U.S. patent application number 14/417685 was filed with the patent office on 2015-07-23 for hot-rolled flat steel product and method for the production thereof.
The applicant listed for this patent is ThyssenKrupp Steel Europe AG. Invention is credited to Brigitte Hammer, Thomas Heller, Frank Hisker, Rudolf Kawalla, Grzegorz Korpala.
Application Number | 20150203946 14/417685 |
Document ID | / |
Family ID | 48874319 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150203946 |
Kind Code |
A1 |
Hammer; Brigitte ; et
al. |
July 23, 2015 |
Hot-Rolled Flat Steel Product and Method For the Production
Thereof
Abstract
A hot-rolled flat steel product having a product of Rm and A80
of .gtoreq.18,000 MPa*%, a composition including (in wt.)
C:0.10-0.60%, Si:0.4-2.0%, Al:.ltoreq.2.0%, Mn:0.4-2.5%,
Ni:.ltoreq.1%, Cu:.ltoreq.2.0%, Mo:.ltoreq.0.4%, Cr.ltoreq.2%,
Ii:.ltoreq.0.2%, Nb:.ltoreq.0.2%, V:.ltoreq.0.5%, remainder Fe and
unavoidable impurities, and a microstructure of bainite and
residual austenite, wherein the microstructure includes .gtoreq.60
vol.% bainite, and wherein at least some of the residual austenite
is in block form and .gtoreq.98% of the residual austenite blocks
have a size of .ltoreq.5 .mu.m. Also, a method where a slab, thin
slab or a cast strip having the aforementioned composition is
hot-rolled at a hot-rolling end temperature of .gtoreq.880.degree.
C., cooled with a cooling rate of .gtoreq.5.degree. C./s to a
coiling temperature between the martensite start temperature and
600.degree. C., coiled, and cooled in the coil while being held
between the bainite start temperature and the martensite start
temperature until .gtoreq.60 vol.% of the hot strip microstructure
is bainite.
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 |
ThyssenKrupp Steel Europe AG |
Duisburg |
|
DE |
|
|
Family ID: |
48874319 |
Appl. No.: |
14/417685 |
Filed: |
July 26, 2013 |
PCT Filed: |
July 26, 2013 |
PCT NO: |
PCT/EP2013/065836 |
371 Date: |
January 27, 2015 |
Current U.S.
Class: |
148/602 ;
148/332 |
Current CPC
Class: |
C21D 6/004 20130101;
C21D 8/0226 20130101; C22C 38/16 20130101; C22C 38/28 20130101;
C21D 8/0263 20130101; C21D 2211/002 20130101; C22C 38/58 20130101;
C22C 38/00 20130101; C21D 8/0463 20130101; C22C 38/04 20130101;
C22C 38/50 20130101; C21D 6/005 20130101; C22C 38/46 20130101; C22C
38/34 20130101; C21D 1/20 20130101; C22C 38/22 20130101; C22C 38/02
20130101; C22C 38/18 20130101; C21D 6/008 20130101; C22C 38/42
20130101; C22C 38/06 20130101; C21D 2211/001 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C21D 6/00 20060101 C21D006/00; C22C 38/50 20060101
C22C038/50; C22C 38/02 20060101 C22C038/02; C22C 38/42 20060101
C22C038/42; C22C 38/34 20060101 C22C038/34; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C21D 8/02 20060101
C21D008/02; C22C 38/46 20060101 C22C038/46 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2012 |
EP |
12178330.2 |
Claims
1. A hot-rolled flat steel product, with the mathematical product
of tensile strength (Rm) and elongation (A80) being at least 18,000
MPa*% and comprising, in addition to iron and unavoidable
impurities (in % by weight): C: 0.10-0.60%, Si: 0.4-2.0%, Al: up to
2.0%, Mn: 0.4-2.5%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%,
Cr: up to 2%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%,
wherein the flat steel product has a microstructure dominated by
two phases, one dominating constituent of the microstructure being
bainite and the second dominating constituent of the microstructure
being residual austenite, wherein the microstructure of the flat
steel product consists of bainite to an extent of at least 50% by
volume and of residual austenite as the remainder, wherein
optionally up to 5% by volume ferrite and up to 10% by volume
martensite may be present in the microstructure of the flat steel
product, and wherein at least part of the residual austenite is
present in block form and at least 98% of the blocks of the
residual austenite being present in block form have a mean diameter
of less than 5 .mu.m.
2. The flat steel product according to claim 1, wherein the
microstructure of said flat steel product contains at least 10% by
volume residual austenite.
3. The flat steel product according to claim 1, wherein the Cu
content of said flat steel product is at least 0.15% by weight.
4. The flat steel product according to claim 1, wherein the C
content of said flat steel product is at least 0.3% by weight.
5. The flat steel product according to claim 1, wherein the Mn, Cr,
Ni, Cu and C contents of said flat steel product 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.
6. The flat steel product according to claim 1, wherein the Si and
Al contents of said flat steel product 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.
7. The flat steel product according to claim 1, wherein the
diameter of the block residual austenite is 1-3 .mu.m.
8. A method for producing a flat steel product, said method
comprising: providing a preliminary product in the form of a slab,
thin slab or a cast strip, which, in addition to iron and
unavoidable impurities, contains (in % by weight): 0.10-0.60% C,
0.4-2.0% Si, up to 2.0% Al, 0.4-2.5% Mn, up to 1% Ni, up to 2.0%
Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to 0.2% Nb and up
to 0.5% V; hot-rolling the preliminary product to form a hot strip
in one or more rolling passes, the hot strip obtained having a
hot-rolling end temperature of at least 880.degree. C. when it
leaves the last rolling pass; accelerated cooling of the hot strip
obtained with a cooling rate of at least 5.degree. C./s to a
coiling temperature lying in the range between the martensite start
temperature (MS) and 600.degree. C.; coiling the hot strip to form
a coil; cooling the coil, the temperature of the coil being held,
during the cooling to form bainite, in a temperature range having
an upper limit which is the same as the bainite start temperature
(BS), from which bainite forms in the microstructure of the hot
strip, and having a lower limit which is the same as the martensite
start temperature (MS), from which martensite forms in the
microstructure of the hot strip, until at least 50% by volume of
the microstructure of the hot strip comprises bainite.
9. The method according to claim 8, wherein the end temperature of
the hot-rolling is at least 900.degree. C.
10. The method according to claim 8, wherein the cooling rate is at
least 10.degree. C./s.
11. The method according to claim 8, wherein the cooling rate is at
most 150.degree. C./s.
12. The method according to claim 8, wherein the cooling rate is at
most 50.degree. C./s.
13. The method according to claim 8, wherein the lower limit of the
coiling temperature at which the cooling begins in the coil is
about 20.degree. C. higher than the martensite start temperature
(MS).
14. The method according to claim 8, wherein the upper limit of the
coiling temperature at which the cooling begins in the coil is
550.degree. C.
15. The method according to claim 8, wherein the coiling
temperature at least corresponds to the temperature HTopt
determined by the following formula: HTopt=MS+(BS-MS)/3.
Description
[0001] The invention relates to a hot-rolled flat steel product
with the mathematical product of tensile strength Rm and elongation
A80 being at least 18 000 MPa*%. Flat steel 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] The product of tensile strength Rm and elongation A80 is
technically also referred to as "quality".
[0006] 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 reduction rate of at least 10%. 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. The strip thus cooled is finally
wound to form a coil. The copper precipitations bring about
precipitation hardening, by virtue of which the desired strength
level of the steel is to be achieved. 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.
[0007] A hot strip having a tensile strength of above 1200 MPa and
an elongation of up to 10% and a method for the production thereof
are known from US 2009/0107588 A1. This known hot strip consists of
a steel which, in addition to iron and unavoidable impurities,
comprises (in % by weight) 0.10-0.25% C, 1-3% Mn, more than 0.015%
Al, up to 1.985% Si, up to 0.30% Mo, up to 1.5% Co and up to 0.005%
B, where the following should apply: 1%.ltoreq.% Si+% Al.ltoreq.2%
(% Al=respective Al content, % Si=respective Si content) and %
Cr+(3.times.% Mo).gtoreq.0.3% (% Cr=respective Cr content, %
Mo=respective Mo content). At the same time, the steel is to have a
microstructure which consists to an extent of at least 75% of
bainite, to an extent of at least 5% of residual austenite and to
an extent of at least 2% of martensite. To produce the hot strip, a
melt of corresponding composition is cast to form a primary or
preliminary product, which is then heated to more than 1150.degree.
C. and then hot-rolled at a hot-rolling end temperature at which
the steel is still entirely austenitic. The hot strip obtained is
then cooled in three steps. In the first step, the cooling is
effected proceeding from a temperature lying above the Ar3
temperature of the steel at a cooling rate of at least 70.degree.
C./s to a first intermediate temperature of above 650.degree. C.
Proceeding from this first intermediate temperature, cooling is
then effected to a second intermediate temperature, which lies
between the bainite start temperature, i.e. the temperature at
which bainite begins to form in the steel, and a lower limit
temperature, which is 50.degree. C. higher than the martensite
start temperature, i.e. the temperature from which martensite forms
in the steel. The cooling rate in this second cooling step is
20-90.degree. C./s. This is followed by a third cooling step, in
which the hot strip is cooled to room temperature. The temperature
from which this third cooling step proceeds is determined here
depending on the respective cooling rate.
[0008] Another method for producing a high-strength and readily
deformable hot strip which is likewise based on the
strength-increasing action of Cu precipitations is described in
U.S. Pat. No. 6,190,469 B1. In this method, a steel comprising (in
% by weight) 0.15-0.3% C, 1.5-2.5% Si, 0.6-1.8% Mn, 0.02-0.10% Al,
0.6-2.0% Cu, 0.6-2.0% Ni and as remainder iron and unavoidable
impurities is cast to form slabs. The slabs are rolled to form hot
strip, the hot-rolling end temperature being 750-880.degree. C. The
hot strip obtained is then cooled by means of water, proceeding
from a start temperature of 680-740.degree. C., to a coiling
temperature which is at least the same as the temperature
calculated on the basis of the formula 240.times.(% Mn+% Ni)-140
(where % Mn=respective Mn content, % Ni=respective Ni content) and
not higher than 540.degree. C. Then, the hot strip cooled to the
coiling temperature is wound to form a coil. The hot strip obtained
has a microstructure which, in addition to ferrite, comprises 5-20%
residual austenite and 20-50% bainite, the microstructure
containing copper precipitations which, through precipitation
hardening, contribute to the strength of the hot strip obtained.
The hot strip produced and provided in this way has an elongation
of up to 23% combined with strengths lying in the region of 1000
MPa, and therefore as a whole high quality values of more than 20
000 MPa*% are achieved.
[0009] Against the background of the prior art explained above, it
was an object of the invention to provide a hot-rolled flat steel
product which can be produced in a simple and operationally
reliable manner and has an optimized combination of a particularly
high strength and good deformability. In addition, the intention
was to provide a method for producing such a flat steel
product.
[0010] In relation to the hot strip, this object has been achieved
according to the invention by the hot-rolled flat steel product
indicated in claim 1.
[0011] 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 8 are performed to produce a hot-rolled
flat steel product according to the invention.
[0012] 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.
[0013] The hot-rolled flat steel product according to the invention
is distinguished by the fact that it contains, in addition to iron
and unavoidable impurities (in % by weight): [0014] C: 0.10-0.60%,
[0015] Si: 0.4-2.0%, [0016] Al: up to 2.0%, [0017] Mn: 0.4-2.5%,
[0018] Ni: up to 1%, [0019] Cu: up to 2.0%, [0020] Mo: up to 0.4%,
[0021] Cr: up to 2%, [0022] Ti: up to 0.2%, [0023] Nb: up to 0.2%,
[0024] V: up to 0.5%.
[0025] A flat steel product according to the invention has a
microstructure dominated by two phases, one dominating constituent
of the microstructure being bainite and the second dominating
constituent of the microstructure being residual austenite. In
addition to these two main components, small proportions of
martensite and ferrite may be present, but the contents thereof are
too small to have an influence on the properties of the hot-rolled
flat steel product. Accordingly, the microstructure of the flat
steel product according to the invention consists of bainite to an
extent of at least 50% by volume, in particular at least 60% by
volume, and of residual austenite as the remainder in addition to
optionally present proportions of up to 5% by volume ferrite and up
to 10% by volume martensite, wherein at least part of the residual
austenite is present in block form and at least 98% of the blocks
of the residual austenite present in block form have a mean
diameter of less than 5 .mu.m.
[0026] The method according to the invention for producing a flat
steel product provided according to the invention includes the
following work steps: [0027] providing a preliminary product in the
form of a slab, thin slab or a cast strip, which, in addition to
iron and unavoidable impurities, contains (in % by weight):
0.10-0.60% C, 0.4-2.0% Si, up to 2.0% Al, 0.4-2.5% Mn, up to 1% Ni,
up to 2.0% Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to
0.2% Nb and up to 0.5% V; [0028] hot-rolling the preliminary
product to form a hot strip in one or more rolling passes, the hot
strip obtained having a hot-rolling end temperature of at least
880.degree. C. when it leaves the last rolling pass; [0029]
accelerated cooling of the hot strip obtained with a cooling rate
of at least 5.degree. C./s to a coiling temperature lying between
the martensite start temperature MS and 600.degree. C.; [0030]
coiling the hot strip to form a coil; [0031] cooling the coil, the
temperature of the coil being held, during the cooling to form
bainite, in a temperature range having an upper limit which is the
same as the bainite start temperature BS, from which bainite forms
in the microstructure of the hot strip, and having a lower limit
which is the same as the martensite start temperature MS, from
which martensite forms in the microstructure of the hot strip,
until 50% by volume, in particular at least 60% by volume, of the
microstructure of the hot strip consists of bainite.
[0032] The invention is based on the knowledge that it is
beneficial to the required properties of the hot-rolled flat steel
product if the residual austenite is present in block form, as long
as the diameter of the residual austenite blocks does not exceed 5
.mu.m. It has been assumed to date in the prior art that residual
austenite present in block form is to be avoided in principle,
since residual austenite in block form has been interpreted to be a
cause of instabilities of the microstructure and an associated
tendency toward the formation of undesirable martensite.
Accordingly, to date the highest possible proportions of film-like
residual austenite have always been sought in the prior art in the
microstructure of a steel of the type in question here (see H. K.
D. H. Bhadeshia and D. V. Edmonds "Bainite in silicon steels: new
composition-property approach" published in Metal Science Vol. 17,
September 1983, pages 411-419 ("Part 1") and pages 420-425 ("Part
2")).
[0033] Reference is made to "block-like" residual austenite in this
context when the ratio of length/width, i.e. longest
extent/thickness, of the microstructure constituents of residual
austenite present in the microstructure is 1 to 5. By contrast,
residual austenite is referred to as "film-like" when the ratio of
length/width of the residual austenite accumulations present in the
microstructure is greater than 5 and the width of the respective
microstructure constituents of residual austenite is smaller than 1
.mu.m. Film-like residual austenite is accordingly typically
present as finely distributed lamellae.
[0034] The outlay which is still required according to the prior
art to avoid residual austenite present in block form can therefore
be avoided in the production of a flat steel product according to
the invention by keeping the residual austenite blocks present in
the microstructure of the obtained flat steel product according to
the invention small, i.e. the extent thereof as expressed by their
mean diameter is limited to less than 5 .mu.m. It has surprisingly
been found in this respect that residual austenite present in block
form and having a diameter of smaller than 5 .mu.m has a positive
effect on the elongation properties of a steel of the type provided
according to the invention. The residual austenite blocks present
in this size prove to be more stable than block-like residual
austenite present in coarser form. At the same time, they are not
as stable as residual austenite present in film-like form and
therefore enable the TRIP effect. The positive influence of
residual austenite present in block form can be utilized
particularly reliably when the extent of the block residual
austenite measures at most 4 .mu.m, in particular at most 3 .mu.m.
In this respect, it has been found in practice that, in flat steel
products with a composition according to the invention and produced
according to the invention, the maximum extent of the residual
austenite present in block form regularly lies in the range of 1-3
.mu.m, the maximum extent of the residual austenite blocks
typically being limited on average to 2 .mu.m. Complex, multi-step
temperature control during the production of the flat steel product
is surprisingly not required for this purpose.
[0035] Accordingly, a hot-rolled flat steel product according to
the invention can be produced without any special expenditure while
at the same time observing the parameters predefined according to
the invention for the production method. In particular, complex
cooling strategies or cooling strategies which require a high
cooling power, as have still been deemed unavoidable in the prior
art, are no longer required.
[0036] The positive influence of the residual austenite contents in
the microstructure of a flat steel product according to the
invention arises particularly reliably when the residual austenite
content is at least 10% by volume, with beneficial effects to be
expected with particular reliability given residual austenite
contents of at least 15% by volume.
[0037] Hot-rolled flat steel products which are produced according
to the invention regularly achieve tensile strengths Rm of more
than 1000 MPa, in particular at least 1200 MPa, with elongations
A80 which similarly regularly lie above 17%, in particular above
19%. Accordingly, the quality Rm*A80 of hot strips according to the
invention is regularly in the range of 18 000-30 000 MPa*%. In
particular, it is regularly at least 20 000 MPa*%. A flat steel
product according to the invention as such has an optimum
combination of extreme strength and good deformability.
[0038] The strength-increasing action of copper can also be
utilized in a hot-rolled flat steel product according to the
invention. In this respect, a minimum Cu content of 0.15% by weight
can be present in the hot-rolled flat steel product according to
the invention.
[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 at at least
0.3% by weight.
[0040] In the steel processed according to the invention, Mn in
contents of up to 2.5% by weight, in particular up to 2.0% 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 1.6% by
weight.
[0041] In addition, 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 a
flat steel product according to the invention arising when the Cr
content is limited to 1.5% by weight.
[0042] The optional addition of Ti, V or Nb can suppress the
formation of a finer-grained microstructure and promote the
ferritic 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 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 0.14% by weight.
[0043] Through the presence of Si and Al, the carbide formation in
the bainite can be suppressed and, associated therewith, the
residual austenite can be stabilized by dissolved carbon. In
addition, primarily Si contributes to the solid solution
solidification. In the steel processed according to the invention,
Al can partly replace the Si content. For this purpose, a minimum
Al content of 0.4% by weight can be provided. This applies in
particular 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.
[0044] The positive influences of the simultaneous presence of Al
and Si can be utilized particularly effectively when the Si and Al
contents within the limits predefined according to the invention
satisfy the condition % Si+0.8% Al>1.2% by weight or even the
condition % Si+0.8% Al>1.5% by weight (where % Si: respective Si
content in % by weight, % Al: respective Al content in % by
weight).
[0045] The formation of the microstructure 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.
[0046] To produce a flat steel product according to the invention,
the 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 at which the hot strip obtained has a completely
recrystallized, austenitic microstructure affording optimum
preconditions for the bainite formation. This is the case when the
hot strip obtained has a hot-rolling end temperature of at least
880.degree. C. when it leaves the last rolling pass, it being
possible for the method according to the invention to be executed
with a particularly high level of operational reliability if the
hot-rolling end temperature is set to at least 900.degree. C. and
does not exceed 1100.degree. C., in particular 1050.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.
before the hot-rolling. If the hot-rolling end temperature falls
below 900.degree. C., widespread softening of the austenite can be
achieved by virtue of the fact that the main deformation of the hot
strip takes place in the last hot-rolling passes. The hot strip
thus obtained likewise has a microstructure with residual austenite
proportions which satisfy the specifications according to the
invention.
[0047] Subsequent to the hot-rolling, the hot strip is subjected to
accelerated cooling with a cooling rate of at least 5.degree. C./s
to a coiling temperature lying in the range of 350-600.degree. C.
The cooling is optimally started here when 50-60% of the austenite
has softened. In practice, a pause of for instance up to 2 s is
provided for this purpose between the end of the hot-rolling and
the start of the cooling. The minimum pause duration tp can be
calculated by means of the following empirical formula:
tp=510.sup.+36T.sup.-12.5,
where tp is the pause duration after the last deformation in
seconds and T is the temperature in .degree. C. The formula gives
the minimum time after which 50-60% of softened austenite is
present. Pause durations calculated therefrom are:
TABLE-US-00001 T [.degree. C.] t [s] 850 1.21 900 0.59 950 0.30
1000 0.16
[0048] Cooling to the coiling temperature is effected here in such
a manner that no transformation of the austenite occurs up until
the coiling. This has the effect that the bainite formation takes
place over a sufficiently long time exclusively in the coil. Once
the hot strip cooled in the manner described above has been wound
to form a coil, for this purpose this coil is cooled in a
temperature range having an upper limit which is the same as the
temperature from which bainite forms from the austenite and having
a lower limit which lies above the temperature from which
martensite forms in the microstructure of the hot strip. The period
of time for which the coil is held in this temperature range is in
this respect chosen in such a way that the bainite proportion of at
least 60% by volume as is desired according to the invention is
achieved. In practice, a period of time of at least 0.5 h is
regularly sufficient for this purpose, with higher bainite contents
being set given a longer period of time.
[0049] Practical investigations have shown that a microstructural
transformation between the end of the hot-rolling and the coiling
can be avoided particularly reliably when the cooling rate is at
least 10.degree. C./s, with practical cooling rates lying in the
range of up to 150.degree. C./s, in particular being 10-50.degree.
C./s.
[0050] The formation of undesired martensite can be avoided
particularly reliably by virtue of the fact that the lower limit of
the coiling temperature is higher than the martensite start
temperature by at least 10.degree. C., in particular at least
20.degree. C.
[0051] At the same time, the desired profile of the bainite
formation can be ensured in practice by virtue of the fact that the
upper limit of the coiling temperature is set at 550.degree. C.
[0052] An optimum profile of the bainite formation taking place
according to the invention in the coil arises when the coiling
temperature at least corresponds to the temperature HTopt
determined by the following formula:
HTMin=MS+(BS-MS)/3
[0053] Here, it is self-evident that the observance of this
temperature is always subject to a certain tolerance under the
operating conditions, i.e. this temperature is generally not
satisfied exactly but instead is observed with a tolerance of
typically +/-20.degree. C.
[0054] The invention will be explained in more detail hereinbelow
on the basis of exemplary embodiments.
[0055] Seven steels S1-S7 were melted, the composition thereof
being shown in Table 1.
[0056] The steel melts of corresponding composition were cast in a
conventional manner to form slabs and then heated similarly in a
conventional manner to a reheating temperature OT.
[0057] The heated slabs were hot-rolled in a similarly conventional
group of hot-rolling stands to form hot strips W1-W10 having a
thickness of 2.0 mm.
[0058] The hot strips W1-W10 emerging from the group of hot-rolling
stands were each at a hot-rolling end temperature ET, proceeding
from which they were subjected to accelerated cooling at a cooling
rate KR to a coiling temperature HT. The hot strips W1-W10 were
wound to form coils at this coiling temperature HT.
[0059] The coils were then each cooled in a temperature range
having an upper limit determined by the respective coiling
temperature HT and a lower limit determined by the martensite start
temperature MS determined for the respective steel S1-S7. The
martensite start temperature MS here was calculated by the
procedure explained in the article "Thermodynamic Extrapolation and
Martensite-Start-Temperature of Substitutionally Alloyed Steels" by
H. Bhadeshia, published in Metal Science 15 (1981), pages
178-180.
[0060] The period of time for which the coil was cooled in the
temperature range defined in the manner described above was of such
a magnitude that the hot strips thus obtained each had a
microstructure consisting of bainite and residual austenite, in
which the proportions of other microstructure constituents were
present at most in ineffective quantities of virtually "0".
[0061] The respective operating parameters of reheating temperature
OT, hot-rolling end temperature ET, cooling rate KR, coiling
temperature HT and martensite start temperature MS are indicated in
Table 2.
[0062] Table 3 additionally shows the mechanical properties
ascertained for the individual hot strips of tensile strength Rm,
yield strength Rp, elongation A80, quality Rm*A80 and also the
respective residual austenite content RA.
[0063] It was found that the tensile strength of at least 1200 MPa
as desired here was not achieved in the case of the hot strip W3,
which was produced from the steel S3 and had a comparatively low Si
content.
[0064] In the case of the hot strip W5, which consisted of the
steel S4 and was not produced according to the invention owing to
the excessively low hot-rolling end temperature ET, up to 12% by
volume of block-like, coarse residual austenite and also coarse
martensite were present in the microstructure, which led to a
considerably impaired elongation A80.
[0065] By contrast, the hot strip W4, which was likewise produced
from the steel S4 but in a manner observing the specifications
according to the invention, merely comprised up to 1% by volume of
coarse block residual austenite with a mean extent of more than 5
.mu.m. The remaining residual austenite was present in film-like
and in finer-block form, with the result that a high elongation A80
was achieved.
[0066] In the case of the hot strip W7 produced from the steel S5
and in the case of the hot strip W10 produced from the steel S7,
the minimum tensile strength of 1200 MPa as desired here was
likewise not achieved. The reason in these cases lay in the
respectively excessively high coiling temperature HT.
TABLE-US-00002 TABLE 1 Steel C Si Al Mn Ni Cu Cr Other S1 0.48 1.5
0.02 1.48 0.034 1.51 0.9 S2 0.51 1.5 0.02 1.58 0.015 1.53 0.9 Ti:
0.013 V: 0.099 S3 0.52 0.4 1.40 1.48 0.030 1.51 0.9 V: 0.09 S4 0.30
1.4 0.02 1.46 0.021 1.47 0.9 Ti: 0.014 V: 0.09 S5 0.51 1.5 0.01
0.40 0.63 0.60 1.3 Ti: 0.011 V: 0.098 Mo: 0.3 S6 0.49 1.5 0.01 0.41
0.60 0.61 1.5 Ti: 0.014 V: 0.1 S7 0.38 2.0 0.02 0.41 0.59 0.57 1.4
Mo: 0.30 Amounts in % by weight, Remainder iron and unavoidable
impurities
TABLE-US-00003 TABLE 2 Hot OT ET KR HT MS strip Steel [.degree. C.]
[.degree. C.] [.degree. C./s] [.degree. C.] [.degree. C.] W1 S1
1150 970 20 350 245 W2 S2 1150 1000 20 500 230 W3 S3 1150 1000 10
450 275 W4 S4 1150 900 10 400 320 W5 S4 1150 850 10 400 320 W6 S5
1200 1000 10 400 270 W7 S5 1200 1000 10 500 270 W8 S6 1200 1000 20
450 270 W9 S7 1200 1000 10 400 315 W10 S7 1200 1000 10 500 315
TABLE-US-00004 TABLE 3 RA Hot Rm Rp A80 RM*A80 [% by strip Steel
[MPa] [MPa] [%] [MPa* %] volume] W1 S1 1357 807 22.2 27387 36 W2 S2
1345 889 21.0 25677 30 W3 S3 1137 807 23.7 24497 32 W4 S4 1346 878
16.5 20190 20 W5 S4 1593 887 6.4 9268 17 W6 S5 1291 778 22.7 26642
29 W7 S5 1166 830 29.1 30846 30 W8 S6 1217 821 25.8 28544 32 W9 S7
1318 751 17.8 21328 17 W10 S7 1164 812 23.4 24761 17
* * * * *