U.S. patent application number 11/816799 was filed with the patent office on 2008-06-19 for coated steel sheet or strip.
This patent application is currently assigned to ThyssenKrupp Steel AG Kaiser-Wilhelm-Str. l00. Invention is credited to Manfred Meurer, Rudolf Schoenenberg, Wilhelm Warnecke, Sabine Zeizinger.
Application Number | 20080142125 11/816799 |
Document ID | / |
Family ID | 34933875 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080142125 |
Kind Code |
A1 |
Meurer; Manfred ; et
al. |
June 19, 2008 |
Coated Steel Sheet or Strip
Abstract
The invention relates to a coated steel sheet or strip with a
ground coating made of steel, onto at least one upper side of which
a coating is applied by hot-dip galvanizing, the coating being
formed from a melt consisting of 0.05-0.30% by weight Al and
0.2-2.0% by weight Mg, the remainder being zinc and unavoidable
impurities, and, with a coating thickness of a maximum of 3.5 .mu.m
on each side and a coating weight of a maximum 25 g/m.sup.2 on each
side, guarantees that the steel sheet, in the salt spray mist test
carried out in accordance with DIN 50021-SS, shows the first
formation of red rust at the earliest after 250 hours. With such a
sheet or strip, a flat steel product is provided which possesses an
optimum combination of high corrosion resistance and optimum
weldability and which is particularly well-suited for use as a
material for motor vehicle chassis construction or for the
construction of domestic appliances.
Inventors: |
Meurer; Manfred; (Rheinberg,
DE) ; Zeizinger; Sabine; (Mulheim, DE) ;
Schoenenberg; Rudolf; (Hemer, DE) ; Warnecke;
Wilhelm; (Hamminkeln, DE) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Assignee: |
ThyssenKrupp Steel AG
Kaiser-Wilhelm-Str. l00
Duisburg
DE
|
Family ID: |
34933875 |
Appl. No.: |
11/816799 |
Filed: |
February 15, 2006 |
PCT Filed: |
February 15, 2006 |
PCT NO: |
PCT/EP2006/050955 |
371 Date: |
November 16, 2007 |
Current U.S.
Class: |
148/533 |
Current CPC
Class: |
C23C 2/06 20130101 |
Class at
Publication: |
148/533 |
International
Class: |
C23C 2/06 20060101
C23C002/06 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2005 |
EP |
05003762.1 |
Claims
1-5. (canceled)
6. A method for the manufacture of a steel sheet or strip coated on
at least one of side with a coating of 0.05-0.30% by weight Al and
0.2-2.0% by weight Mg, the remainder being zinc and unavoidable
impurities, the coated steel strip, in a salt spray mist test
carried out in accordance with DIN 50021-SS, shows the first red
rust formation at the earliest after 250 hours, the method
comprising: annealing an uncoated steel strip; conducting the
annealed uncoated steel strip through a galvanizing bath including
0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, the remainder
being zinc and unavoidable impurities, to form the coated steel
strip; and adjusting a coating layer thickness and a coating weight
on the coated steel strip emerging from the galvanizing bath using
a stripper nozzle; wherein (i) a surface of the galvanizing bath is
kept under an inert gas flow in relation to the surrounding
atmosphere, to inhibit formation of slag or inter-metallic phases;
and (ii) the coating layer thickness is up to 3.5 .mu.m on each
side and the coating weight is up to 25 g/m.sup.2 on each side.
7. The method of claim 6 wherein the coating includes 0.4-1.0% by
weight Mg.
8. The method of claim 7 wherein the coating includes more than
0.5% by weight Mg.
9. The method of claim 6 wherein the Al content of the galvanizing
bath amounts to 0.12-0.14% by weight.
10. The method of claim 7 wherein the Al content of the galvanizing
bath amounts to 0.12-0.14% by weight.
11. The method of claim 8 wherein the Al content of the galvanizing
bath amounts to 0.12-0.14% by weight.
12. The method of claim 6 wherein the Al content of the galvanizing
bath amounts to at least 0.15% by weight.
13. The method of claim 7 wherein the Al content of the galvanizing
bath amounts to at least 0.15% by weight.
14. The method of claim 8 wherein the Al content of the galvanizing
bath amounts to at least 0.15% by weight.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a National Phase Application of
International Application No. PCT/EP2006/050955, filed Feb. 15,
2006, which claims the benefit of and priority to European
Application No. 05003762.1, filed Feb. 22, 2005, which is owned by
the assignee of the instant application. The disclosure of each of
the above applications is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The invention relates to a coated steel sheet or strip, with
a ground coating made of steel, onto at least one upper side of
which a zinc coating is applied by hot-dip galvanizing.
BACKGROUND
[0003] On steel sheets or steel strips of this type the zinc
coating ensures a high degree of corrosion resistance. The thicker
the coating is, the greater the resistance. Accordingly, with a
conventionally alloyed zinc coating, in a spray test according to
DIN 50021 carried out on a bright, unpainted specimen, with a
coating of 25 g/m.sup.2, red rust already occurs after 24 hours,
while with a coating of 70 g/m red rust does not occur until after
120 hours.
[0004] The thickness of the coating required with the prior art for
adequate corrosion resistance does, however, incur problems with
regard to weldability. This applies in particular if the intention
is that through-welding is to be produced by means of laser welding
in the overlap joint without any joint gaps at high welding speeds,
such as is required in the motor vehicle chassis construction
sector or the domestic engineering sector. The seam produced by
such welding should be free of passage holes, should be largely
free of cratering, and should not have any open pores.
[0005] One possibility for manufacturing hot-dip galvanized steel
sheets with increased corrosion resistance with simultaneously
reduced coating weight is described in EP 0 038 904 B1. According
to this prior art, a zinc coating containing 0.2% by weight Al and
0.5% by weight Mg is applied on a steel substrate by hot-dip
galvanizing. As a result of the magnesium content, the hot-dip
galvanized steel strip obtained in this way, with a coating weight
of 44 g/m.sup.2 per side, in the painted state, had first rust
formation only after a spray duration of more than 2,000 hours
under a salt spray test in which the individual specimen was
sprayed with an NaCl solution under the conditions specified in
Japanese Industrial Standard JIS Z 2371. This long period before
the occurrence of rust was achieved thanks to the combined
corrosion protection of the zinc coating and painting.
[0006] Despite the reduction in the coating weight achieved in
accordance with EP 0 038 904 B1 and the simultaneous good corrosion
resistance, the hot-dip galvanized steel sheets produced in this
way still do not fulfil the requirements imposed in the motor
vehicle construction sector with regard to weldability.
SUMMARY OF THE INVENTION
[0007] The invention, in one embodiment, features a flat steel
product which possesses an optimum combination of high corrosion
resistance and optimum weldability and which is particularly
well-suited for use as a material for motor vehicle chassis
construction or for the production of domestic appliances.
[0008] In one aspect, the invention features a coated steel sheet
or strip, which has a ground coating made of steel, onto at least
one upper side of which a coating is applied by hot-dip
galvanizing, the coating being formed from a melt consisting of
0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, the remainder
being zinc and unavoidable impurities, and on each side, with a
coating thickness of a maximum of 3.5 .mu.m and a coating weight of
a maximum of 25 g/m.sup.2, guarantees that the steel sheet, in the
salt spray mist test carried out in accordance with DIN 50021-SS,
shows the first formation of red rust at the earliest after 250
hours.
DESCRIPTION OF THE INVENTION
[0009] A hot-dip galvanized flat steel product according to the
invention possesses surprisingly good corrosion resistance with a
coating weight minimised in relation to the prior art coating
weight of a maximum of 25 g/m.sup.2 on each side. The low coating
weight and the low thickness of the coating associated with this,
of a maximum of 3.5 .mu.m on each side, in combination with the
high corrosion resistance, makes sheet or strip according to the
invention particularly well-suited for the production of components
which are manufactured by the welding of individual sheet elements.
Accordingly, with steel sheets produced in accordance with the
invention, elements for motor vehicle chassis or domestic appliance
technology can be manufactured in particular, in that the
individual sheet components formed from sheet or strip according to
the invention can be welded to one another by laser beam welding at
high welding speeds economically and with optimum results.
[0010] The corrosion resistance according to the invention is
determined on the basis of a salt spray mist test in accordance
with DIN 50021-SS in a corrosion short-term test process on bright
unpainted steel sheet, in which a neutral 5% NaCl solution, as the
corrosive agent, is sprayed continuously at a temperature of
35.+-.2.degree. C. in a chamber. The steel sheet samples are in
this situation placed in the chamber at an angle of inclination to
the horizontal of 65 to 75.degree.. In the practical test carried
out in this manner, it has been demonstrated that sheets and strips
coated in accordance with the invention regularly do not show any
red rust formation until after a test duration of 300 hours.
[0011] The magnesium content in the melt intended for the coating
remains essentially unchanged in the coating. The Al content of the
coating, in the finished steel strip according to the invention, by
contrast, is as a rule 1.8 to 3.2 and in particular 2 to 3 times
higher than in the melt. An optimum corrosion protection is
attained when the coating has an Mg content of 0.4-1.0% by weight,
in particular at least 0.5% by weight.
[0012] If it is intended that the coating of the steel base
material is to be carried out in the galvanealed process, the melt
contains preferably less than 0.15% by weight of aluminium. Al
contents of the melt which are suitable for standard practice are
in this case in the range of 0.12-0.14% by weight.
[0013] If, by contrast, a conventionally hot-dip galvanized steel
sheet according to the invention is provided, the Al content of the
melt is preferably at least 0.15% by weight.
[0014] A further surprising property which makes a flat product
according to the invention particularly suitable for use in chassis
construction becomes apparent when such a sheet or strip is
painted. Accordingly, a mandrel bend test carried out on the basis
of DIN EN ISO 6860 for sheets or strips according to the invention
at room temperature and at -20.degree. C. produces good paint
adherence capacity. In particular, at a temperature of -20.degree.
C. there is no indication of paint flaking or of flaking of the
coating from the base material.
[0015] For the test carried out to determine the paint adherence
capacity, a full paint structure was applied onto a steel sheet
specimen after alkaline cleaning and phosphating, this structure
comprising a 20 .mu.m thick cathodic dip paint coating, a 32 .mu.m
thick filler paint coating applied onto this and a 40 .mu.m thick
base coating. The bending carried out over the conical mandrel did
not lead to any detachment of the paint coating at room temperature
or at -20.degree. C.
[0016] In addition to a high corrosion resistance and a good paint
adherence capacity, sheets or strips according to the invention
have outstandingly good resistance to stone impact. Thus, for
example, in the stone impact test carried out in accordance with
DIN 65996-1B, it was possible to demonstrate that, with steel
sheets according to the invention, stone impact did not cause any
flaking of the coating from the base coat.
[0017] To manufacture sheets according to the invention, a fine
steel strip is subjected to a continuous hot-dip galvanizing
process in a galvanizing plant operating at a strip speed of a
typical 60 to 150 m/min. To do this, the sheet or strip to be
galvanized is firstly annealed in a furnace, such as a DFF furnace
(Direct Fired Furnace) or, preferably, an RTF furnace (Radiant Type
Furnace). Following on from the furnace, the sheet or strip runs
through the reduction furnace section, in which it is held under a
protective gas atmosphere with 3.5-75% hydrogen. The temperatures
attained in the course of the annealing lie in the range from
720-850.degree. C.
[0018] The sheet or strip annealed in this way is then conducted
via what is referred to as a nozzle, with the exclusion of air,
into the zinc bath, which is formed from a melt containing
0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, in particular
0.4-1.0% by weight and 0.5-1.0% by weight respectively, the
remainder being zinc and unavoidable impurities.
[0019] After the sheet or strip emerges from the melt bath, the
thickness of the coating is restricted in an inherently known
manner by means of stripper nozzles to a value of a maximum of 3.5
.mu.m on each side, with the result that, with the flat product
obtained according to the invention, the coating weight is
restricted to a maximum of 25 g/m.sup.2 per side.
[0020] In order to prevent an over-proportionate formation of slags
and inter-metallic phases on the melt bath, it may be expedient to
conduct an inert gas flow over the surface of the bath. This inert
gas flow can be derived from the stripper nozzles which are used to
adjust the thickness of the coating or can be supplied from
separate nozzles, which distribute the inert gas in mist fashion
over the surface of the bath. As an alternative, the entire melt
bath can be surrounded by a housing enclosure, in which an inert
atmosphere is maintained. Especially suitable as the inert gas for
this purpose is nitrogen.
[0021] The slag formation can also be reduced by adjusting the bath
temperature to a range of 380-450.degree. C. For the same purpose,
the temperature of the strip can be restricted on immersion to
360-500.degree. C. in order in particular to minimise the
inclination to oxidise in the immersion area.
[0022] After emerging from the melt bath the coated strip is cooled
at a cooling speed of at least 10 K/s.
[0023] By means of subsequent in-line rolling at rolling degrees of
0.3-1.5%, if required, the desired texturing of the surface can
then be carried out.
[0024] Inasmuch as the coated is likewise subjected in-line to
subsequent heating in the temperature range of 300-600.degree. C.,
either a redistribution takes place inside the ZnMg coating or
through-alloying is achieved into a ZnFeMg coating. The melts used
to produce such a coating preferably have an Al content of less
than 0.15% by weight, and in particular 0.12-0.14% by weight.
[0025] To extend the range of application, it is also possible then
to apply in an inherently known manner a thin-film layer on the
coating.
[0026] The effects achieved by the invention have been confirmed on
the basis of an experiment, in which a steel strip, 0.82 mm thick,
hard-rolled from conventional IF steel, was initially subjected to
an alkaline spray cleaning, a brush cleaning, and an electrolytic
cleaning.
[0027] This was then followed by annealing, during which the
cleaned steel strip was annealed under protective gas (5% H.sub.2,
remainder N.sub.2) to a temperature of 800.degree. C. The annealing
time was 60 s.
[0028] The steel strip annealed in this way is then cooled, such
that it is immersed at a melt bath immersion temperature of
465.degree. C. into the melt bath, contained in a housing enclosure
under a protective gas atmosphere containing a maximum of 10 ppm
oxygen. The melt bath consisted of a Zn melt, which as well as
unavoidable impurities (e.g. Fe contents, which are drawn into the
melt bath by the strip), contained 0.2% by weight Al, and 0.8% by
weight Mg. Immersion time was two seconds.
[0029] After being conducted out of the melt bath, the coating
thickness on the steel strip applied on both sides was adjusted,
while still inside the melt bath housing enclosure, by means of
stripper nozzles, likewise arranged in the housing enclosure, to a
coating thickness of 3 .mu.m on each side (corresponding to a
coating weight of 21 g/m.sup.2 per side). The stripping was
likewise carried out by means of nitrogen gas.
[0030] To conclude, the steel strip was dressed. The melt
bath-coated steel strip obtained had Ra values of 1.8 .mu.m, with
Pc values of 46 cm.sup.-1 determined in accordance with the
StahlEisen Test Datasheet SEP 1940.
[0031] The ball impact hardness test in accordance with the
StahlEisen Test Datasheet SEP 1931 was carried out on specimens
taken from the finished coated steel strip in order to determine
the adherence of the coating and its formability. The result could
be classified as Stage 1, which corresponds to good adherence and
likewise good ability to forming.
[0032] In a deep drawing test, in which a hat-shaped component was
drawn in a suitable mould out of a round steel sheet blank, a very
low friction value was achieved, of a maximum of 0.45
g/m.sup.2.
[0033] The evaluation of weldability produced, for a laser-beam
welded seam, a very good result in each case. Thus, for example,
with laser beam welding with a joint gap "0" with welding speeds of
up to 5 m/min, error-free results are achieved.
[0034] The salt spray test in accordance with DIN 50021 SS carried
out on an unpainted bright specimen, coated in the manner explained
above, did not have first red rust formation until after a spray
duration of 312 hours. With a conventional Zn coating on a sheet
with a coating weight of 25 g/m.sup.2 per side, red rust formation
already appeared after 24 hours.
[0035] The paint adherence on samples coated according to the
invention, in the conical mandrel bending test based on DIN EN ISO
6860, was good both at room temperature as well as at -20.degree.
C. The stone impact test according to DIN 55996-1B likewise did not
result in any flaking of the coating from the steel base layer.
* * * * *