U.S. patent application number 12/673388 was filed with the patent office on 2010-10-28 for dual-phase steel, flat product made of a dual-phase steel of this type and processes for the production of a flat product.
This patent application is currently assigned to THYSSENKRUPP STEEL EUROPE AG. Invention is credited to Ekaterina Bocharova, Thomas Heller, Dorothea Mattissen, Thomas Nickels, Gunter Stich, Silke Strauss.
Application Number | 20100273024 12/673388 |
Document ID | / |
Family ID | 38984114 |
Filed Date | 2010-10-28 |
United States Patent
Application |
20100273024 |
Kind Code |
A1 |
Bocharova; Ekaterina ; et
al. |
October 28, 2010 |
DUAL-PHASE STEEL, FLAT PRODUCT MADE OF A DUAL-PHASE STEEL OF THIS
TYPE AND PROCESSES FOR THE PRODUCTION OF A FLAT PRODUCT
Abstract
A dual-phase steel, a flat product produced therefrom and a
process for the production thereof. The dual-phase steel has, in
addition to a strength of at least 950 MPa and good deformability,
a surface finish which, when a simple production process is used,
makes it possible for the flat product produced from this steel to
be formed into a complexly formed component, such as a part of a
car bodywork, in an uncoated state or in a state provided with an
anti-corrosion coating. The steel according to the invention
comprises 20-70% martensite, up to 8% retained austenite and the
remainder ferrite and/or bainite and comprises (in % by weight): C:
0.10-0.20%, Si: 0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20-0.80%, Ti:
0.02-0.08%, B: <0.0020%, Mo: <0.25%, Al: <0.10%, P:
.ltoreq.0.2%, S: .ltoreq.0.01%, N: .ltoreq.0.012%, the remainder
iron and unavoidable impurities.
Inventors: |
Bocharova; Ekaterina;
(Duisburg, DE) ; Heller; Thomas; (Duisburg,
DE) ; Mattissen; Dorothea; (Mulheim An Der Ruhr,
DE) ; Stich; Gunter; (Bochum, DE) ; Strauss;
Silke; (Hunxe, DE) ; Nickels; Thomas;
(Ratingen, DE) |
Correspondence
Address: |
THE WEBB LAW FIRM, P.C.
700 KOPPERS BUILDING, 436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
THYSSENKRUPP STEEL EUROPE
AG
Duisburg
DE
|
Family ID: |
38984114 |
Appl. No.: |
12/673388 |
Filed: |
August 7, 2008 |
PCT Filed: |
August 7, 2008 |
PCT NO: |
PCT/EP2008/060381 |
371 Date: |
July 7, 2010 |
Current U.S.
Class: |
428/659 ;
148/330; 148/333; 148/334; 148/546; 164/76.1; 428/684 |
Current CPC
Class: |
C21D 8/02 20130101; Y10T
428/12799 20150115; Y10T 428/12972 20150115; C22C 38/02 20130101;
C22C 38/38 20130101; C22C 38/28 20130101 |
Class at
Publication: |
428/659 ;
164/76.1; 148/546; 148/330; 148/333; 148/334; 428/684 |
International
Class: |
B32B 15/01 20060101
B32B015/01; C21D 8/02 20060101 C21D008/02; C22C 38/32 20060101
C22C038/32; C22C 38/00 20060101 C22C038/00; C22C 38/22 20060101
C22C038/22; B32B 15/00 20060101 B32B015/00; B32B 15/18 20060101
B32B015/18 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 15, 2007 |
EP |
07114398.6 |
Claims
1-30. (canceled)
31. A dual-phase steel, having a structure comprising 20-70%
martensite, up to 8% retained austenite and the remainder of
ferrite and/or bainite and a tensile strength of at least 950 MPa,
comprising (in % by weight): TABLE-US-00006 C: 0.10-0.20%, Si:
0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20-0.80%, Ti: 0.02-0.08%, B:
<0.0020%, Mo: <0.25%, Al: <0.10%, P: .ltoreq.0.2%, S:
.ltoreq.0.01%, N: .ltoreq.0.012%
the remainder being iron and unavoidable impurities.
32. The dual-phase steel according to claim 31, wherein the yield
strength thereof is at least 580 MPa.
33. The dual-phase steel according to claim 31, wherein the
elongation A.sub.80 thereof is at least 10%.
34. The dual-phase steel according to claim 31, wherein the P
content thereof is <0.1% by weight.
35. The dual-phase steel according to claim 31, wherein the C
content thereof is from 0.12 to 0.18% by weight.
36. The dual-phase steel according to claim 31, wherein the Si
content thereof is from 0.20 to 0.40% by weight.
37. The dual-phase steel according to claim 31, wherein the Mn
content thereof is from 1.50 to 2.35% by weight.
38. The dual-phase steel according to claim 31, wherein the Cr
content thereof is from 0.30 to 0.70% by weight.
39. The dual-phase steel according to claim 31, wherein the Ti
content thereof is from 0.030 to 0.055% by weight.
40. The dual-phase steel according to claim 31, wherein in the
presence of N, the Ti content of said dual-phase steel is more than
5.1 times greater than the respective N content.
41. The dual-phase steel according to claim 31, wherein the B
content thereof is from 0.0005 to 0.0020% by weight.
42. The dual-phase steel according to claim 41, wherein the B
content thereof is from 0.0007 to 0.0016% by weight.
43. The dual-phase steel according to claim 31, wherein the Mo
content thereof is from 0.05 to 0.22% by weight.
44. The dual-phase steel according to claim 43, wherein the Mn
content thereof is 1.5 to 1.7% by weight.
45. The dual-phase steel according to claim 43, wherein the Cr
content thereof is 0.20 to 0.4% by weight.
46. The dual-phase steel according to claim 31, wherein the Mo
content thereof is from 0.065 to 0.150% by weight.
47. The dual-phase steel according to claim 31, wherein the Al
content thereof is from 0.01 to 0.06% by weight.
48. The dual-phase steel according to claim 31, wherein the S
content thereof is <0.003% by weight.
49. The dual-phase steel according to claim 31, wherein the N
content thereof is <0.007% by weight.
50. The dual-phase steel according to claim 31, wherein the
retained austenite content thereof is less than 7%.
51. A flat product comprising a dual-phase steel according to claim
31.
52. The flat product according to claim 51, wherein it is a hot
strip which has only been hot-rolled.
53. The flat product according to claim 51, wherein it is a cold
strip obtained by cold rolling.
54. A flat product according to claim 51, further comprising a
protective metallic coating.
55. The flat product according to claim 54, wherein the protective
metallic coating is produced by hot-dip galvanisation.
56. The flat product according to claim 54, wherein the protective
metallic coating is produced by galvannealing.
57. A process for the production of a hot strip having a tensile
strength of at least 950 MPa and a dual-phase structure comprising
20-70% martensite, up to 8% retained austenite and the remainder
ferrite and/or bainite, comprising the following steps: melting a
dual-phase steel obtained according to claim 31, casting the melt
into a pre-product, such as slab or thin slab, reheating to or
keeping the pre-product at a starting hot rolling temperature of
1100-1300.degree. C., hot rolling the pre-product at a final hot
rolling temperature of 800-950.degree. C. into a hot strip, and
reeling the hot strip at a reeling temperature of up to 570.degree.
C.
58. A process for the production of a cold strip having a tensile
strength of at least 950 MPa and a dual-phase structure comprising
20-70% martensite, up to 8% retained austenite and the remainder
ferrite and/or bainite, comprising the following steps: melting a
dual-phase steel composed according to claim 31, casting the melt
into a pre-product, such as slab or thin slab, reheating to or
keeping the pre-product at a starting hot rolling temperature of
1100-1300.degree. C., hot rolling the pre-product at a final hot
rolling temperature of 800-950.degree. C. into a hot strip, reeling
the hot strip at a reeling temperature of 500-650.degree. C.,
cold-rolling the hot strip after reeling, annealing the cold strip
at an annealing temperature of 700-900.degree. C., and cooling the
annealed cold strip in a controlled manner.
59. A process according to claim 58, wherein the hot strip is
cold-rolled into a cold strip with a degree of cold-rolling of from
40 to 70%.
60. A process according to claim 58, wherein the controlled cooling
is carried out within a temperature range of from 550 to
650.degree. C. at a cooling rate of at least 10 K/s.
Description
[0001] The invention relates to a dual-phase steel, the structure
of which substantially consists of martensite and ferrite and
respectively bainite, it being possible for portions of retained
austenite to be present and the dual-phase steel having a tensile
strength of at least 950 MPa. The invention also relates to a flat
product produced from a dual-phase steel of this type as well as to
processes for the production of this flat product.
[0002] The generic term "flat product" as used herein typically
includes steel strips and sheets of the type according to the
invention.
[0003] In the field of vehicle body construction, there is a demand
for steels which on the one hand have a high strength with a low
weight, but on the other hand also have a good deformability.
Numerous attempts are known at producing steels which combine these
contradictory characteristics.
[0004] Thus, for example, EP 1 431 107 A1 discloses a steel which
is not only to have an effective deep-drawing property but also a
high tensile strength, and a flat product produced therefrom and a
process for the production thereof. The known steel contains, in
addition to iron and unavoidable impurities (in % by weight)
0.08-0.25% C, 0.001-1.5% Si, 0.01-2.0% Mn, 0.001-0.06% P, up to
0.05% S, 0.001-0.007% N and 0.008-0.2% Al. At the same time, it
should have an average r-value of at least 1.2, an r-value in the
rolling direction of at least 1.3, an r-value in a direction of
45.degree. based on the rolling direction of at least 0.9 and an
r-value transversely to the rolling direction of at least 1.2. In
the known steel, a strength-increasing effect is attributed to
silicon, the upper limit of 1.5% by weight having been chosen in
respect of an effective coatability of the steel. The positive
influence of Mn on the strength is also emphasised. In this
respect, the upper limit of the Mn content of 1.5% was set in
respect of the decrease in the r-values which accompany any
exceeding of this limit, and to optimise the r-values of the known
steel sheet, Mn contents ranging from 0.04 to 0.8% by weight, in
particular from 0.04 to 0.12% by weight were considered
advantageous.
[0005] To further increase the strength of the known steel, it can
optionally also contain, in addition to other selectively added
alloying elements, contents of B of 0.0001-0.01% by weight, of Ti,
Nb and/or V in a total quantity of 0.001-0.2% by weight as well as
contents of Sn, Cr, Cu, Ni, Co, W and/or Mo in a total quantity of
0.001-2.5% by weight. The total content of these elements is
restricted to the respectively stated upper limit for reasons of
cost.
[0006] If the steels described in EP 1 431 407 A1 have strengths of
more than 850 MPa, they no longer have a dual-phase structure, but
their structure either consists only of martensite or only of
ferrite and respectively bainite. Furthermore, EP 1 431 407 A1 does
not provide an example by which, for example the effects of Cr, Mo,
Ti or B could be reproduced at the same time with small amounts of
Si or relatively high contents of Mn. Instead, the examples given
in EP 1 431 407 A1 prove that according to this prior art, the
strength has been substantially adjusted by an appropriate
coordination of the Mn and Si contents with the respective steel
alloy.
[0007] A further possibility of producing flat products which
consist of relatively high-strength dual-phase steels and which
still have good mechanical-technological characteristics even after
undergoing an annealing process with the inclusion of an overaging
treatment is disclosed in EP 1 200 635 A1. In the process known
from this document, a steel strip or sheet is produced which has a
predominantly ferritic-martensitic structure in which the
martensitic proportion is from 4 to 20%, the steel strip or sheet
containing, in addition to Fe and melt-induced impurities (in % by
weight) 0.05-0.2% C, up to 1.0% Si, up to 2.0% Mn, up to 0.1% P, up
to 0.015% S, 0.02-0.4% Al, up to 0.005% N, 0.25-1.0% Cr,
0.002-0.01% B. The martensitic proportion of the respective steel
preferably amounts to approximately 5 to 20% of the predominantly
martensitic-ferritic structure. A flat product produced in this
manner has strengths of at least 500 N/mm.sup.2 with a
simultaneously good forming ability without requiring, for this
purpose, particularly high contents of specific alloying
elements.
[0008] To increase the strength, the transformation-influencing
effect of the element boron is drawn on in the case of the steel
described in EP 1 200 635 A1. In the known steel, the
strength-increasing effect of boron is ensured in that at least one
alternative nitride former, preferably Al and additionally Ti is
added to the steel material. The effect of adding titanium and
aluminium is to bind the nitrogen contained in the steel, such that
boron is available to form hardness-increasing carbides. Supported
by the necessarily present Cr content, a higher strength level is
achieved in this manner compared to comparable steels. However, the
maximum strength of the steels stated by way of example in EP 1 200
635 is less than 900 MPa in each case.
[0009] Finally, EP 1 559 797 A1 discloses a relatively
high-strength dual-phase steel which has a structure comprising
more than 60% ferrite and from 5-30% martensite and which contains,
in addition to iron and unavoidable impurities (in % by weight)
0.05-0.15% C, up to 0.5% Si, 1-2% Mn, 0.01-0.1% Al, up to 0.009% P,
up to 0.01% S and up to 0.005% N. In order to further increase the
strength of this known steel, it is possible to add thereto
0.01-0.3% Mo, 0.001-0.05% Nb, 0.001-0.1% Ti, 0.0003-0.002% B, and
0.05-0.49% Cr. The known steel alloyed and obtained in this manner
achieves tensile strengths of up to 700 MPa with a good
deformability and surface finish. The objective of the development
described in EP 1 559 797 A1 was an improvement in the mechanical
characteristics of a steel of this type while avoiding an alloying
of relatively large amounts of alloying elements, such as Si, P and
Al which are critical in respect of surface finish, weldability and
deformability.
[0010] Against the background of the prior art described above, the
object of the invention was to develop a steel and a flat product
produced therefrom which has a strength of at least 950 MPa and a
good deformability. Furthermore, the steel should have a surface
finish which, when using a simple production process, enables a
flat product produced from this steel to be deformed in an uncoated
state or in a state provided with an anti-corrosion coating, into a
complexly formed component, such as a part of a car bodywork. In
addition, a process is also to be provided which makes it easily
possible to produce flat products obtained in the manner described
above.
[0011] With respect to the material, this object is achieved
according to the invention by the dual-phase steel stated in claim
1. Advantageous embodiments of this steel are set out in the claims
referring to claim 1.
[0012] A flat product which achieves the aforementioned object is
characterised according to the invention in accordance with claim
20 in that it consists of a steel which is composed and obtained
according to the invention.
[0013] Finally, with respect to the production process, the
aforementioned object is achieved according to the invention by the
production methods stated in claims 26 and 27, the process stated
in claim 26 relating to the production according to the invention
of a hot strip and the procedural method stated in claim 27
relating to the production according to the invention of a cold
strip. The claims referring to claims 26 and 27 each contain
advantageous variants of the processes according to the invention.
In addition, particularly advantageous embodiments are described
below for the practical application of the processes according to
the invention and of the variants thereof stated in the claims.
[0014] A steel according to the invention is characterised by high
strengths of at least 950 MPa, in particular more than 980 MPa,
while strengths of 1000 MPa and above are also routinely achieved.
This steel simultaneously has a yield strength of at least 580 MPa,
in particular at least 600 MPa, and has an elongation A.sub.80 of
at least 10%.
[0015] Due to the combination of high strength and good
deformability, the steel according to the invention is particularly
suitable for the production of complexly formed components which
are heavily stressed in practical use, as required for example in
the field of car body construction.
[0016] Due to its dual-phase structure, the steel according to the
invention has a high strength with a simultaneously good
elongation. Thus, the alloy of a steel according to the invention
is composed such that it has a martensitic proportion of at least
20%, preferably more than 30%, up to a maximum of 70%. At the same
time, retained austenite portions of up to 8% can be advantageous,
while smaller retained austenite proportions of at most 7% or less
are generally preferred. The remainder of the structure of a
dual-phase steel according to the invention consists respectively
of ferrite and/or bainite (bainitic ferrite+carbides).
[0017] The high strengths and good elongation characteristics are
achieved by the adjustment according to the invention of the
dual-phase structure. This is enabled by a narrow choice of the
contents of the individual alloying elements which are present in a
steel according to the invention in addition to iron and
unavoidable impurities.
[0018] Thus, the invention provides a C content of from 0.10-0.20%
by weight. The minimum content of carbon of 0.10% by weight is
selected in order to obtain the formation of the martensitic
structure with sufficient hardness and to adjust the desired
combination of characteristics of the steel according to the
invention. However, where there are contents of more than 0.20% by
weight, carbon hinders the formation of the desired
ferritic/bainitic structural portion. Higher contents of carbon
also have a negative effect on the welding suitability, which is
particularly significant for the application of the material
according to the invention in the field of automotive engineering,
for example. The advantageous effect of carbon in a steel according
to the invention can be used in a particularly reliable manner when
the carbon content of a steel according to the invention is from
0.12 to 0.18% by weight, in particular from 0.15 to 0.16% by
weight.
[0019] Si also serves in a steel according to the invention to
increase the strength by hardening the ferrite or bainite. In order
to be able to use this effect, a minimum Si content of 0.10% by
weight is provided, the effect of Si emerging in a particularly
reliable manner when the Si content of a steel according to the
invention is at least 0.2% by weight, in particular at least 0.25%
by weight. In respect of the fact that a flat product produced from
a steel according to the invention is to have a surface finish
which is optimum for further processing and, if necessary, for
applied coatings, the upper limit of the Si content is
simultaneously set at 0.6% by weight. The risk of grain boundary
oxidation is also minimised when this upper limit is observed. An
unfavourable influence of Si on the characteristics of the steel
according to the invention can be avoided with even greater
reliability by restricting the Si content of the steel according to
the invention to 0.4% by weight, in particular to 0.35% by
weight.
[0020] The Mn content of a steel according to the invention is
within a range of from 1.5 to 2.50% by weight, in particular from
1.5 to 2.35% by weight in order to use the strength-increasing
effect of this element. Thus, the presence of Mn promotes the
formation of martensite. If a cold strip is produced from the steel
according to the invention and said cold strip is annealed at the
end of processing, the contents of Mn provided according to the
invention prevent the formation of pearlite during cooling after
annealing. These positive effects due to the presence of Mn in a
steel according to the invention can be used in a particularly
reliable manner when the Mn content is at least 1.7% by weight, in
particular at least 1.80% by weight. However, in order to avoid a
negative influence of Mn on the deformability, welding suitability
and coatability, the upper limit for the contents of Mn is set at
2.5% by weight in the steel according to the invention. The
possibly negative influences of Mn on a steel according to the
invention can be ruled out with greater reliability by restricting
the Mn content to 2.20% by weight, in particular 2.00% by
weight.
[0021] Cr also has a strength-increasing effect in a dual-phase
steel according to the invention in contents of from 0.2 to 0.8% by
weight. This effect appears in particular when the Cr content is at
least 0.3% by weight, in particular at least 0.5% by weight.
However, the Cr content of a steel according to the invention is
restricted at the same time to 0.8% by weight to reduce the risk of
grain boundary oxidation and to ensure good elongation
characteristics of the steel according to the invention.
Furthermore, when this upper limit is observed, a surface is
achieved which can be effectively provided with a metallic coating.
Negative influences of the Cr contents are avoided in particular
when the upper limit of the chromium content of a steel according
to the invention is set at a maximum of 0.7% by weight, in
particular at 0.6% by weight.
[0022] The presence of titanium in contents of at least 0.02% by
weight also contributes to the increase in the strength of a steel
according to the invention in that it forms fine deposits of TiC or
Ti (C,N) and contributes to the grain refining. A further positive
effect of Ti is the binding of nitrogen which may be present,
thereby preventing the formation of boron nitrides in the steel
according to the invention. These would have a strong negative
influence on the elongation characteristics and also on the
deformability of a flat product according to the invention. Thus,
when boron is added to increase the strength, the presence of Ti
also ensures that the boron can fully develop its effect. For this
purpose, it can be favourable for Ti to be added in a quantity
which is more than 5.1 times the respective N content (i.e. Ti
content>1.5 (3.4.times.N content)). Excessively high Ti contents
result, however, in unfavourably high recrystallisation
temperatures, which has a particularly negative effect when
cold-rolled flat products are produced from steel according to the
invention which are annealed in the final processing stage. For
this reason, the upper limit of the Ti content is restricted to
0.08% by weight, in particular to 0.06% by weight. The positive
effect of Ti can be used in a particularly reliable manner on the
characteristics of a steel according to the invention when its Ti
content is from 0.03 to 0.055% by weight, in particular from 0.040
to 0.050% by weight.
[0023] The strength of the steel according to the invention is also
increased by the contents of B of up to 0.002% by weight, which are
optionally provided according to the invention and, as by the
respective addition of Mn, Cr and Mo, when cold strip is produced
from steel according to the invention, the critical cooling rate is
reduced after annealing. For this reason, according to a
particularly practice-oriented embodiment of the invention, the B
content is at least 0.0005% by weight. However, at the same time
excessively high contents of B can reduce the deformability of the
steel according to the invention and adversely affect the
development of the dual-phase structure which is desired according
to the invention. Therefore, optimised effects of boron are
provided in a steel according to the invention with contents of
0.0007-0.0016% by weight, in particular 0.0008-0.0013% by
weight.
[0024] Like Boron or Cr in the aforementioned content ranges, the
contents of molybdenum which are optionally present according to
the invention also contribute to increasing the strength of a steel
according to the invention. In this respect, according to
experience, the presence of Mo does not have a negative effect on
the coatability of the flat product with a metallic coating or on
its extensibility. Practical tests have shown that the positive
influences of Mo can be used particularly effectively up to
contents of 0.25% by weight, in particular 0.22% by weight, also
from a financial point of view. Thus, even contents of 0.05% by
weight of Mo have a positive effect on the characteristics of the
steel according to the invention. Where there are sufficient
quantities of other strength-increasing elements, the desired
effect of molybdenum in a steel according to the invention emerges
in particular when its Mo content is from 0.065 to 0.18% by weight,
in particular from 0.08 to 0.13% by weight. However, if the steel
according to the invention contains less than 1.7% by weight of Mo
and/or less than 0.4% by weight of Cr, it is advantageous to add
from 0.05 to 0.22% by weight of Mo to ensure the required strength
of the steel according to the invention.
[0025] When a steel according to the invention is melted, aluminium
is used for deoxidisation and for binding nitrogen which may be
contained in the steel. For this purpose, Al can be added if
necessary in contents of less than 0.1% by weight to the steel
according to the invention, the desired effect of Al ensuing in a
particularly reliable manner when the contents thereof are within a
range of from 0.01 to 0.06% by weight, in particular from 0.020 to
0.050% by weight.
[0026] Nitrogen is permitted in the steel according to the
invention only in contents of up to 0.012% by weight particularly
to avoid the formation of boron nitrides when B is simultaneously
present. To reliably prevent the respectively present titanium from
bonding completely with N and no longer being effective as a
micro-alloying element, the N content is preferably restricted to
0.007% by weight.
[0027] Low contents of P which are below the upper limit provided
according to the invention contribute to the good welding ability
of the steel according to the invention. Therefore, according to
the invention, the P content is preferably restricted to <0.1%
by weight, in particular to <0.02% by weight, particularly good
results being obtained with P contents of <0.010% by weight.
[0028] Where there are contents of sulphur below the upper limit
provided according to the invention, the formation of MnS or
(Mn,Fe) S is suppressed, thereby ensuring a good extensibility of
the steel according to the invention and of the flat products
produced therefrom. This is particularly the case when the S
content is below 0.003% by weight.
[0029] In a manner according to the invention, flat products
consisting of a dual-phase steel according to the invention can be
delivered directly, i.e. without a subsequently performed cold
rolling process, for further processing as a hot strip obtained
after hot rolling. Thus, highly stress-resistant components in an
uncoated state can be formed from the hot strip obtained according
to the invention. If these components are to be protected in
particular against corrosion, the hot strips can be provided with a
protective metallic coating before or after they are formed into
the respective component.
[0030] If, on the other hand, flat products of a relatively low
thickness are required, the hot strips produced from the steel
according to the invention can firstly undergo cold rolling and
subsequently annealing in order to then be further processed as a
cold strip, optionally after the application of a metallic
anti-corrosion coating.
[0031] If the flat product according to the invention is provided
with a protective metallic coating, this can be performed, for
example by hot-dip galvanising, by a galvannealing treatment or by
electrolytic coating. If required, a pre-oxidation process can be
carried out before coating, in order to ensure a reliable bonding
of the metallic coating on the substrate to be respectively
coated.
[0032] To produce according to the invention a flat product which
is present as a hot strip and has a tensile strength of at least
950 MPa and a dual-phase structure consisting to 20 to 70% of
martensite, up to 8% of retained austenite and for the remainder of
ferrite and/or bainite, a dual-phase steel, composed according to
the invention, is firstly melted, the melt is cast into a
pre-product, such as a slab or thin slab, the pre-product is
reheated to or kept at a hot rolling starting temperature of from
1100 to 1300.degree. C., the pre-product is hot rolled into the hot
strip at a hot rolling final temperature of from 800 to 950.degree.
C. and the resulting hot strip is reeled at a reeling temperature
of up to 570.degree. C.
[0033] By suitably adjusting the reeling temperature within a range
of from room temperature to 570.degree. C., the dual-phase
structure of the hot strip which is not then rolled any further as
such can be adjusted in order to obtain the respectively desired
combination of characteristics.
[0034] If the hot strip, obtained in the manner according to the
invention, is to remain uncoated or is to be electrolytically
coated as a hot strip with a metallic coating, the flat product
does not have to be annealed. If, on the other hand, the hot strip
is to be coated with a metallic coating by hot-dip galvanisation,
it is firstly annealed at a maximum annealing temperature of
600.degree. C. and then cooled to the temperature of the coating
bath, which can be, for example, a zinc bath. After passing through
the zinc bath, the coated hot strip can be cooled to room
temperature in a conventional manner.
[0035] If a flat product according to the invention is to be
provided in the form of a cold strip, then for this purpose a
dual-phase steel composed according to the invention is melted, the
corresponding steel melt is cast into a pre-product, such as a slab
or thin slab, the pre-product is reheated to or kept at a hot
rolling starting temperature of from 1100 to 1300.degree. C., the
pre-product is hot rolled into a hot strip at a hot rolling final
temperature of from 800 to 950.degree. C., the hot strip is reeled
at a reeling temperature of from 500 to 650.degree. C., the hot
strip is then cold rolled, the resulting cold strip is annealed at
an annealing temperature of from 700 to 900.degree. C. and
thereafter the cold strip is cooled in a controlled manner.
[0036] Reeling temperatures ranging up to 580.degree. C. have
proved to be particularly advantageous in connection with the
production of a cold strip, because if the reeling temperature of
580.degree. C. is exceeded, the risk of grain boundary oxidation
increases. With low reeling temperatures, the strength and yield
strength of the hot strip increase such that it becomes
increasingly difficult to cold roll the hot strip. Accordingly, the
hot strip which is to be cold rolled into a cold strip is
preferably reeled at a temperature of at least 530.degree. C., in
particular at least 550.degree. C.
[0037] If the cold strip produced according to the invention is to
remain uncoated or is to be coated electrolytically, an annealing
treatment is carried out in a continuous annealing furnace as a
separate working step. The maximum annealing temperatures which are
achieved are within a range of from 700 to 900.degree. C. at
heating rates of from 1 to 50 K/s. Subsequently, for the
intentional adjustment of the combination of characteristics
desired according to the invention, the annealed cold strip is
preferably cooled such that cooling rates of at least 10 K/s are
achieved within a temperature range of from 550 to 650.degree. C.
in order to suppress the formation of pearlite. After reaching the
temperature in this critical range, the strip can be kept for a
period of 10 to 300 s or can be cooled directly to room temperature
at a cooling rate of from 0.5 to 30 K/s.
[0038] However, if the cold strip is to be coated by hot-dip
galvanisation, the annealing and coating steps can be combined. In
this case, the cold strip passes in a continuous sequence through
various furnace sections of a hot-dip coating line, different
temperatures prevailing in the individual furnace sections and
reaching a maximum of from 700 to 900.degree. C., in which case
heating rates ranging from 2 to 100 K/s should be selected. After
the respective annealing temperature has been attained, the strip
is then kept at this temperature for 10 to 200 s. The strip is then
cooled to the temperature, usually below 500.degree. C., of the
respective coating bath which is typically a zinc bath, and in this
case as well the cooling rate should be more than 10 K/s within a
temperature range of from 550 to 650.degree. C. After reaching this
temperature stage, the cold strip can optionally be kept at the
respective temperature for 10 to 300 s. The annealed cold strip
then passes through the respective coating bath which is preferably
a zinc bath. Subsequently, the cold strip is either cooled to room
temperature in order to obtain a conventional hot-dip galvanised
cold strip, or is rapidly heated, then cooled to room temperature
to produce a galvannealed cold strip.
[0039] If the hot strip is cold rolled into a cold strip, it has
proved to be favourable to adjust cold rolling degrees of from 40
to 70%, in particular from 50 to 60% to achieve sufficiently high
strengths of the rolled strip with an optimum utilisation of the
respectively available installation engineering. A cold strip
according to the invention which is cold-rolled in this manner
typically has thicknesses of from 0.8 to 2.5 mm.
[0040] If necessary, the cold strip can undergo a skin pass rolling
in a coated or uncoated state, with the adjustment of skin pass
rolling degrees ranging up to 2%.
[0041] The invention will be described in detail in the following
with reference to practical examples.
[0042] Sixteen steel melts 1 to 16, the compositions of which are
stated in Table 1 were melted in conventional manner and cast into
slabs. The slabs were then reheated in a furnace to 1200.degree. C.
and hot rolled in conventional manner starting from this
temperature. The final rolling temperature was 900.degree. C.
[0043] For a first series of tests, the hot strips obtained thus
were reeled at a reeling temperature of 550.degree. C. which was
adjusted with an accuracy of +/-30.degree. C., before they were
cold rolled with a cold rolling degree of 50%, 65% and 70% into a
cold strip having a thickness of from 0.8 mm to 2 mm.
[0044] The cold strips which were obtained then underwent annealing
and controlled cooling procedures in the manner described above in
a general form for a cold strip which is to be delivered
uncoated.
[0045] Table 2 states the structural state, the mechanical
characteristics and the respectively adjusted degrees of cold
rolling and the strip thicknesses for the cold strips produced in
the first series of tests from melts 1 to 16.
[0046] In three further series of tests, the hot strips produced
from melts 1 to 16 in the manner described above were reeled at a
reeling temperature below 100.degree. C., at a temperature of
500.degree. C. and at a temperature of 650.degree. C. The
characteristics determined for these hot strips are stated in Table
3 (reeling temperature 20.degree. C.), Table 4 (reeling
temperature=500.degree. C.) and Table 5 (reeling
temperature=570.degree. C.). The hot strips obtained thus were not
intended for cold rolling, but were forwarded for further
processing into components, optionally after being provided with a
protective metallic coating.
TABLE-US-00001 TABLE 1 Melt C Si Mn Al Mo Ti Cr B P S N 1 0.149
0.30 1.97 0.007 -- -- 0.45 0.0004 0.003 0.004 0.0013 2 0.150 0.30
1.97 <0.005 -- 0.023 0.45 0.0021 0.005 0.004 0.015 3 0.152 0.30
1.99 0.005 -- -- 0.46 0.0004 0.004 0.004 0.0014 4 0.157 0.30 1.97
0.005 -- -- 0.81 0.0005 0.004 0.004 0.0017 5 0.153 0.30 1.50 0.005
-- -- 0.81 0.0004 0.004 0.004 0.0015 6 0.150 0.02 1.98 <0.005 --
0.023 0.80 0.0022 0.004 0.005 0.0015 7 0.152 0.60 1.97 <0.005 --
0.021 0.45 0.0022 0.004 0.004 0.0024 8 0.154 0.19 2.07 0.004 --
0.022 0.60 0.0011 0.004 0.007 0.0052 9 0.16 0.29 1.8 0.032 0.08
0.046 0.52 0.0009 0.013 0.001 0.004 10 0.152 0.28 1.7 0.028 0.15
0.051 0.3 0.0012 0.008 0.001 0.0045 11 0.145 0.21 1.7 0.036 0.19
0.035 0.45 0.0010 0.011 0.0015 0.0042 12 0.148 0.24 1.83 0.031 0.22
0.035 0.65 0.0012 0.010 0.0015 0.0042 13 0.153 0.29 2.2 0.029 0.08
0.090 0.59 0.0018 0.012 0.0013 0.0051 14 0.19 0.22 1.75 0.033 0.18
0.052 0.51 0.0009 0.007 0.0020 0.0031 15 0.12 0.27 2.35 0.027 --
0.051 0.5 0.0012 0.014 0.0012 0.0029 16 0.1 0.31 2.31 0.031 0.22
0.086 0.66 0.0016 0.013 0.0016 0.0047
[0047] Amounts in % by weight, remainder iron and unavoidable
impurities
TABLE-US-00002 TABLE 2 Structural proportions Degree of Retained
cold- Sheet R.sub.p0.2 R.sub.m A.sub.80 Martensite Bainite
austenite rolling thickness Melt [MPa] [%] Matrix [%] [%] [%]
Carbides [%] [mm] 1 580 955 15.2 Ferrite 20 some -- -- 50 2.0
bainite 2 598 1057 8.3 Ferrite 50 Some -- -- 65 1.2 bainite 3 581
970 14.9 Ferrite 30-40 Bainite -- Carbides 50 2.0 4 590 1023 12.5
Ferrite 20-0 10 Carbides 70 0.8 5 585 960 17.1 Ferrite 20 -- --
Carbides 50 2.0 6 601 997 8.6 Bainite 50 -- -- Carbides 50 2.0 7
607 1038 10.8 Bainite + 10% 50 -- -- Carbides 70 0.8 Ferrite 8 602
992 14 Bainite 40-45 -- 6.5 -- 50 2.0 9 645 1071 14.8 Ferrite 50-60
-- 2.5 -- 50 2.0 10 635 1054 15.1 Ferrite 45 -- 2.0 -- 70 0.8 11
618 1035 15.3 Ferrite 30-40 -- 1 -- 65 1.2 12 626 1047 14.2
Ferrite/ 40-50 -- -- -- 70 0.8 Bainite 13 675 1102 10.5 Bainite/
60-70 -- -- -- 50 2.0 Ferrite 14 609 1031 15.4 Ferrite 35-45 -- 3
-- 50 2.0 15 612 1010 11.3 Ferrite 40 -- 1.5 -- 65 1.2 16 603 1016
13.6 Ferrite 55-65 -- 3 -- 65 1.2
TABLE-US-00003 TABLE 3 Structual proportions R.sub.p0.2 R.sub.m
A.sub.80 Martensite Melt [MPa] [MPa] [%] Matrix [%] 1 580 950 12.3
Bainite/ 20 Ferrite 2 621 1023 11.5 Bainite 20-30 3 614 985 13.4
Bainite/ 25-30 Ferrite 4 639 1012 12.9 Bainite 25 5 580 950 14.5
Bainite 20 6 725 996 13.7 Bainite 25 7 594 998 13.5 Bainite 20-30 8
731 1005 13.9 Bainite 25-35 9 1070 1129 12.1 Bainite 45-55 10 642
1014 13.4 Bainite 30-35 11 626 1007 14.8 Bainite 25-35 12 640 1017
15.7 Bainite 20-30 13 854 1121 10.7 Bainite 60-70 14 674 1014 12.8
Bainite 25-35 15 685 1027 12.7 Bainite 35-45 16 691 1031 13.8
Bainite 30-40
TABLE-US-00004 TABLE 4 Structural proportions Retained R.sub.p0.2
R.sub.m A.sub.80 Martensite austenite Melt [MPa] [MPa] [%] Matrix
[%] [%] 1 580 950 14 Bainite/ 20 -- Ferrite 2 600 985 12 Bainite 25
3 3 630 970 14 Bainite/ 20 1 Ferrite 4 580 950 15 Bainite/ 25 5.5
Ferrite 5 600 1005 15.2 Bainite 25 <1 6 642 1012 12.1 Bainite 20
1 7 585 970 13.8 Bainite 20-25 5.5 8 855 1002 13 Bainite 20 3 9 801
1079 10.6 Bainite 20-25 2.5 10 634 970 13 Bainite/ 20 3.5 Ferrite
11 671 954 14.2 Bainite 20 3 12 678 1021 10.6 Bainite 30 1 13 716
1069 11.8 Bainite 25-30 6 14 681 1012 13.2 Bainite/ 35 3 Ferrite 15
706 1010 13.1 Bainite 30 1 16 724 986 15.6 Bainit 30 5
TABLE-US-00005 TABLE 5 Structual proportions Retained R.sub.p0.2
R.sub.m A.sub.80 Martensite austenite Melt [MPa] [MPa] [%] Matrix
[%] [%] 1 580 950 13 Ferrite 20 -- 2 590 980 13.6 Ferrite/ 20 6
Bainite 3 610 965 15.8 Ferrite 20 -- 4 580 950 17.2 Ferrite/ 25 3
Bainite 5 585 995 18.4 Ferrite 20 -- 6 580 1003 16.4 Ferrite/ 20 4
Bainite 7 590 960 15.9 Ferrite/ 35 3 Bainite 8 654 1003 15 Ferrite/
30 3 Bainite 9 618 1006 15.4 Ferrite/ 30 8 Bainite 10 580 940 17.1
Ferrite/ 25 6 Bainite 11 595 911 18.4 Ferrite/ 25 6 Bainite 12 641
1011 13.7 Bainite/ 30 2 Ferrite 13 698 1021 13.4 Bainite 35 6 14
585 921 16.7 Ferrite/ 25 5 Bainite 15 712 1001 15.4 Bainite/ 30 7
Ferrite 16 722 1015 16.3 Bainite/ 35 2 Ferrite
* * * * *