U.S. patent application number 12/447621 was filed with the patent office on 2010-02-25 for method for manufacturing flat steel products from boron microalloyed multi-phase steel.
This patent application is currently assigned to ThyssenKrupp Steel AG. Invention is credited to Brigitte Hammer, Thomas Heller, Johann Wilhelm Schmitz, Jochen Wans.
Application Number | 20100043513 12/447621 |
Document ID | / |
Family ID | 37782637 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100043513 |
Kind Code |
A1 |
Hammer; Brigitte ; et
al. |
February 25, 2010 |
Method for manufacturing flat steel products from boron
microalloyed multi-phase steel
Abstract
A method, which allows high-tensile flat steel products to be
manufactured with less effort includes a steel that forms a
multi-phase microstructure and contains (in wt. %) 0.08-0.12% C,
1.70-2.00% Mn, up to 0.030% P, up to 0.004% S, up to 0.20% Si,
0.01-0.06% Al, up to 0.0060% N, 0.20-0.50% Cr, 0.010-0.050% Ti,
0.0010-0.0045% B, remainder iron and unavoidable impurities, being
cast into a cast strip having a thickness of 1-4 mm. The cast strip
is hot-rolled in-line into a hot-rolled strip having a thickness of
0.5-3.2 mm in a continuous process at a final hot-rolling
temperature ranging from 800 to 1100.degree. .C, the deformation
degree being greater than 20%. The hot-rolled strip is coiled at a
coiling temperature ranging from 250 to 570.degree. C., so as to
obtain a hot-rolled strip, which has a minimum tensile strength
R.sub.m of 800 MPa at a minimum breaking elongation A.sub.80 of
5%.
Inventors: |
Hammer; Brigitte; (Voerde,
DE) ; Heller; Thomas; (Duisburg, DE) ;
Schmitz; Johann Wilhelm; (Baesweiler, DE) ; Wans;
Jochen; (Dusseldorf, DE) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Assignee: |
ThyssenKrupp Steel AG
Duisburg
DE
|
Family ID: |
37782637 |
Appl. No.: |
12/447621 |
Filed: |
October 24, 2007 |
PCT Filed: |
October 24, 2007 |
PCT NO: |
PCT/EP07/61390 |
371 Date: |
June 16, 2009 |
Current U.S.
Class: |
72/200 |
Current CPC
Class: |
B22D 11/001 20130101;
C21D 8/0426 20130101; C22C 38/28 20130101; C22C 38/06 20130101;
C22C 38/38 20130101; C21D 8/0436 20130101; C21D 8/0473 20130101;
C21D 8/0415 20130101; C21D 2211/008 20130101; B22D 11/06
20130101 |
Class at
Publication: |
72/200 |
International
Class: |
B21B 47/00 20060101
B21B047/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2006 |
EP |
06123139.5 |
Claims
1. Method for manufacturing flat steel products, wherein a steel
that forms a multi-phase microstructure with the following
composition (in wt. %) C: 0.08-0.12% Mn: 1.70-2.00%
P:.ltoreq.0.030% S:.ltoreq.0.004% Si:.ltoreq.0.20% Al: 0.01-0.06%
N:.ltoreq.0.0060% Cr: 0.20-0.50% Ti: 0.010-0.050% B: 0.0010-0.0045%
remainder iron and unavoidable impurities is cast into a cast strip
having a thickness of 1-4 mm, wherein the cast strip is hot-rolled
in-line into a hot-rolled strip having a thickness ranging from 0.5
to 3.2 mm in a continuous process at a final hot-rolling
temperature ranging from 800 to 1100.degree. C., the deformation
degree being greater than 20%, and wherein the hot-rolled strip is
coiled at a coiling temperature ranging from 250 to 570.degree. C.
so as to obtain a hot-rolled strip, which has a minimum tensile
strength R.sub.m of 800 MPa at a minimum breaking elongation
A.sub.80 of 5%.
2. Method according to claim 1, wherein the width of the hot-rolled
strip is greater than 1,200 mm.
3. Method according to claim 1, wherein the thickness of the
hot-rolled strip is 1.5 mm at most.
4. Method according to claim 1, wherein the hot-rolled strip is
cold-rolled into cold-rolled strip having a thickness of 0.5-1.4
mm.
5. Method according to claim 4, wherein the cold-rolled strip is
annealed at an annealing temperature of 750-850.degree. C.
6. Method according to claim 4, wherein a minimum tensile strength
of the cold-rolled strip is 800 MPa.
7. Method according to claim 4, wherein the cold-rolled strip has a
minimum breaking elongation A.sub.50 of 10%.
8. Method according claim 1, wherein the hot-rolled strip is
provided with a metallic coating.
9. Method according to claim 8, wherein the metallic coating is a
zinc coating.
10. Method according to claim 1, wherein with a minimum breaking
elongation A.sub.80 of the obtained hot-rolled strip of 10%, the
final hot-rolling temperature is 900-1020.degree. C. and the
coiling temperature is 420-490.degree. C.
11. Method according to claim 1, wherein with a minimum tensile
strength R.sub.m of the obtained hot-rolled strip of 1000 MPa, the
final hot-rolling temperature is 900-1100.degree. C. and the
coiling temperature is 450-570.degree. C.
12. Method according to claim 1, wherein with a minimum tensile
strength R.sub.m of the obtained hot-rolled strip of 1200 MPa, the
final hot-rolling temperature is 800-1000.degree. C. and the
coiling temperature is 250-550.degree. C.
13. Method according to claim 4, wherein the cold-rolled strip is
provided with a metallic coating.
14. Method according to claim 13, wherein the metallic coating is a
zinc coating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a National Phase Application of
International Application No. PCT/EP2007/061390, filed on Oct. 24,
2007, which claims the benefit of and priority to European patent
application no. EP 06 123 139.5, filed on Oct. 30, 2006. The
disclosures of the above applications are incorporated herein by
reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a method for manufacturing flat
steel products, such as strips or sheet metal blanks, from
high-tensile, boron microalloyed steels. Such steels belong to the
group of multi-phase steels. These are usually steels, the
properties of which are determined by type, quantity and alignment
of the phases of the microstructure. Therefore at least two phases
exist in the microstructure (ferrite, martensite, bainite for
example). As a result, they have a superior strength/formability
combination compared to conventional steels.
BACKGROUND
[0003] Because of these special features, multi-phase steels are of
major interest for automotive construction, since due to their high
strength on the one hand they allow the use of smaller material
thicknesses and consequently at the same time a reduction in the
vehicle weight and on the other hand improve the safety of the
vehicle body in the event of a collision (crash behavior). Thus,
multi-phase steels with at least equal strength of the overall body
permit a reduction in the sheet metal thickness of a component made
from such multi-phase steels compared to a body made from
conventional steels.
[0004] Usually, multi-phase steels are melted in a converter steel
mill and cast on a continuous casting machine into slabs or thin
slabs, which are then hot-rolled into hot-rolled strip and coiled.
In this case the mechanical properties of the hot-rolled strip can
be varied by selectively controlled cooling of the hot-rolled strip
after hot-rolling with the aim of adjusting certain microstructural
fractions. The hot-rolled strip can also be cold-rolled into
cold-rolled strip in order to also obtain thinner sheet metal
thicknesses (EP 0 910 675 B1, EP 0966547 B1, EP 1 169486 B1, EP 1
319725 B1, EP 1 398390 A1).
[0005] This manufacturing route creates problems particularly with
respect to casting compositions solidifying peritectically. In the
case of these steel grades there is a risk of longitudinal cracks
arising during continuous casting. The emergence of such
longitudinal cracks can lower the quality of the hot-rolled strip
produced from cast slabs or thin slabs so severely that they become
unusable. In order to prevent this risk, extensive measures are
necessary, such as increased flame treatment, which can go as far
as making the conversion of such steel grades uneconomic. When
casting steel with high Al content, unwanted effects also come
about due to interaction with the powdered fluxes, as the result of
which the quality of a flat product made from this steel is also
negatively affected.
[0006] A problem with manufacturing flat products from high-tensile
multi-phase steels with a tensile strength of more than 800 MPa is
that high rolling forces must be applied when rolling such steels.
This requirement has the consequence that normally with the
production machines at present generally available, high-tensile
hot-rolled strip made from steel of the type under discussion can
often only be manufactured in a width and thickness, which no
longer fully meet the requirements demanded today by the automotive
industry. In particular, strip of narrow thickness with sufficient
width cannot be produced very well in conventional installations.
Also, with conventional methods it is shown in practice that it is
difficult to manufacture cold-rolled strip with a strength of more
than 800 MPa from multi-phase steel.
[0007] An alternative way to produce steel strip from a multi-phase
steel has been proposed in the European Patent EP 1 072 689 B1 (DE
600 09 611 T2). In accordance with this known method firstly a
steel melt, which (in wt. %) contains 0.05 and 0.25% C, in total
0.5-3% Mn, Cu and Ni, in total 0.1-4% Si and Al, in total up to
0.1% P, Sn, As and Sb, in total less than 0.3% Ti, Nb, V, Zr and
REM (i.e., rare earth metals) as well as in each case less than 1%
Cr, Mo and V, remainder iron and unavoidable impurities, is cast
into cast strip having a thickness of 0.5-10 mm, in particular 1-5
mm. The cast strip is subsequently hot-rolled in-line into a
hot-rolled strip in one or more passes, the deformation degree
ranging between 25% and 70%. The final hot-rolling temperature in
this case is above the Ar.sub.3 temperature. At the end of
hot-rolling, the obtained hot-rolled strip is then cooled down in
two steps. In the first step of this cooling a cooling rate of
5-100.degree. C. per second is maintained until a temperature
ranging between 400-550.degree. C. is reached. The hot-rolled strip
is then held at this temperature for a dwell time, which is needed
in order to allow bainitic transformation of the steel with a
residual austenite content greater than 5%. The formation of
pearlite in this case is to be avoided. After a dwell time
sufficient to obtain the required microstructure, the
transformation process is interrupted by the beginning of the
second cooling step, wherein the hot-rolled strip is brought to a
temperature below 400.degree. C., in order then to wind it into a
coil at a coiling temperature below 350.degree. C.
[0008] With the method described in EP 1 072 689 B1 it should be
possible to produce hot-rolled strip with bainitic microstructural
fractions in a simple way from a multi-phase steel, which has TRIP
characteristics ("TRIP"="Transformation Induced Plasticity"). Such
steel has relatively high strength with good formability. However,
the strength is not sufficient for many applications, particularly
in the field of automotive construction.
SUMMARY OF THE INVENTION
[0009] In general, an aspect of the invention is to provide a
method, which allows high-tensile flat steel products to be
manufactured with less effort in a wide range of geometrical
dimensions.
[0010] In one embodiment, the aspect indicated above has been
achieved by a method for manufacturing flat steel products, wherein
according to the invention a steel that forms a multi-phase
microstructure, which (in wt. %) contains 0.08-0.12% C, 1.70-2.00%
Mn, up to 0.030% P, up to 0.004% S, up to 0.20% Si, 0.01-0.06% Al,
up to 0.0060% N, 0.20-0.50% Cr, 0.010-0.050% Ti, 0.0010-0.0045% B
and remainder iron and unavoidable impurities, is cast into a cast
strip having a thickness of 1-4 mm, wherein the cast strip is
hot-rolled in-line into a hot-rolled strip having a thickness of
0.5 to 3.2 mm in a continuous process at a final hot-rolling
temperature ranging from 800 to 1100.degree. C., the deformation
degree being greater than 20%, and wherein the hot-rolled strip is
coiled at a coiling temperature ranging from 250 to 570.degree. C.,
so as to obtain a hot-rolled strip, which has a minimum tensile
strength R.sub.m of 800 MPa at a minimum breaking elongation
A.sub.80 of 5%.
[0011] The invention provides a method of casting to convert a
particularly high-tensile, possibly peritectically solidifying
multi-phase steel into a hot-rolled strip. Since the cast strip
itself in this case already possesses a narrow thickness, only
relatively low deformation degrees must be maintained in the course
of hot-rolling this strip, in order to manufacture flat products
with narrow thicknesses, as they are needed particularly in the
field of automotive construction. Thus it is possible with the
method according to the invention, by specifying a corresponding
initial thickness of the cast strip, to produce without any
problems hot-rolled strip, which with an optimal characteristic
distribution has a maximum thickness of 1.5 mm and from which
components for the support structure of a vehicle for example can
be manufactured.
[0012] Due to the low deformation degrees during hot-rolling, the
rolling forces necessary for this, compared to the forces necessary
for hot-rolling slabs or thin slabs with the conventional method,
are low, so that hot-rolled strip of large width, which lies
substantially above the width of hot-rolled strip of the same
strength and thickness cast in the conventional way, can be
produced without any problems with the method according to the
invention. Thus, the invention permits high-tensile hot-rolled
strip, consisting of a martensitic steel with the composition
indicated and processed according to the invention, the width of
which is greater than 1,200 mm, in particular greater than 1,600
mm, to be reliably produced.
[0013] The application according to the invention of the strip
casting process for converting high-tensile steels of the type
composed according to the invention, apart from the advantages
mentioned above, due to their characteristics and process variables
specific to the method (hot-rolling final temperature, cooling,
coiling temperature for example) offers the possibility, also in
respect to their solidification behavior, of reliably casting
critical steel compositions of the type processed according to the
invention. Thus very rapid solidification of the cast strip,
characteristic of strip casting, leads to a substantially reduced
risk of the emergence of center liquations, compared to
conventional production, with the consequence that the hot-rolled
strip produced according to the invention has a particularly
uniform characteristic distribution and microstructure over its
cross section and its length.
[0014] A further special advantage of the method according to the
invention is that the hot-rolled strip produced according to the
invention has a high strength of at least 800 MPa, without in
addition a special cooling cycle of the hot-rolled strip having to
be maintained between the end of hot-rolling and coiling, which is
prescribed for example in EP 1 072 689 B I as the result of the
need for a cooling interruption. In carrying out the method
according to the invention, it must only be ensured that
hot-rolling is terminated in a relatively closely confined
temperature window and also that coiling is carried out in a
precisely defined temperature range. Single-step cooling takes
place in the interim.
[0015] A further advantage of the method according to the invention
is that an extension in the range of mechanical properties of the
strip produced according to the invention can be achieved, based on
a single steel analysis, by varying the cooling and rolling
conditions.
[0016] Hot-rolled strip produced according to the invention is
particularly suitable for subsequent conversion into cold-rolled
strip. Accordingly, one practical embodiment of the invention makes
provision for the hot-rolled strip to be cold-rolled into
cold-rolled strip having a thickness of 0.5-1.4 mm, in particular
0.7 mm up to 1.3 mm, as is needed for constructing automotive
bodies. In order to eliminate solidifications arising during
cold-rolling, the cold-rolled strip can be annealed at an annealing
temperature of 750-850.degree. C. For cold strip produced in this
way from hot-rolled strip manufactured according to the invention,
a minimum tensile strength of 800 MPa can be reliably ensured. At
the same time just as reliably the minimum breaking elongation
A.sub.50 of the cold-rolled strip is 10%.
[0017] In accordance with a further advantageous embodiment of the
invention the cold-rolled strip is provided in the way known per se
with a metallic coating, in which, for example, this can be a zinc
coating.
[0018] The strength and elongation values of hot-rolled strip
produced according to the invention can be adjusted over a large
range by corresponding coordination of the final hot-rolling and
coiling temperatures. If for example hot-rolled strip, which has a
minimum breaking elongation A.sub.80 of the obtained hot-rolled
strip of 10% and a minimum tensile strength R.sub.m of 800 MPa, is
to be manufactured, this can be achieved due to the final
hot-rolling temperature being 900-1000.degree. C. and the coiling
temperature being 420-510.degree. C.
[0019] On the other hand, if a hot-rolled strip with a higher
tensile strength R.sub.m of at least 1000 MPa at a minimum breaking
elongation A.sub.80 of 5% is to be manufactured, in order to do
this a final hot-rolling temperature ranging from 900 to
1100.degree. C. and a coiling temperature ranging from 450 to
570.degree. C. are selected.
[0020] Even higher tensile strengths R.sub.m of the obtained
hot-rolled strip of at least 1200 MPa with a minimum breaking
elongation A.sub.80 of 5% can be achieved due to the final
hot-rolling temperature being 800-1000.degree. C. and the coiling
temperature being 250-550.degree. C.
[0021] The invention is described in detail below on the basis of
exemplary embodiments.
DESCRIPTION
[0022] In trials carried out to demonstrate the effect of the
invention, two steels A and B composed according to the invention
with the composition indicated in Table 1 are melted and, in a
conventional two-roll casting machine, cast into cast strip, which
was 1.6 mm thick.
TABLE-US-00001 TABLE 1 (data in wt. %) C Mn P S Si Al N Cr Ti B A
0.102 1.76 0.005 0.004 0.14 0.014 0.0057 0.24 0.016 0.0027 B 0.098
1.81 0.005 0.003 0.19 0.060 0.0048 0.37 0.045 0.0044
[0023] The strips cast out of the steels A and B have been
hot-rolled in-line directly after the strip was cast into a
hot-rolled strip, having a thickness of 1.25 mm, at a final
hot-rolling temperature WET. Subsequently, the obtained hot-rolled
strip in each case was directly cooled in a cooling step to a
coiling temperature HT and coiled. After coiling the hot-rolled
strips produced from the steels A and B in each case had a tensile
strength R.sub.m and a breaking elongation A.sub.80, which are
indicated in Table 2 as the final hot-rolling temperature WET and
coiling temperature HT maintained in each case during their
production.
TABLE-US-00002 TABLE 2 WET HT R.sub.m A.sub.80 Trial Steel
[.degree. C.] [.degree. C.] [MPa] [%] 1 B 950 500 878 11.3 2 B 1050
480 1073 5.5 3 A 830 285 1234 6.2 4 B 950 540 1041 5.3 5 B 950 510
1263 5.5 6 A 950 440 1244 5.1
[0024] The hot-rolled strip produced according to Trial 4 from
steel B, after coiling and pickling, was cold-rolled into a 0.7 mm
thick cold-rolled strip and annealed in-line at a temperature of
800.degree. C., in order to recrystallize the strip.
[0025] With a breaking elongation A.sub.50 of 11.5%, the tensile
strength R.sub.m of the cold-rolled strip obtained in this way was
835 MPa.
* * * * *