U.S. patent application number 14/417659 was filed with the patent office on 2015-08-06 for cold-rolled flat steel product and method for the production thereof.
The applicant listed for this patent is ThyssenKruppe Steel Europe AG. Invention is credited to Brigitte Hammer, Thomas Heller, Frank Hisker, Rudolf Kawalla, Grzegorz Korpala.
Application Number | 20150218684 14/417659 |
Document ID | / |
Family ID | 48877247 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150218684 |
Kind Code |
A1 |
Hammer; Brigitte ; et
al. |
August 6, 2015 |
Cold-Rolled Flat Steel Product and Method for the Production
Thereof
Abstract
A cold-rolled flat steel product where Rm.gtoreq.1400 MPa and
A80.gtoreq.5% and also a method for producing such product. The
product includes, in addition to Fe and unavoidable impurities (in
wt. %), 0.10-0.60% C, 0.4-2.5% Si, .ltoreq.3.0% Al, 0.4-3.0% Mn,
.ltoreq.1.0% Ni, .ltoreq.2.0% Cu, .ltoreq.0.4% Mo, .ltoreq.% Cr,
.ltoreq.1.5% Co, .ltoreq.0.2% Ti, .ltoreq.0.2% Nb, .ltoreq.0.5% V.
The microstructure includes (in vol. %) .gtoreq.20% bainite, 10-35%
residual austenite and martensite as the remainder. A slab, thin
slab or a cast strip having said composition, is hot-rolled to form
hot strip with a hot-rolling end temperature .gtoreq.830.degree.
C., coiled at a coiling temperature .ltoreq.560.degree. C.,
cold-rolled at .gtoreq.30% reduction and heat-treated by firstly
being heated to an annealing temperature .gtoreq.800.degree. C.,
then being cooled at a cooling rate of .gtoreq.8.degree. C./s to a
holding temperature of 470.degree. C. to greater than the
martensite start temperature and then being held at the holding
temperature until at least 20 vol. % bainite is present in the
microstructure.
Inventors: |
Hammer; Brigitte; (Voerde,
DE) ; Heller; Thomas; (Duisburg, DE) ; Hisker;
Frank; (Bottrop, DE) ; Kawalla; Rudolf;
(Bobritzsch, DE) ; Korpala; Grzegorz; (Freiberg,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ThyssenKruppe Steel Europe AG |
Duisburg |
|
DE |
|
|
Family ID: |
48877247 |
Appl. No.: |
14/417659 |
Filed: |
July 26, 2013 |
PCT Filed: |
July 26, 2013 |
PCT NO: |
PCT/EP2013/065838 |
371 Date: |
January 27, 2015 |
Current U.S.
Class: |
148/537 ;
148/332; 148/603 |
Current CPC
Class: |
C21D 8/0226 20130101;
C22C 38/28 20130101; C21D 2211/002 20130101; C21D 6/002 20130101;
C21D 6/004 20130101; C21D 8/0263 20130101; C22C 38/50 20130101;
C21D 9/52 20130101; C22C 38/58 20130101; C22C 38/04 20130101; C21D
8/0236 20130101; C21D 9/46 20130101; C22C 38/001 20130101; C22C
38/34 20130101; C22C 38/46 20130101; C22C 38/48 20130101; C21D
6/008 20130101; C22C 38/02 20130101; C21D 2211/008 20130101; C22C
38/06 20130101; C21D 8/0247 20130101; C22C 38/20 20130101; C22C
38/44 20130101; C22C 38/38 20130101; C21D 6/005 20130101; C22C
38/24 20130101; C22C 38/42 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C21D 9/46 20060101 C21D009/46; C21D 9/52 20060101
C21D009/52; C21D 6/00 20060101 C21D006/00; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/34 20060101 C22C038/34; C22C 38/28 20060101
C22C038/28; C22C 38/24 20060101 C22C038/24; C22C 38/20 20060101
C22C038/20; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 8/02 20060101 C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2012 |
EP |
12178332.8 |
Claims
1. A cold-rolled flat steel product, having a tensile strength Rm
of at least 1400 MPa and an elongation A80 of at least 5% and
comprising, in addition to iron and unavoidable impurities (in % by
weight): C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%,
Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co:
up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, and V: up to 0.5%,
wherein the microstructure of the flat steel product consists of
bainite to an extent of at least 20% by volume, of residual
austenite to an extent of 10-35% by volume and of martensite as the
remainder.
2. The flat steel product according to claim 1, wherein the C
content thereof is at least 0.25% by weight.
3. The flat steel product according to either of claim 1, wherein
the C content thereof is at least 0.27% by weight.
4. The flat steel product according to claim 1, wherein the Si
content thereof is at least 1.0% by weight.
5. The flat steel product according to claim 1, wherein the Al
content thereof is at least 0.01% by weight.
6. The flat steel product according to claim 1, wherein the Cu
content thereof is at least 0.2% by weight.
7. The flat steel product according to claim 5, wherein the Cu
content thereof is at least 0.55% by weight.
8. The flat steel product according to claim 1, wherein the Cr
content thereof is at least 0.3% by weight.
9. The flat steel product according to claim 1, wherein the Mn, Cr,
Ni, Cu and C contents thereof satisfy the following condition:
1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2, where %
Mn: respective Mn content in % by weight, % Cr: respective Cr
content in % by weight, % Ni: respective Ni content in % by weight,
% Cu: respective Cu content in % by weight, % C: respective C
content in % by weight.
10. The flat steel product according to claim 1, wherein the
microstructure thereof comprises at least 50% by volume
bainite.
11. The flat steel product according to claim 1, wherein the
microstructure thereof comprises 10-25% by volume residual
austenite.
12. A method for producing a flat steel product, said method
comprising the following work steps: providing a preliminary
product in the form of a slab, thin slab or a cast strip, which, in
addition to iron and unavoidable impurities, comprises (in % by
weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%,
Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co:
up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%;
hot-rolling the preliminary product to form a hot strip in one or
more rolling passes, wherein the hot strip obtained has a
hot-rolling end temperature of at least 830.degree. C. when it
leaves the last rolling pass; coiling the hot strip obtained at a
coiling temperature which lies between the hot-rolling end
temperature and 560.degree. C.; cold-rolling the hot strip to form
a cold strip with a degree of cold-rolling of at least 30%;
heat-treating the cold strip obtained, wherein, during the course
of the heat treatment, the cold strip is heated to an annealing
temperature amounting to at least 800.degree. C., is cooled
proceeding from the annealing temperature at a cooling rate
amounting to at least 8.degree. C./s to a holding temperature which
lies in a holding temperature range having an upper limit of
470.degree. C. and having a lower limit which is higher than the
martensite start temperature (MS), from which martensite forms in
the microstructure of the cold strip, and is held at the holding
temperature for a period of time which is sufficient to form at
least 20% by volume bainite in the microstructure of the cold
strip.
13. The method according to claim 12, wherein the hot-rolling end
temperature is 850-950.degree. C.
14. The method according to claim 12, wherein the holding
temperature is 300-420.degree. C.
15. The method according to claim 12, wherein the cold strip is
coated with a metallic protective layer after the heat treatment.
Description
[0001] The invention relates to a cold-rolled flat steel product
having a tensile strength Rm of at least 1400 MPa and an elongation
A80 of at least 5%. Products of this type are distinguished by a
very high strength in combination with good elongation properties,
and are suitable as such in particular for the production of
components for motor vehicle bodies.
[0002] The invention similarly relates to a method for producing a
flat steel product according to the invention.
[0003] The term "flat steel product" is to be understood here as
meaning steel sheets or steel strips produced by a rolling process
and also sheet bars and the like separated therefrom.
[0004] Where alloy contents are stated here merely in "%", this
always means "% by weight", unless expressly stated otherwise.
[0005] EP 1 466 024 B1 (DE 603 15 129 T2) discloses a method for
producing a flat steel product which is intended to have tensile
strengths of considerably more than 1000 MPa. In order to achieve
this, a steel melt comprising (in % by weight) 0.0005-1% C, 0.5-10%
Cu, up to 2% Mn, up to 5% Si, up to 0.5% Ti, up to 0.5% Nb, up to
5% Ni, up to 2% Al and as remainder iron and impurities which are
unavoidable for production-related reasons is produced. The melt is
cast to form a strip, the thickness of which is at most 10 mm and
which is cooled rapidly to a temperature of at most 1000.degree. C.
by sprinkling with water or a water-air mixture. Then, the cast
strip is hot-rolled with a conventional reduction rate. The
hot-rolling is ended at an end temperature at which all of the
copper is still in a solid solution in the ferrite and/or austenite
matrix. Then, the strip is subjected to a step of rapid cooling, in
order to keep the copper in a supersaturated solid solution in the
ferrite and/or austenite solution. After coiling to form a coil, a
cold strip can be rolled from the hot strip thus obtained with a
degree of cold-rolling amounting to 40-80%. This cold strip is then
subjected to recrystallization annealing, during which it is
brought as rapidly as possible to an annealing temperature lying in
the region of 840.degree. C. and held at said temperature, in order
to bring the greatest possible proportion of the copper present in
the steel into solution. This is followed by rapid cooling to a
temperature amounting to 400-700.degree. C., at which Cu
precipitations form once again. In this way, precipitation
hardening is intended to achieve the desired strength level of the
steel. At the same time, the copper content is intended to increase
the corrosion and embrittlement resistance of the steel through the
formation of a protective oxide layer.
[0006] A further method for producing a cold strip of extreme
strength is known from U.S. Pat. No. 7,591,977 B2. According to
this method, a hot strip comprising (in % by weight) 0.1-0.25% C,
1.0-2.0% Si and 1.5-3.0% Mn is rolled with a degree of cold-rolling
of 30-70% to form a cold strip, which is then subjected to a heat
treatment completed in a continuous pass. In this heat treatment,
the cold strip is heated, in a first annealing step, to a first
annealing temperature lying above the Ar3 temperature thereof, in
order to bring carbides present in the cold strip into solution.
This is followed by cooling, proceeding from the first annealing
temperature and being effected at a cooling rate of at least
10.degree. C./s, to a second annealing temperature. This
temperature is selected such that bainite forms in the cold strip,
and typically lies in the range of 300-450.degree. C. This second
annealing step carried out to form bainite is performed until the
microstructure of the cold strip consists of bainite to an extent
of at least 60% and of residual austenite to an extent of at least
5% and also of polygonal ferrite as remainder. The aim here is for
the microstructure to be bainitic to the fullest possible extent
and for other microstructure constituents to be present at most in
traces. The cold strip thus provided achieves tensile strengths of
up to 1180 MPa combined with an elongation of at least 9% and can
be coated, if required, with a metallic layer affording protection
against corrosion.
[0007] Against the background of the prior art explained above, it
was an object of the invention to provide a cold-rolled flat steel
product which can be produced in a simple and operationally
reliable manner and has an optimized combination of a further
increased strength and good deformability. In addition, the
intention was to provide a method for producing such a cold-rolled
flat steel product.
[0008] In relation to the cold-rolled flat steel product, this
object has been achieved according to the invention by the flat
steel product indicated in claim 1.
[0009] In relation to the method, the object mentioned above is
achieved according to the invention in that at least the working
steps indicated in claim 12 are performed to produce a cold-rolled
flat steel product according to the invention.
[0010] Advantageous configurations of the invention are indicated
in the dependent claims and will be explained in detail hereinbelow
as the general concept of the invention.
[0011] The cold-rolled flat steel product according to the
invention is distinguished by the fact that it comprises, in
addition to iron and unavoidable impurities (in % by weight):
[0012] C: 0.10-0.60%, [0013] Si: 0.4-2.5%, [0014] Al: up to 3.0%,
[0015] Mn: 0.4-3.0%, [0016] Ni: up to 1.0%, [0017] Cu: up to 2.0%,
[0018] Mo: up to 0.4%, [0019] Cr: up to 2%, [0020] Co: up to 1.5%,
[0021] Ti: up to 0.2%, [0022] Nb: up to 0.2%, [0023] V: up to
0.5%.
[0024] Here, in the cold-rolled state, the microstructure of the
flat steel product according to the invention consists of bainite
to an extent of at least 20% by volume, of residual austenite to an
extent of 10-35% by volume and of martensite as remainder, it being
self-evident that technically unavoidable traces of other
microstructure constituents may be present in the microstructure of
the flat steel product. A cold-rolled flat steel product according
to the invention provided in this way regularly achieves tensile
strengths Rm of at least 1400 MPa and an elongation A80 of at least
5%. The C content of the residual austenite is typically more than
1.0% by weight.
[0025] The method according to the invention for producing a flat
steel product provided or being composed according to the invention
comprises the following working steps: [0026] providing a
preliminary product in the form of a slab, thin slab or a cast
strip, which, in addition to iron and unavoidable impurities,
comprises (in % by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to
3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%,
Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up
to 0.5%; [0027] hot-rolling the preliminary product to form a hot
strip in one or more rolling passes, wherein the hot strip obtained
has a hot-rolling end temperature of at least 830.degree. C. when
it leaves the last rolling pass; [0028] coiling the hot strip
obtained at a coiling temperature which lies between the
hot-rolling end temperature and 560.degree. C.; [0029] cold-rolling
the hot strip to form a cold strip with a degree of cold-rolling of
at least 30%; [0030] heat-treating the cold strip obtained,
wherein, during the course of the heat treatment, the cold strip
[0031] is heated to an annealing temperature amounting to at least
800.degree. C., [0032] is optionally held at the annealing
temperature for an annealing duration of 50-150 s, [0033] is cooled
proceeding from the annealing temperature at a cooling rate
amounting to at least 8.degree. C./s to a holding temperature which
lies in a holding temperature range having an upper limit of
470.degree. C. and having a lower limit which is higher than the
martensite start temperature MS, from which martensite forms in the
microstructure of the cold strip, and [0034] is held in the holding
temperature range for a period of time which is sufficient to form
at least 20% by volume bainite in the microstructure of the cold
strip.
[0035] A steel strip according to the invention has a three-phase
microstructure, the dominant constituent of which is bainite and
which moreover consists of residual austenite and also of
martensite as remainder. It is optimal here that the bainite
proportion is at least 50% by volume, in particular at least 60% by
volume, and that the residual austenite proportion lies in the
range of 10-25% by volume, with the remainder of the microstructure
being made up here too in each case by martensite. The optimum
martensite proportion is at least 10% by volume. A microstructure
having such a composition brings about the best combination of
Rm*A80 with the required tensile strength.
[0036] In addition to the main components "bainite", "residual
austenite" and "martensite", it is possible for contents of other
microstructure constituents to be present, but the proportions of
these are too low to have an influence on the properties of the
cold strip according to the invention. The residual austenite is
present in a cold strip according to the invention predominantly in
film form with small, globular islands of block residual austenite
having a grain size of <5 .mu.m, such that the residual
austenite has a high stability in the initial state and, associated
therewith, a low tendency towards undesirable transformation into
martensite. At higher degrees of deformation, martensite is formed
from this residual austenite (TRIP effect), and this increases the
elongation at break.
[0037] Cold strip produced according to the invention regularly
achieves tensile strengths Rm of more than 1400 MPa, with
elongations A80 which similarly regularly lie above 5%.
Accordingly, the quality Rm*A80 of flat steel products according to
the invention is regularly above 7000 MPa*%, with qualities Rm*A80
of at least 13 500 MPa*% typically being achieved. A cold strip
according to the invention as such has an optimum combination of
extreme strength and sufficient deformability.
[0038] The martensite start temperature, i.e. the temperature from
which martensite forms in steel processed according to the
invention, can be calculated on the basis of the procedure
explained in the article entitled "Thermodynamic Extrapolation and
Martensite-Start-Temperature of Substitutionally Alloyed Steels" by
H. Bhadeshia, published in Metal Science 15 (1981), pages
178-180.
[0039] In the steel according to the invention, carbon delays the
transformation to ferrite/pearlite, reduces the martensite start
temperature MS and contributes to an increase in the hardness. In
order to utilize these positive effects, the C content of the flat
steel product according to the invention can be set to at least
0.25% by weight, in particular at least 0.27% by weight or at least
0.28% by weight, it being possible for the effects achieved by the
comparatively high carbon content to be utilized particularly
reliably when the C content lies in the range of >0.25-0.5% by
weight, in particular 0.27-0.4% by weight or 0.28-0.4% by
weight.
[0040] The strength-increasing action of copper can also be
utilized in a cold-rolled flat steel product according to the
invention. In this respect, a minimum Cu content of 0.15% by
weight, in particular at least 0.2% by weight Cu, can be present in
the flat steel product according to the invention. Cu makes a
particularly effective contribution to the strength if it is
present in the flat steel product according to the invention in
contents of at least 0.55% by weight, it being possible for
negative effects of the presence of Cu to be limited by virtue of
the fact that the Cu content is limited to at most 1.5% by
weight.
[0041] In the steel processed according to the invention, Mn in
contents of at least 0.4% by weight and up to 3% by weight, in
particular up to 2.5% by weight, promotes the bainite formation,
the Cu, Cr and Ni contents which are optionally additionally
present likewise contributing to the formation of bainite.
Depending on the respective other constituents of the steel
processed according to the invention, it can be expedient here to
limit the Mn content to at most 2% by weight or to increase the
minimum Mn content to 1.5% by weight.
[0042] The optional addition of Cr can also lower the martensite
start temperature and suppress the tendency of the bainite to
transform into pearlite or cementite. Moreover, in contents up to
the upper limit of at most 2% by weight as predefined according to
the invention, Cr promotes the ferritic transformation, with
optional effects of the presence of Cr in the cold-rolled flat
steel product according to the invention arising when the Cr
content is limited to 1.5% by weight. The positive influence of Cr
can be utilized particularly effectively if at least 0.3% by weight
Cr is present in the flat steel product according to the
invention.
[0043] The addition of Ti, V or Nb, which is likewise optional, can
support the formation of a finer-grained microstructure and promote
the bainitic transformation. In addition, these microalloying
elements contribute to an increase in the hardness through the
formation of precipitations. The positive effects of Ti, V and Nb
can be utilized in a particularly effective manner in the
cold-rolled flat steel product according to the invention when the
content of each of these elements lies in the range of 0.002-0.15%
by weight, in particular does not exceed 0.1% by weight.
[0044] Si is present in a flat steel product according to the
invention in contents of 0.4-2.5% by weight and brings about a
considerable solid solution solidification. In order to utilize
this effect in a particularly reliable manner, the Si content can
be set to at least 1.0% by weight. Similarly, to avoid negative
influences, it may be expedient to limit the Si content to at most
2% by weight.
[0045] In the steel processed according to the invention, Al can
partly replace the Si content. At the same time, Al, like Si, has a
deoxidizing action during the steel production. For this purpose, a
minimum Al content of 0.01% by weight can be provided. Higher
contents of Al prove to be expedient, for example, when the
addition of Al is intended to set the hardness or tensile strength
of the steel to a relatively low value in favour of improved
deformability.
[0046] A further function of Si and Al consists in suppressing the
carbide formation in the bainite and therefore stabilizing the
residual austenite by dissolved C down to low temperatures.
[0047] The positive influences of the simultaneous presence of Al
and Si can thereby be utilized particularly effectively when the Si
and Al contents within the limits predefined according to the
invention satisfy the following condition: % Si+0.8% Al>1.2% by
weight (where % Si: respective Si content in % by weight, % Al:
respective Al content in % by weight).
[0048] The formation of the microstructure predefined according to
the invention can be ensured in particular by virtue of the fact
that the Mn, Cr, Ni, Cu and C contents of the steel processed
according to the invention and accordingly the Mn, Cr, Ni, Cu and C
contents of the flat steel product according to the invention
satisfy the following condition
1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2
where % Mn denotes the respective Mn content in % by weight, % Cr
denotes the respective Cr content in % by weight, % Ni denotes the
respective Ni content in % by weight, % Cu denotes the respective
Cu content in % by weight and % C denotes the respective C content
in % by weight.
[0049] To produce a flat steel product according to the invention,
the primary or preliminary product cast from a steel having a
composition according to the invention is firstly brought to a
temperature or held at a temperature which is sufficient to end the
hot-rolling carried out proceeding from this temperature at a
hot-rolling end temperature lying in the range of 830-1000.degree.
C. After it leaves the last rolling stand used for the hot-rolling,
the hot strip cools down on the roller table adjoining the rolling
stand in question. Subsequent to the roller table, the hot strip
passes into a coiling device, in which it is wound to form a
coil.
[0050] The coiling temperature has to be at least 560.degree. C.,
so that a relatively soft hot strip microstructure consisting of
ferrite and pearlite is formed. A temperature profile which is
optimal for this purpose arises if the hot-rolling end temperature
lies in the range of 850-950.degree. C., in particular in the range
of 880-950.degree. C. To this end, it is typically the case that
the preliminary product is heated to a temperature lying in the
range of 1100-1300.degree. C. or is held at this temperature before
the hot-rolling. The microstructure of the hot strip thus obtained
consists primarily of ferrite and pearlite. The risk of grain
boundary oxidation arising can be minimized by virtue of the fact
that the coiling temperature is limited to at most 750.degree.
C.
[0051] After the coiling, the hot strip is cold-rolled, it going
without saying that the hot strip can be conventionally descaled by
chemical or mechanical means before the cold-rolling.
[0052] The cold-rolling is effected with a degree of cold-rolling
of at least 30%, in particular at least 45%, in order to accelerate
the recrystallization and transformation during the subsequent
annealing. It is generally the case that a better surface quality
is also obtained by observing a correspondingly high degree of
cold-rolling. Degrees of cold-rolling of at least 50% have proved
to be particularly favourable for this purpose.
[0053] After the cold-rolling, the cold strip obtained according to
the invention completes an annealing cycle in a continuous pass,
during which it is heated in a first annealing phase to a
temperature of at least 800.degree. C., preferably at least
830.degree. C. This first annealing phase lasts at least for such a
period of time that the cold strip is completely austenitized.
50-150 s are typically required for this.
[0054] At the end of the first annealing phase, the product is
quenched, the cooling rate being at least 8.degree. C./s, in
particular 10.degree. C./s. The target temperature for this
quenching is a holding temperature which is at most 470.degree. C.
and is higher than the martensite start temperature MS, from which
martensite forms in the microstructure of the cold strip. In
practice, the range of 300-420.degree. C., in particular
330-420.degree. C., can be used as an indication of the range in
which the holding temperature is to lie.
[0055] Proceeding from the respective holding temperature, the cold
strip is held in the holding temperature range in the second
annealing phase, to be precise until at least 20% by volume of the
microstructure of the cold strip has transformed into bainite. The
hold can be carried out here as an isothermal hold at the holding
temperature reached during the cooling or as a slow decrease in
temperature within the holding temperature range.
[0056] The flat steel product produced according to the invention
can be coated in a conventional manner with a metallic protective
layer. This can be effected by hot-dip coating, for example. If
annealing is required before the application of the metallic
coating, the heat treatment provided according to the invention can
be carried out in the course of this annealing.
[0057] The invention will be explained in more detail hereinbelow
on the basis of exemplary embodiments.
[0058] Five steels S1-S5 were melted, the composition thereof being
shown in Table 1.
[0059] The steel melts of corresponding composition were cast in a
conventional manner to form a strand, from which slabs were
separated. The slabs were then heated in a similarly conventional
manner to a reheating temperature.
[0060] The heated slabs were hot-rolled in a similarly conventional
group of hot-rolling stands to form hot strips having a thickness
of 2 mm.
[0061] The hot-rolling end temperature was in the range of
830-900.degree. C. in each case. The hot strips were cooled
proceeding from this temperature to a coiling temperature lying
above 560.degree. C. and then coiled to form coils.
[0062] The hot strips thus obtained were descaled after the coiling
and cold-rolled to form cold strip with degrees of cold-rolling of
50% after the descaling.
[0063] A relatively large number of specimens of these cold strips
were then subjected to a heat treatment, in which they were heated
in a first annealing step at a heating rate of at least 1.9.degree.
C./s to a first annealing temperature in the range of
830-850.degree. C. The cold strips were held at this temperature
for a period of time of 120 s, until they had been completely
heated through.
[0064] This was followed by quenching, during which cold strips
were quenched at a cooling rate amounting to at least 8.degree.
C./s to a holding temperature T2 in the range of 350-420.degree. C.
Specifically, the holding temperatures T2 in a first batch of tests
were 300.degree. C., 310.degree. C., 330.degree. C., 340.degree.
C., 375.degree. C., 390.degree. C. and 410.degree. C. The cold
strip specimens were held at the respective holding temperature T2
for an annealing duration t2.
[0065] In FIG. 1, the tensile strengths Rm achieved are plotted
against the respective annealing temperature T2. It can be seen
that the cold strip specimens produced from the steel S5 each
achieve the required minimum tensile strength of 1400 MPa only
under certain annealing conditions, whereas the tensile strengths
of the cold strip specimens produced from the other steels were
always reliably above the minimum limit of 1400 MPa. The
comparatively low carbon content of the steel S5, lying at the
lower limit of the content range predefined according to the
invention, has been identified as the reason for this.
[0066] In FIG. 2, the tensile strengths of the cold strip specimens
produced from the steel S4 are plotted against the annealing
duration t2 of the second annealing stage. It can be seen that the
cold strip specimens held at a holding temperature of 310.degree.
C., 330.degree. C. and 350.degree. C., i.e. in the holding
temperature range of 310-350.degree. C., achieved the required
tensile strength Rm of 1400 MPa, irrespective of the respective
annealing duration t2.
[0067] In FIG. 3, the tensile strengths of the cold strip specimens
produced from the steel S5 are similarly plotted against the
annealing duration t2 of the second annealing stage. It can be seen
here that the cold strip specimens held at a holding temperature of
350.degree. C. and 390.degree. C., i.e. in the holding temperature
range of 350-390.degree. C., achieve the required tensile strength
Rm of 1400 MPa if the annealing duration t2 is shorter than 145
s.
[0068] In FIG. 4, the elongation A80 of the cold strip specimens
produced from the steel S4 is plotted against the annealing
duration t2 of the second annealing stage. The cold strip specimens
held at a holding temperature of 310.degree. C., 330.degree. C. and
350.degree. C., i.e. in the holding temperature range of
310-350.degree. C., achieved the required minimum elongation A80,
irrespective of the respective annealing duration t2.
[0069] In FIG. 5, the elongation A80 of the cold strip specimens
produced from the steel S5 is plotted against the annealing
duration t2 of the second annealing stage. Here, too, it can be
seen that the cold strip specimens achieve the required elongation
A80 of at least 5% irrespective of the respective holding
temperature T2 thereof and irrespective of the respective annealing
duration t2. Accordingly, if a short annealing duration and
suitably low holding temperatures T2 are observed, it is also
possible for a cold-rolled flat steel product according to the
invention in which a high tensile strength Rm is combined with a
sufficient elongation A80 to be produced from the steel S5, in
spite of the comparatively low C content thereof.
[0070] FIG. 6 shows, in a section, a magnified view of a cross
section of a cold strip according to the invention. In this figure,
by way of example, residual austenite blocks RA-b are marked and a
point at which film-like residual austenite RA-f is present in a
lamellar stratification is emphasized by being circled.
TABLE-US-00001 TABLE 1 Steel C Mn Si Cu Cr Ti Nb V Al N Other S1
0.52 1.48 0.40 1.51 0.88 0.009 -- 0.093 1.400 -- -- S2 0.301 1.41
1.46 1.47 0.87 0.014 0.005 0.09 0.021 0.0015 Ni: 0.021 Mo:
<0.002 S3 0.505 1.50 0.40 0.6 1.30 0.011 -- 0.098 0.012 0.002
Ni: 0.63 Mo: 0.30 S4 0.384 1.97 0.41 0.57 1.37 0.0016 -- <0.0005
0.018 0.0014 Ni: 0.59 Mo: 0.30 S5 0.252 1.47 2.15 0.32 0.41 0.020
-- 0.11 0.009 -- Ni: 0.02 Mo: <0.002 Amounts, in % by weight
Remainder iron and unavoidable impurities
* * * * *