U.S. patent application number 12/514716 was filed with the patent office on 2010-01-07 for process for producing a steel strip comprising a relatively high strength dual phase steel.
This patent application is currently assigned to Salzgitter Flachstahl GmbH. Invention is credited to Thomas Evertz, Thorsten Maiwald, Manuel Otto, Sven Schulz, Jurgen Spehr.
Application Number | 20100000634 12/514716 |
Document ID | / |
Family ID | 39128645 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100000634 |
Kind Code |
A1 |
Spehr; Jurgen ; et
al. |
January 7, 2010 |
PROCESS FOR PRODUCING A STEEL STRIP COMPRISING A RELATIVELY HIGH
STRENGTH DUAL PHASE STEEL
Abstract
A relatively high strength dual phase steel for a cold-rolled or
hot-rolled steel strip with excellent forming properties, in
particular for lightweight vehicle construction, contains the
elements (contents in mass-%): 0.1 to <0.16 of C, 0.02 to
<0.05 of Al, 0.40 to <0.60 of Si, 1.5 to <2.0 of Mn,
<0.020 of P, <0.003 of S, <0.01 of N, 0.01 of Nb, 0.02 of
V, remainder iron including common incidental steel elements with
optional addition of Ti. The demanded dual phase microstructure is
produced during continuous annealing, wherein the cold-rolled or
hot-rolled steel strip is heated in the continuous annealing
furnace in a one-step process to a temperature in the range of 820
to 1000.degree. C., preferably 840 to 1000.degree. C., and the
annealed steel strip is then cooled down from the annealing
temperature with a rate of cooling between 15 and 30.degree.
C./s.
Inventors: |
Spehr; Jurgen; (Sickte,
DE) ; Maiwald; Thorsten; (Seesen, DE) ;
Evertz; Thomas; (Peine, DE) ; Otto; Manuel;
(Magdeburg, DE) ; Schulz; Sven; (Lengede,
DE) |
Correspondence
Address: |
HENRY M FEIEREISEN, LLC;HENRY M FEIEREISEN
708 THIRD AVENUE, SUITE 1501
NEW YORK
NY
10017
US
|
Assignee: |
Salzgitter Flachstahl GmbH
Salzgitter
DE
|
Family ID: |
39128645 |
Appl. No.: |
12/514716 |
Filed: |
November 13, 2007 |
PCT Filed: |
November 13, 2007 |
PCT NO: |
PCT/DE2007/002074 |
371 Date: |
September 8, 2009 |
Current U.S.
Class: |
148/537 ;
148/645 |
Current CPC
Class: |
C21D 8/1222 20130101;
C21D 8/1233 20130101; C22C 38/02 20130101; C22C 38/12 20130101;
C22C 38/04 20130101; C22C 38/06 20130101 |
Class at
Publication: |
148/537 ;
148/645 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2006 |
DE |
10 2006 054 300.9 |
Claims
1.-8. (canceled)
9. A process for producing a cold-rolled or hot-rolled steel strip
of a high strength dual phase steel having excellent forming
property, comprising the steps of: selecting a cold-rolled or
hot-rolled steel strip with a composition comprising, in mass-%,
0.1 to .ltoreq.0.16 of C, 0.02 to .ltoreq.0.05 of Al 0.40 to
.ltoreq.0.60 of Si 1.5 to .ltoreq.2.0 of Mn .ltoreq.0.020 of P
.ltoreq.0.003 of S .ltoreq.0.01 of N 0.01 of Nb 0.02 of V,
remainder iron including common incidental steel elements; heating
the steel strip in a continuous annealing furnace in a one-step
process to an annealing temperature in the range of 820 to
1000.degree. C.; and cooling the annealed steel strip from the
annealing temperature with a rate of cooling between 15 and
30.degree. C./s.
10. The process of claim 9, further comprising the step of adding
Ti.
11. The process of claim 9, wherein the steel strip is heated to a
temperature of 840 to 1000.degree. C.
12. The process of claim 9, wherein the content of V is 0.06%.
13. The process of claim 9, wherein the content of V is 0.08%.
14. The process of claim 9, wherein the content of Nb is 0.02%.
15. The process of claim 9, wherein the content of Nb is 0.04%.
16. The process of claim 10, wherein the content of Ti is
.ltoreq.0.01%.
17. The process of claim 9, further comprising the step of refining
the steel strip by hot dipping.
18. The process of claim 17, further comprising the step of
dressing the steel strip.
19. The process of claim 9, wherein the cold-rolled or hot-rolled
steel strip of dual phase steel is used for lightweight vehicle
construction.
Description
[0001] The invention relates to a process for producing a
cold-rolled or hot-rolled steel strip of a relatively high strength
dual phase steel with excellent forming properties, in particular
for lightweight vehicle construction according to the preamble of
claim 1.
[0002] The hotly contested automobile market forces the
manufacturer i.a. to look continuously for solutions to lower the
fleet consumption while maintaining a highest possible comfort and
greatest possible occupant protection. A crucial role plays hereby
weight saving of all vehicle components, on the one hand, but also
a beneficial behavior of the individual components when exposed to
high static and dynamic stress during operation and in the event of
a crash, on the other hand. Suppliers attempt to take this
requirement into account in such a way that the wall thickness can
be reduced through use of high strength and super high strength
steels while at the same time improving the component behavior
during its manufacture thereof and at operation. Such steels have
to meet therefore comparably high standards with respect to
strength, stretching capacity, toughness, energy consumption and
workability, for example by cold forming, welding and/or surface
treatment.
[0003] So-called dual phase steels find increasingly application in
this area as a result of their excellent formability and high
strength values at the same time. Dual phase steels have hereby
mainly ferritic-martensitic structure.
[0004] Considered in this context are steel strips of dual phase
steel which are cold-rolled as well as hot-rolled.
[0005] For economic reasons, cold-rolled steel strips are normally
subjected to recrystallization annealing by way of a continuous
annealing process into a metal sheet that is easy to shape.
[0006] The furnace parameters (run-through speed, annealing
temperature, rate of cooling) are adjusted in dependence on the
alloy composition and strip thickness in accordance with the
demanded microstructure and mechanical-technological
properties.
[0007] The dual phase microstructure is adjusted by heating the
cold bath in the continuous annealing furnace to such a temperature
that the required ferritic-martensitic microstructure is formed
during cooling.
[0008] When high corrosion standards demand that the surfaces of
hot or cold strips should be galvanized through hot dipping, the
annealing treatment is normally carried out in a continuous
annealing furnace upstream of the galvanizing bath.
[0009] Also in the case of the hot strip, the required dual phase
microstructure is occasionally adjusted depending on the alloying
concept only during annealing treatment in the continuous furnace
in order to be able to realize the demanded mechanical properties
on the basis of an austenitic microstructure which is as homogenous
as possible.
[0010] The alloying concepts for dual phase steels known for
example from the documents EP 0 152 665 B1, EP 0691 415 B1, and EP
0510 718 B1, for use in continuous annealing of hot-rolled or
cold-rolled steel strips are problematic because of the presence of
only a narrow process window for the annealing parameters to
ensure-uniform mechanical properties over the length of the
strip.
[0011] In order for the steels to attain a transformation inertia
that is sufficient for realizing the demanded dual phase
microstructure, when the cold strip undergoes recrystallizing
annealing, the known steels have respective contents, e.g. of Cr,
Mo, Nb, or B. In particular the costly elements Cr and Mo have an
adverse impact on the manufacturing costs of the dual phase
steel.
[0012] A narrow process window is to be understood in this context
as a need to adjust the run-through speed in dependence on
thickness of the strip to be annealed in order to attain a
homogenous temperature distribution in the strip and the demanded
dual phase microstructure and the mechanical-technological
properties during cooling.
[0013] When the process windows are wide, the demanded strip
properties can be realized even when the strips to be annealed have
different thickness while the furnace parameters remain the
same.
[0014] During manufacture, it is oftentimes required to anneal
successive strips of different thickness, e.g. 1.5 and 2.0 mm,
depending on specification.
[0015] A homogenous temperature distribution is difficult to
realize in particular in the transition zone from one strip to
another, when different thicknesses are involved, and lead in the
event of alloy compositions with too small process window to a
situation in which the advance of the thinner strip through the
furnace is too slow, causing a lower productivity, or the advance
of the thicker strip through the annealing furnace is too fast,
posing the risk of failure to realize a homogenous temperature
distribution and thus the demanded mechanical-technological
properties. As a result, increasing waste and even customer
complaints are encountered.
[0016] The problem of an excessively narrow process window is
especially egregious during annealing treatment when load-optimized
components of hot or cold strip should be produced which have
varying sheet thicknesses in length and, optionally, across the
width of the strip, i.e. have been rolled flexibly. A process for
producing a steel strip of varying thickness over the strip length
is described, e.g., in DE 100 37 867 A1.
[0017] When applying the known alloying concepts for dual phase
steels, the presence of the narrow process window renders the
realization of uniform mechanical properties difficult to achieve
over the entire strip length of the respective strip when a
continuous annealing of strips of varying thicknesses is already
involved.
[0018] When the process window is too small, the regions of smaller
sheet thickness in flexibly rolled hot or cold strips of steel of
known compositions have strengths that are too low as a result of
the substantial proportion of ferrite in view of the transformation
processes during cooling, or the regions of greater sheet thickness
reach values that are too high as a result of the substantial
proportion of martensite. Homogenous mechanical-technological
properties over the strip length or across the strip width are
virtually impossible to attain, when using the known alloying
concepts during continuous annealing.
[0019] The invention is therefore based on the object to provide a
different more cost-efficient alloying concept for a relatively
high-strength steel with dual phase microstructure that allows a
broadening of the process window for continuous annealing of hot or
cold strips in such a way that in addition to strips of varying
thickness also steel strips of varying thickness over the strip
length and, optionally, across the strip width can be produced
having mechanical-technological properties which are as homogenous
as possible.
[0020] According to the teaching of the invention, this object is
solved by a steel having the following contents in mass-%:
C 0.1 to .ltoreq.0.16
Al 0.02 to .ltoreq.0.05
Si 0.40 to .ltoreq.0.60
Mn 1.5 to .ltoreq.2.0
P.ltoreq.0.020
S.ltoreq.0.003
N.ltoreq.0.01
Nb=0.01
V=0.02
[0021] remainder iron including common incidental steel elements
with optional addition of Ti, wherein the demanded dual phase
microstructure is produced during continuous annealing, and wherein
the cold-rolled or hot-rolled steel strip is heated in the
continuous annealing furnace in a one-step process to a temperature
in the range of 820 to 1000.degree. C., preferably 840 to
1000.degree. C., and the annealed steel strip is then cooled down
from the annealing temperature with a rate of cooling between 15
and 30.degree. C./s.
[0022] The relatively high strength dual phase steel in accordance
with the invention for the lightweight vehicle construction is
characterized in that the targeted addition of V and Nb while
omitting the cost-intensive alloying elements Mo or CR results in a
transformation inertia which is high enough to enable with very
high process reliability an adjustment of the demanded dual phase
microstructure with homogenous mechanical-technological properties
during continuous annealing from a completely austenitic matrix
even when strips are involved having a thickness which varies over
the strip length or across the strip width.
[0023] Comprehensive laboratory experiments have surprisingly found
that a targeted addition of V in combination with Nb provides a
dual phase steel which allows a significantly broader process
window during continuous annealing. Same microstructure formations
and mechanical-technological properties of the strips can be
realized even when strips of different thickness or strips with
varying thickness are annealed at otherwise constant furnace
parameters.
[0024] The steel according to the invention offers the benefit of a
significantly greater process window compared to known steels. As a
result, process reliability is enhanced during continuous annealing
or hot dip galvanizing of cold and hot strips with dual phase
microstructure. Thus, homogenous mechanical-technological
properties in the strip can be assured in hot-galvanized as well as
continuously annealed hot or cold strips. This applies for
continuous annealing of successive strips with different strip
thickness and in particular for strips with varying sheet thickness
over the strip length and/or strip width.
[0025] When in accordance with the invention relatively
high-strength hot or cold strips of varying sheet thicknesses are
produced by a continuous annealing process, load-optimized
components can be advantageously manufactured from this material
through shaping.
[0026] In accordance with the invention, a dual phase steel is
involved having approx. 20% martensite embedded in the form of
islands in the strength class of about 800 MPa. in particular for
hot dip galvanizing as well as for the application in a continuous
annealing facility.
[0027] As a consequence of the optional addition of Ti in contents
of .ltoreq.0.01%, the fine-grained configuration of the
microstructure and the mechanical-technological properties can be
adjusted in accordance with the invention via the formation of
nitrides or carbonitrides in dependence on the N-content of the
steel.
[0028] The field of application of the steel for rolling with
flexible strip thicknesses in longitude and transverse directions
with respect to the rolling direction is opened up as a result of
its insensitivity against process fluctuations during heat
treatment.
[0029] This insensitivity is effectuated by the use of Nb and in
particular V which cause a transformation-inert or
transformation-free zone during cooling.
[0030] In order to attain a respective effect, the steel has in
accordance with the invention a V content of at least 0.02% and a
Nb content of at least 0.01%. Nb acts hereby as grain refining
element, with the extent of the Nb addition being suited to the
actual C and N contents of the steel.
[0031] The addition of V is also adjusted in accordance with the
invention to the contents of C and N, with the extent of the
addition being suited however in such a way that enough V is kept
in solution in order to realize a sufficient transformation
inertia. When desiring a behavior that is as transformation-inert
as possible and thus to realize a broadest possible process window
during continuous annealing, the V content amounts to at least 0.06
to 0.10% and the Nb content to more than 0.02 to 0.05%. Further
increase of the contents of V and Nb does not provide any further
benefits as far as a further retarded transformation of the steel
is concerned and thus for the broadness of the process window
during continuous annealing.
[0032] In order to attain a substantially homogenous starting
microstructure for adjustment of the dual phase microstructure, the
annealed strip is first heated to a temperature that causes a
completely austenitic microstructure. The annealing temperatures
range hereby for the steel according to the invention between
approx. 820 and approx. 1000.degree. C., depending on the concrete
alloy composition.
[0033] Performed tests have shown that this steel has a zone which
does not undergo a reverse transformation of austenite into
ferrite, bainite, or martensite despite temperatures of less than
800.degree. C. Important is hereby in particular the temperature
range of about 450.degree. C. because the galvanizing bath
temperature is hereby at a level for hot dip galvanizing.
[0034] The adjusted content of ferrite and (residue) austenite
during cooling is maintained until after the process step
"galvanizing". The still present proportion of austenite is then
fully transformed into martensite during continued cooling. The
galvanizing parameters may vary over a wide range. The galvanizing
speeds range between 60 and 120 m/min depending on the sheet
thickness. The rate of cooling before and after the galvanizing
bath ranges at fairly low 10 to 30.degree. C./sec.
[0035] The produced material may be processed as cold bath as well
as also hot bath, in dressed and undressed but also heat-treated
state (intermediate annealing) via a hot dip galvanizing line or a
pure continuous annealing facility.
[0036] At the same time, there is the possibility to vary the
cooling conditions in a targeted manner before the galvanizing bath
in order to increase or decrease the proportion of ferrite. As a
result, it is possible to, e.g., produce partly martensitic steels
(PM).
* * * * *