U.S. patent application number 13/058085 was filed with the patent office on 2011-06-23 for method for producing a steel strip having a dual-phase microstructure.
This patent application is currently assigned to SMS SIEMAG AG. Invention is credited to Christian Bilgen, Christian Klinkenberg.
Application Number | 20110146850 13/058085 |
Document ID | / |
Family ID | 41353805 |
Filed Date | 2011-06-23 |
United States Patent
Application |
20110146850 |
Kind Code |
A1 |
Klinkenberg; Christian ; et
al. |
June 23, 2011 |
Method for Producing A Steel Strip Having A Dual-Phase
Microstructure
Abstract
A method for producing semi-finished product, particularly steel
strip, having a dual-phase microstructure and a tensile strength
between 500 and 1000 MPa. Elements such as carbon and nitrogen
which are interstitially dissolved in a ferritic matrix are bound
by alloying elements such as Al, Mo, Nb, Ti and V which form
carbide, nitride or carbonitride in order to adjust a
microstructure of hard components such as martensite, bainite,
carbide with low fractions of residual austenite in a ferritic
matrix, which is substantially free from interstitially dissolved
elements (IF character).
Inventors: |
Klinkenberg; Christian;
(Herdecke, DE) ; Bilgen; Christian; (Duesseldorf,
DE) |
Assignee: |
SMS SIEMAG AG
Duesseldorf
DE
|
Family ID: |
41353805 |
Appl. No.: |
13/058085 |
Filed: |
August 7, 2009 |
PCT Filed: |
August 7, 2009 |
PCT NO: |
PCT/DE2009/001136 |
371 Date: |
March 9, 2011 |
Current U.S.
Class: |
148/527 ;
148/559 |
Current CPC
Class: |
C22C 38/12 20130101;
C21D 8/02 20130101 |
Class at
Publication: |
148/527 ;
148/559 |
International
Class: |
C23C 2/02 20060101
C23C002/02; C21D 1/00 20060101 C21D001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 8, 2008 |
DE |
10 2008 038 865.3 |
Claims
1.-6. (canceled)
7. A method for producing semi-finished product having a dual-phase
microstructure and a tensile strength between 500 and 1000 MPa,
comprising: interstitially dissolving elements including one or
more of carbon and nitrogen in a ferritic matrix; binding the
interstitially dissolved elements by alloying elements including
one or more of Al, Mo, Nb, Ti and V, which form carbide, nitride,
or carbonitride to adjust a microstructure of hard components
including one or more of martensite, bainite, carbide having low
fractions of residual austenite in a ferritic matrix that is
substantially free from interstitially dissolved elements; and
adapting an alloy content of the alloying elements
stoichiometrically to a ferrite content and to a solubility of the
carbon and the nitrogen in the ferrite at a temperature of at least
one of a subsequent heat treatment and heating.
8. The method according to claim 7, wherein for production of a hot
strip with a DP microstructure, the hot strip is cooled in a
two-phase region after exiting a finishing mill train of a rolling
mill to adjust a suitable amount of residual austenite to achieving
a desired strength class, and the hot strip is subsequently cooled
in an accelerated manner to a temperature below a martensite start
and finish temperature.
9. The method according to claim 7, wherein for producing a cold
strip with a DP microstructure, the cold strip is heated in a
two-phase region to adjust a suitable amount of residual austenite
and the cold strip is subsequently cooled in an accelerated manner
to a temperature below a martensite start and finish
temperature.
10. The method according to claim 9, wherein the heat treatment for
adjusting the DP microstructure in the cold strip is performed in a
continuous annealing installation.
11. The method according to claim 7, wherein the heat treatment is
performed between 400.degree. C. and A.sub.3, where A.sub.3 is the
temperature at which grains of ferrite start to form.
12. The method according to claim 7, wherein the semi-finished
product is steel strip.
13. The method according to claim 7, wherein the at least one of
the subsequent heat treatment and the heating is a hot-dip
galvanization.
Description
PRIORITY CLAIM
[0001] This is a U.S. national stage of Application No.
PCT/DE2009/001136, filed on Aug. 7, 2009, which claims priority to
German Application No: 10 2008 038 865.3, filed: Aug. 8, 2008, the
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention is directed to a method for producing
semi-finished product, particularly steel strip, having a
dual-phase microstructure (DP steel strip) and a tensile strength
between 500 and 1000 MPa.
[0004] 2. Related Art
[0005] High-strength steel sheet with good formability, stability
of microstructures, and mechanical characteristics up to
approximately 600.degree. C. is required in many fields.
[0006] One such field is the automotive industry, for example, in
which efforts are made to reduce emissions by reducing the weight
of structural chassis parts because the consumption of fuel can be
reduced by reducing weight. On the other hand, there is also a
demand for achieving a greater strength of the sheets that are
used.
[0007] Finally, the sheets should also be suitable for hot-dip
galvanization.
[0008] However, steels with a dual-phase microstructure that has
already been adjusted have the disadvantage that an unwanted,
pronounced yield strength generally results when heated above
200.degree. C. as occurs during or in connection with hot-dip
galvanization.
[0009] Owing to this absence of temperature stability in the
materials above approximately 200.degree. C., semi-finished
products such as strip, sheet, pipe and other structural components
or workpieces comprising DP steel are not suitable for hot-dip
galvanization.
SUMMARY OF THE INVENTION
[0010] Therefore, it is the object of the invention to provide a
method for the production of a semi-finished product, particularly
DP steel strip, which is economical and by which a DP steel strip
can be produced which not only has optimal formability properties
but also has a temperature stability up to 600.degree. C. and is
therefore also suitable for hot-dip galvanization.
[0011] According to one embodiment of the invention, a method for
producing semi-finished product, particularly steel strip, having a
dual-phase microstructure and a tensile strength between 500 and
1000 MPa, is disclosed. Elements such as carbon and nitrogen which
are interstitially dissolved in the ferritic matrix are bound by
alloying elements such as Al, Mo, Nb, Ti and V which form carbide,
nitride or carbonitride in order to adjust a microstructure of hard
components such as martensite, bainite, carbide with low fractions
of residual austenite in a ferritic matrix which is substantially
free from interstitially dissolved elements (IF character).
[0012] The alloy content of the alloying elements is adapted
stoichiometrically to the ferrite content and to the solubility of
carbon and nitrogen at the temperature of a subsequent heating or
heat treatment of the DP microstructure.
[0013] In this way, the semi-finished product or a structural
component part manufactured therefrom acquires the properties
characteristic of a DP steel with respect to microstructure,
strength, minimum elongation, yield strength ratio, and strain
hardening exponent also after a subsequent heat treatment or
heating, particularly hot-dip galvanization and possibly subsequent
dressing.
[0014] For the production of hot strip with a DP microstructure,
the hot strip is cooled in the two-phase region after exiting the
finishing mill train in order to adjust a suitable amount of
residual austenite and is subsequently cooled in an accelerated
manner to a temperature below the martensite start and finish
temperature.
[0015] Conversely, for producing cold strip with a DP
microstructure, the cold strip is heated in the two-phase region to
adjust a suitable amount of residual austenite and is subsequently
cooled in an accelerated manner to a temperature below the
martensite start and finish temperature. This heat treatment is
preferably carried out in a continuous annealing installation.
EXAMPLE
[0016] In order to exclude the influence of dissolved N on the
aging behavior, the N content should be fixated as low as possible
and by adding Al and possibly Ti. A correspondingly increased V
content can also be used for nitrogen fixation. Table 1 contains
some possible chemical compositions:
TABLE-US-00001 TABLE 1 Chemical composition concept IF-DP in
percent by mass No. Concept C Si Mn P S Al Mo Ti Nb V Cr N 1
reference 0.06 0.35 1.1 0.02 0.004 0.025 -- -- -- -- 0.65 min. 2 Mo
0.2 -- 3 MoV 0.06 0.35 1.1 0.02 0.004 0.025 0.1 -- -- 0.05 0.65
min. 4 V -- 0.1
BRIEF DESCRIPTION OF DRAWINGS
[0017] In the accompanying diagrams:
[0018] FIG. 1 is a cooling curve of steel sheet after hot rolling
for adjusting a dual-phase microstructure; and
[0019] FIG. 2 is a typical stress-strain diagram for a DP steel
with low yield strength ratio (.ltoreq.75%) and without a
pronounced yield strength.
DETAILED DESCRIPTION OF THE DRAWINGS
[0020] In FIG. 1, the austenite fraction is plotted over the time
axis on the left-hand side and the core temperature is plotted over
the time axis on the right-hand side.
[0021] The cooling curve of a steel sheet after hot rolling for
adjusting a dual-phase microstructure is shown by way of
example.
[0022] In so doing, the ferrite region is cooled initially, and
most of the austenite is transformed to ferrite within a very short
time period. This first cooling stage can be followed by a holding
period, or further cooling to temperatures below the martensite
start temperature is carried out directly. In so doing, any
remaining residual austenite transforms to martensite and generates
the second, hard phase. A coiling temperature of about 200.degree.
C. is aimed for.
[0023] The solid line shows the austenite dissociation as cooling
time increases. The curve in dashes shows the lowering of the core
temperature, also over the cooling time. It can be seen that
cooling is accelerated between approximately 600.degree. C. and the
holding temperature of 200.degree. C.
[0024] The method provides a semi-finished product, particularly
steel strip, having a dual-phase microstructure and a tensile
strength between 500 and 1000 MPa elements such as carbon and
nitrogen that are interstitially dissolved in the ferritic matrix
are bound by alloying elements such as Al, Mo, Nb, Ti and V which
form carbide, nitride or carbonitride in order to adjust a
microstructure of hard components such as martensite, bainite,
carbide with low fractions of residual austenite in a ferritic
matrix which is substantially free from interstitially dissolved
elements (IF character). The alloy content of the aforesaid
alloying elements is adapted stoichiometrically to the ferrite
content and to the solubility of carbon and nitrogen in the ferrite
at the temperature of a subsequent heat treatment or heating,
particularly hot-dip galvanization. The production of hot strip
with a DP microstructure, the hot strip is cooled in the two-phase
region after exiting the finishing mill train of the rolling mill
in order to adjust a suitable amount of residual austenite for
achieving the desired strength class, and is subsequently cooled in
an accelerated manner to a temperature below the martensite start
and finish temperature.
[0025] For producing cold strip with a DP microstructure, the cold
strip is heated in the two-phase region to adjust a suitable amount
of residual austenite and is subsequently cooled in an accelerated
manner to a temperature below the martensite start and finish
temperature. Heat treatment for adjusting the DP microstructure in
cold strip is preferably carried out in a continuous annealing
installation. The heat treatment is carried out between 400.degree.
C. and A.sub.3. The A3 point is the temperature at which grains of
ferrite start to form.
[0026] Thus, while there have shown and described and pointed out
fundamental novel features of the invention as applied to a
preferred embodiment thereof, it will be understood that various
omissions and substitutions and changes in the form and details of
the devices illustrated, and in their operation, may be made by
those skilled in the art without departing from the spirit of the
invention. For example, it is expressly intended that all
combinations of those elements and/or method steps which perform
substantially the same function in substantially the same way to
achieve the same results are within the scope of the invention.
Moreover, it should be recognized that structures and/or elements
and/or method steps shown and/or described in connection with any
disclosed form or embodiment of the invention may be incorporated
in any other disclosed or described or suggested form or embodiment
as a general matter of design choice. It is the intention,
therefore, to be limited only as indicated by the scope of the
claims appended hereto.
* * * * *