of
0.001 wt. % and a maximum content of 0.08 wt. % are set, with which
the strength-increasing effect of boron is advantageously used. A
minimum content of 0.002 wt. % and a maximum content of 0.01 wt. %
are preferred which render possible optimum use for increasing
strength whilst at the same time improving the conversion
behaviour.
[0056] Tellurium Te: improves the corrosion-resistance and the
mechanical properties and machinability. Furthermore, Te increases
the solidity of MnS which, as a result, is lengthened to a lesser
extent in the rolling direction during hot-rolling and
cold-rolling. Contents above 0.5 wt. % impair the elongation and
toughness properties, for which reason a maximum content of 0.5 wt.
% is set. Optionally, a minimum content of 0.005 wt. % and a
maximum content of 0.3 wt. % are set, which advantageously improve
the mechanical properties and increase the solidity of MnS present.
Furthermore, a minimum content of 0.01 wt. % and a maximum content
of 0.1 wt. % are preferred which render possible optimisation of
the mechanical properties whilst at the same time reducing alloy
costs.
[0057] Calcium Ca: 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. % may be necessary. Contents above
0.1 wt. % 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.
[0058] A production route in accordance with the invention from the
melting of the steel to the finished steel strip with a required
end thickness of less than 10 mm, preferably less than 4 mm, from a
high-strength, manganese-containing steel comprises the following
steps:
[0059] melting a steel melt containing (in wt. %):
C: 0.0005 to 0.9
[0060] Mn: more than 3.0 to 12 with the remainder being iron
including unavoidable steel-associated elements, with optional
addition by alloying of one or more of the following elements (in
wt. %):
Al: to 10
Si: to 6
Cr: to 6
Nb: to 1.5
V: to 1.5
Ti: to 1.5
Mo: to 3
Cu: to 3
Sn: to 0.5
W: to 5
Co: to 8
Zr: to 0.5
Ta: to 0.5
Te: to 0.5
B: to 0.15
[0061] P: max. 0.1, in particular <0.04 S: max. 0.1, in
particular <0.02 N: max. 0.1, in particular <0.05
Ca: to 0.1
[0062] 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,
[0063] 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 re-heating the pre-strip, produced to approximately
the final dimensions, to 1000.degree. C. to 1200.degree. C. and
then hot-rolling the pre-strip to form a hot strip, or hot-rolling
the pre-strip without re-heating from the casting heat to form a
hot strip with optional intermediate heating between individual
rolling passes of the hot-rolling,
[0064] reeling the hot strip at a reeling temperature between
820.degree. C. and ambient temperature,
[0065] optionally annealing the hot strip with the following
parameters:
annealing temperature: 580 to 820.degree. C., annealing duration: 1
minute to 48 hours,
[0066] while avoiding cold-rolling at ambient temperature, rolling
the hot strip with a required end thickness of less than 10 mm to a
rolled steel strip at a temperature prior to the first pass above
50.degree. C. to 400.degree. C.
[0067] optionally annealing the steel strip with the following
parameters:
annealing temperature: 580 to 820.degree. C., annealing duration: 1
minute to 48 hours.
[0068] optionally acid-cleaning and/or skin-pass rolling the steel
strip,
[0069] optionally coating the steel strip with a corrosion
protection coating.
[0070] In relation to further advantages, reference is made to the
above statements.
[0071] Typical 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 having a thickness of 20 mm to 1.5 mm or the pre-strip,
cast to approximately the final dimensions, is hot-rolled to form a
hot strip having a thickness of 8 mm to 1 mm. The cold-rolled steel
strip produced in accordance with the invention has a thickness of
e.g. >0.15 mm to 10 mm.
[0072] 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 the individual rolling passes of the hot-rolling
process. If only a few rolling passes are then necessary, the
re-heating temperature can be selected at the lower end of the
range.
[0073] The hot strip can optionally be subjected to a heat
treatment in the temperature range between 580.degree. C. and
820.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 optional annealing serves to
reduce the strength and/or to increase the residual austenite
proportion of the hot strip prior to the cold-rolling process,
whereby the deformation properties are advantageously improved for
the subsequent process.
[0074] After the hot-rolling process, cold-rolling takes place with
the hot strip at a temperature raised in accordance with the
invention with the aim of setting the thicknesses of .gtoreq.0.15
mm to 10 mm for the steel strip as required for the end use.
Subsequent thereto, a further annealing process can optionally be
performed, if need be coupled with a coating process and finally a
skin-pass rolling process, by means of which the surface structure
required for the end use is set.
[0075] Preferably, the steel strip is galvanised by hot-dipping or
electrolytically or is coated metallically, inorganically or
organically.
[0076] A steel strip produced by the method in accordance with the
invention has a tensile strength Rm >800 to 2000 MPa and an
elongation at fracture A80 of 3 to 40%, preferably >8 to 40%. In
this case, high levels of strength tend to be associated with lower
elongations at fracture and vice versa.
[0077] The cold-rolled steel strip produced in accordance with the
invention can then be processed e.g. as a sheet metal portion, coil
or panel by cold-forming at ambient temperature or by warm-forming
at temperatures of 60.degree. C. to below the AC3, preferably
<450.degree. C., to form a component wherein by means of the
considerable residual deformation capability it is possible to
dispense with intermediate annealing depending on usage.
[0078] In further processing steps, the cold-rolled steel strip
produced in accordance with the invention can be processed to form
pipes with longitudinal or spiral weld seams, wherein in this case
also, by means of the considerable residual deformation capability
of the steel strip, it is possible to dispense with intermediate
annealing depending on usage. The pipe can thus comprise an outer
and/or inner metallic, organic or inorganic coating.
[0079] The pipe produced in this way can then be deformed further,
e.g. drawn or expanded or deformed using internal high pressure and
processed further to form a component.
[0080] Areas of usage are thus primarily the automotive or utility
vehicle industry and engineering, white goods, construction and
uses at temperatures below 0.degree. C. and as ballistic steel.
Ballistic steels are used in order to protect vehicles and
buildings against shelling and explosions, and have a high level of
hardness and toughness.
[0081] Trials have been carried out to investigate the mechanical
properties of the steel strips produced in accordance with the
invention, using e.g. alloys 1 to 4. The alloys 1 to 4 contain the
following elements in the stated quantities in wt. %:
TABLE-US-00001 Alloy C Mn Al Si Cr Mo 1 0.2 7.0 2.0 0.5 1.0 -- 2
0.2 7.0 0.9 0.5 -- -- 3 0.27 7.4 2.2 0.5 1.2 -- 4 0.21 7.2 2.5 0.5
1.2 0.16
[0082] For the purposes of comparison the steel strips produced
from the above-mentioned alloys 1 to 4 were cold-rolled, i.e. at
ambient temperature and therefore below 50.degree. C., and also
rolled in accordance with the invention at 250.degree. C. The
measured rolling forces are given as follows:
TABLE-US-00002 Rolling force Rolling force Degree of [kN] [kN]
deformation Reduction in cumulative - cumulative - (e =
.DELTA.d/d0) rolling force Alloy cold-rolling at 250.degree. C. [%]
[%] 1 103000 59000 44 ca. 43 2 144000 55000 44 ca. 62 3 161000
63000 44 ca. 60 4 107000 56000 44 ca. 43
[0083] Cumulative rolling force is understood to be the adding up
of the rolling forces of the individual passes in order to obtain a
comparable measure for the expenditure of force. The rolling force
was standardised to a band width of 1000 mm. The degree of
deformation e is defined as the quotient of the change in thickness
.DELTA.d of the steel strip under investigation and the initial
thickness d0 of the steel strip under investigation. The reduction
in rolling force is the calculated decrease in the rolling force at
250.degree. C. compared with the rolling force during
cold-rolling.
[0084] The elongation at fracture A50 was also evaluated:
TABLE-US-00003 Elongation at Elongation at fracture A50 [%]
fracture A50 [%] Alloy cold-rolled rolled at 250.degree. C. 1 2.0
15.5 2 2.5 20.5 3 3.5 19.0 4 3.0 18.5
[0085] The elongation characteristic values represent the
elongation in the rolling direction.
* * * * *