U.S. patent application number 13/505253 was filed with the patent office on 2012-11-08 for manufacturing galvannealed sheets by heat-treating electrolytically finished sheets.
This patent application is currently assigned to VOESTALPINE STAHL GMBH. Invention is credited to Josef Faderl, Johann Gerdenitsch.
Application Number | 20120279868 13/505253 |
Document ID | / |
Family ID | 42942044 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120279868 |
Kind Code |
A1 |
Gerdenitsch; Johann ; et
al. |
November 8, 2012 |
MANUFACTURING GALVANNEALED SHEETS BY HEAT-TREATING ELECTROLYTICALLY
FINISHED SHEETS
Abstract
The invention relates to a method for manufacturing a steel
strip with a cathodic corrosion protection coating, in which the
steel strip is hot rolled and then cold rolled; the steel strip is
subjected to an electrolytic galvanization and after the
electrolytic galvanization, the steel strip is heat treated in a
bell-type annealing furnace at temperatures of 250.degree. C. to
350.degree. C. for a period of 4 to 48 hours and this produces a
zinc/iron layer.
Inventors: |
Gerdenitsch; Johann; (Linz,
AT) ; Faderl; Josef; (Steyr, AT) |
Assignee: |
VOESTALPINE STAHL GMBH
Linz
AT
|
Family ID: |
42942044 |
Appl. No.: |
13/505253 |
Filed: |
September 13, 2010 |
PCT Filed: |
September 13, 2010 |
PCT NO: |
PCT/EP2010/063351 |
371 Date: |
July 26, 2012 |
Current U.S.
Class: |
205/207 ;
205/206 |
Current CPC
Class: |
C21D 2221/00 20130101;
C21D 8/0478 20130101; C25D 5/36 20130101; C21D 1/70 20130101; C23C
28/02 20130101; C23C 28/021 20130101; C25D 5/50 20130101; C25D 3/22
20130101; C25D 7/0614 20130101; C23C 28/025 20130101; C21D 1/72
20130101; C21D 9/663 20130101 |
Class at
Publication: |
205/207 ;
205/206 |
International
Class: |
C25D 5/36 20060101
C25D005/36 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 3, 2009 |
DE |
102009051673.5-45 |
Claims
1. A method for manufacturing a steel strip with a cathodic
corrosion protection coating, comprising: hot rolling the steel
strip and subsequently cold rolling the steel strip; subjecting the
steel strip to an electrolytic galvanization; and after the
electrolytic galvanization, heat treating the steel strip in a
bell-type annealing furnace at temperatures of 250.degree. C. to
350.degree. C. for a period of 4 to 48 hours to produce a zinc/iron
layer.
2. The method as recited in claim 1, comprising flexibly cold
rolling the steel strip in such a way that the steel strip has
periodically different thicknesses and/or widths over its length
and/or a steel strip with isotropic or higher-strength properties
is used.
3. The method as recited in claim 1, wherein before the
electrolytic galvanization, the flexibly rolled strip is subjected
to a recrystallization annealing at 550.degree. C. to 650.degree.
C. in the bell-type annealing furnace or in a continuous annealing
furnace.
4. The method as recited in claim 1, wherein a layer thickness of
the electrolytically deposited zinc coating is between 2 .mu.m and
10 .mu.m.
5. The method as recited in claim 4, comprising annealing the steel
strip in order to convert the zinc layer into a zinc/iron layer in
such a way that the zinc/iron layer contains a maximum of 30% iron.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method for manufacturing coated
steel sheets.
BACKGROUND OF THE INVENTION
[0002] It is known to produce steel sheets with a coating composed
of a metal to protect the steel from corrosion.
[0003] These are basically divided into so-called barrier
protection coatings and cathodically acting protection
coatings.
[0004] Barrier protection coatings are in particular protective
coatings that are composed of aluminum, tin, or chromium; to
produce them, for example an aluminum alloy is deposited onto a
steel strip by means of a so-called hot-dip coating process.
[0005] The most widely used cathodically acting corrosion
protection layer is a zinc coating; in particular, its action is
based on the fact that when the zinc layer on the steel is damaged
all the way down to the steel substrate, the zinc, as the
chemically less noble metal, is corroded first, thus protecting the
steel substrate.
[0006] Zinc coatings include pure zinc coatings, zinc coatings with
a low aluminum content, zinc coatings with an aluminum content of
about 5% (Galfan), and zinc aluminum coatings with approximately
half zinc and half aluminum.
[0007] These coatings are likewise applied using the hot dipping
method in which a preheated steel strip is conveyed through a bath
of zinc or zinc alloy.
[0008] So-called galvannealed layers represent a special case in
which first, a hot-dip galvanization is used to deposit a zinc
layer or zinc alloy layer onto a steel substrate and then an
annealing step is carried out so that a diffusion reaction occurs
between the zinc on the one hand and the iron of the steel
substrate on the other so that a zinc/iron alloy layer forms. Such
a layer is referred to as the galvannealed layer.
[0009] Such a galvannealed layer is produced at annealing
temperatures of 480.degree. C. to 600.degree. C. in continuous
annealing furnaces through which the strip passes after the
galvanization.
[0010] DE 10 2007 031 91 96 A1 relates to a method for producing
flexibly rolled strip stock with a cathodic corrosion protection
layer; in this method, flexibly rolled material, i.e. steel strip
with different steel strip thicknesses over its length, is among
other things electrolytically galvanized and then annealed, with
the annealing treatment being performed at <420.degree. C.
[0011] DE 10 2007 013 739 has disclosed a method for flexibly
rolling coated steel strips; these steel strips can also be
electrolytically galvanized and continuously annealed.
[0012] DE 10 2004 023 886 B4 has disclosed a method and an
apparatus for finishing flexibly rolled strip stock in which
flexibly rolled strip stock with a periodically varying material
thickness is conveyed continuously through a processing line
composed of an annealing section, a quenching unit, a preheating
unit, and a zinc pot, and is thus heat treated and hot-dip
galvanized; the inlet temperature of the flexibly rolled strip into
the zinc pot is regulated to a constant value by varying the heat
energy in the preheating unit as a function of the strip thickness
and the zinc depositing thickness is regulated to a constant value
by varying the distance of outlet nozzles from the flexibly rolled
strip as a function of the strip thickness.
[0013] DE 10 2004 023 886 B4 has disclosed a method and an
apparatus for finishing flexibly rolled strip stock in which the
cold-rolled strip is to have a property profile that is matched to
its thickness progression, where a first annealing treatment is
carried out at a temperature between 500.degree. C. and 600.degree.
C., after which the cold-rolled strip is rolled to a predefined
thickness progression so that in the rolling direction, the
flexible cold-rolled strip has at least one region of greater
thickness and one region of lesser thickness; this is then followed
by a second annealing treatment in which the temperature is higher
than in the first annealing treatment.
[0014] It is, however, problematic to apply a so-called
galvannealed layer onto strips that have been flexibly rolled in
this way. The application of the galvannealed layer is usually
carried out by means of a hot-dip galvanization followed by a
continuous inline heat treatment. But this method cannot be used
for providing a galvannealed coating to steel grades that are
impossible or very expensive to manufacture by means of a hot-dip
galvanization. These include isotropic steels, higher strength
steels with strengths >1,000 MPa, and the aforementioned
flexibly rolled strips. Specifically with flexibly rolled strips in
a hot-dip galvanizing system, the different sheet thicknesses
result in both irregular mechanical properties and differing
zinc/iron growth rates since in inline processes, the thick regions
naturally heat up differently than the thin regions, necessarily
resulting in correspondingly different diffusion speeds between
zinc and iron.
[0015] There are other reasons, however, why manufacturing a
flexibly rolled strip with a galvannealed coating in a hot-dip
galvanizing system seems hardly feasible. Even with an induction
galvannealing system, the required accuracy of the output control
(variation of the coil power by a factor of up to two and more
within a few centimeters of positioning accuracy) would be
difficult to achieve.
[0016] The object of the invention is to create a method for
manufacturing flexibly rolled or isotropic or higher strength
galvannealing-coated steel strips.
SUMMARY OF THE INVENTION
[0017] According to the invention, a method for manufacturing a
steel strip with a cathodic corrosion protection coating includes
hot rolling the steel strip and subsequently cold rolling the steel
strip; subjecting the steel strip to an electrolytic galvanization;
and, after the electrolytic galvanization, heat treating the steel
strip in a bell-type annealing furnace at temperatures of
250.degree. C. to 350.degree. C. for a period of 4 to 48 hours to
produce a zinc/iron layer.
[0018] The method may also include flexibly cold rolling the steel
strip in such a way that the steel strip has periodically different
thicknesses and/or widths over its length and/or a steel strip with
isotropic or higher-strength properties is used.
[0019] In certain embodiments, before the electrolytic
galvanization, the flexibly rolled strip may be subjected to a
recrystallization annealing at 550.degree. C. to 650.degree. C. in
the bell-type annealing furnace or in a continuous annealing
furnace.
[0020] A layer thickness of the electrolytically deposited zinc
coating may be between 2 .mu.m and 10 .mu.m.
[0021] The method may also include annealing the steel strip in
order to convert the zinc layer into a zinc/iron layer in such a
way that the zinc/iron layer contains a maximum of 30% iron.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] According to the invention, the problems arising due to
different temperatures in the flexibly rolled strip are
circumvented in that the strip is not hot-dip galvanized but
instead electrolytically galvanized, thus avoiding different
build-up heights of the zinc and iron phases due to temperature
differences in the strip during the hot-dip galvanization. The same
approach can be used to avoid problems when hot-dip galvanizing
isotropic or higher strength steels.
[0023] Also according to the invention, the galvannealed coating is
produced not continuously in an inline fashion, but instead, the
galvannealed layer formation takes place under protective gas in a
bell-type annealing furnace. Also according to the invention, the
temperature of the annealing is reduced; according to the
invention, temperatures of about 300.degree. C. are maintained for
holding times of approximately 20 hours.
[0024] The slow heating in the bell-type annealing furnace results
in a uniform heating of the strip so that the zinc iron-reaction
also occurs very uniformly in strip regions with different
thicknesses.
[0025] According to the invention, a strip is flexibly cold rolled,
is subjected to a recrystallization annealing in a bell-type
annealing furnace at approximately 650.degree. C. for 24 hours,
following which the strip is then temper rolled and
electrolytically galvanized and then undergoes a galvannealing step
in the bell-type annealing furnace.
[0026] The invention therefore has the advantage that a steel strip
with periodically varying sheet thickness (flexibly rolled strip
(tailor rolled blank)) can be provided with a high-quality cathodic
corrosion protection, which has good weldability; advantageously,
both isotropic and higher strength steels as well as other steel
grades can likewise be provided with a galvannealed layer.
[0027] It is also advantageous that steel sheets that are
galvannealed according to the invention can be produced with very
thin galvannealed coatings, which is made possible on the one hand
by the electrolytic galvanization and on the other hand, by the
gentle cooling.
[0028] According to the invention, isotropic steels and higher
strength steels that can only be hot-dip galvanized with difficulty
can be provided with a galvanized coating if the electrolytic
galvanization is followed by the above-described bell-type
annealing furnace step according to the invention.
* * * * *