U.S. patent application number 16/047581 was filed with the patent office on 2018-11-29 for method for manufacturing a product from a flexibly rolled strip material.
This patent application is currently assigned to Muhr und Bender KG. The applicant listed for this patent is Muhr und Bender KG. Invention is credited to Jorg Dieter Brecht, Wolfgang Eberlein.
Application Number | 20180340266 16/047581 |
Document ID | / |
Family ID | 49515225 |
Filed Date | 2018-11-29 |
United States Patent
Application |
20180340266 |
Kind Code |
A1 |
Eberlein; Wolfgang ; et
al. |
November 29, 2018 |
Method for Manufacturing a Product from a Flexibly Rolled Strip
Material
Abstract
A method for manufacturing a product from a flexibly rolled
strip material includes the steps of: providing a strip material
made from sheet steel; flexibly rolling the strip material such
that a variable thickness is produced along the length of the strip
material; electrolytically coating the strip material with a
metallic coating material containing at least 93% of zinc by mass
after the flexible rolling; heat treating at temperature above
350.degree. C. and below a solidus line of the coating material
after the electrolytic coating; working a blank from the flexibly
rolled strip material; and hot forming the blank.
Inventors: |
Eberlein; Wolfgang;
(Wilnsdorf, DE) ; Brecht; Jorg Dieter; (Olpe,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Muhr und Bender KG |
Attendorn |
|
DE |
|
|
Assignee: |
Muhr und Bender KG
Attendorn
DE
|
Family ID: |
49515225 |
Appl. No.: |
16/047581 |
Filed: |
July 27, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15670041 |
Aug 7, 2017 |
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16047581 |
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14078025 |
Nov 12, 2013 |
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15670041 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B21B 2261/05 20130101;
C21D 1/673 20130101; C25D 5/36 20130101; C21D 8/0205 20130101; C21D
8/0405 20130101; C21D 2221/00 20130101; C21D 8/0442 20130101; Y10T
428/12389 20150115; C21D 9/48 20130101; C25D 5/50 20130101; B21B
2205/02 20130101; Y10T 29/49986 20150115; B21D 39/02 20130101; C25D
7/0614 20130101; B21B 37/26 20130101; C25D 5/48 20130101 |
International
Class: |
C25D 5/36 20060101
C25D005/36; C21D 1/673 20060101 C21D001/673; C25D 7/06 20060101
C25D007/06; C21D 9/48 20060101 C21D009/48; C21D 8/04 20060101
C21D008/04; C21D 8/02 20060101 C21D008/02; B21B 37/26 20060101
B21B037/26; C25D 5/50 20060101 C25D005/50; C25D 5/48 20060101
C25D005/48; B21D 39/02 20060101 B21D039/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 14, 2012 |
DE |
10 2012 110 972.9 |
Claims
1. A method for manufacturing a product from a flexibly rolled
strip material comprising the steps of: providing a strip material
made from hardenable sheet steel, flexible rolling the strip
material, wherein a variable thickness is produced along the length
of the strip material, electrolytically coating the strip material
with a metallic coating material that contains at least 93% by mass
of zinc, wherein the electrolytic coating is carried out after the
flexible rolling, after the electrolytically coating, heat treating
the strip material at temperatures above 350.degree. C. and below a
solidus line of the coating material, wherein diffusion takes place
between iron contained in the strip material and zinc contained in
the metallic coating material so as to form a zinc-iron layer,
working a blank from the flexibly rolled strip material, and hot
forming the blank such that a formed and hardened product is
produced.
2. The method according to claim 1 wherein the metallic coating
material has a minimum of 5% by mass of iron and a maximum of 7% by
mass of iron.
3. The method according to claim 2 wherein the proportions of zinc
and iron in the coating material are selected such that at least
partially .delta.1-phase is present after the step of
electrolytically coating the strip of material.
4. The method according to claim 2 wherein the temperature is
increased during the heat treatment.
5. The method according to claim 2 wherein the heat treatment is
carried out inductively or by means of annealing in a bell-type
annealing furnace, wherein the annealing is carried out with a
holding time of 10 hours to 80 hours.
6. The method according to claim 1 wherein before the
electrolytically coating step, the strip material is coated with an
intermediate layer.
7. The method according to claim 6 wherein the intermediate layer
contains nickel or aluminum or manganese.
8. The method according to claim 1 wherein after the
electrolytically coating step, a scaling prevention is
deposited.
9. The method according to claim 1 wherein the hot forming includes
the steps of: cold pre-forming of the blank to a cold pre-formed
component, heating at least a partial area of the cold pre-formed
component up to austenitization temperature, and hot post-forming
of the cold pre-formed component for producing a final contour.
10. The method according to claim 1 wherein the hot forming step
includes the steps of: heating at least a partial area of the blank
up to the austenitization temperature, and hot forming of the blank
for producing a final contour.
11. The method according to claim 1 wherein at a point of time when
initiating the hot forming step, the coating material is in a solid
state.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 15/670,041, filed on Aug. 7, 2017; which is a
continuation patent application of U.S. patent application Ser. No.
14/078,025, filed Nov. 12, 2013; which claims priority from German
Patent Application No. 10 2012 110 972.9, filed Nov. 14, 2012. The
disclosures of all the aforementioned applications are expressly
incorporated herein by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a method for manufacturing coated
steel sheets made from a flexibly rolled strip material. The steel
sheet should be protected against corrosion by means of the
coating.
[0003] Different methods for coating components made from steel
with a zinc or zinc alloy layer are known, like hot galvanization
(hot-dip galvanization) or galvanic (electrolytic) galvanization.
Hot galvanization means plating of steel parts with a solid
metallic zinc coating by means of dipping of the pretreated steel
parts into a melt of liquid zinc. Galvanic galvanization is carried
out by dipping the workpieces into a zinc electrolyte. Electrodes
of zinc serve, because of their less precious metal, as a
"sacrificial anode". The workpiece to be galvanized serves as a
cathode, because of which the coating is also characterized as a
cathodic corrosion protection.
[0004] From DE 10 2007 013 739 B3 a method for the flexible rolling
of coated steel strips is known. A hot or cold strip is
electrolytically coated and subsequently flexibly rolled, wherein
the coated steel strips receive different sheet thicknesses along
the length. The coating is adjusted to the sheet thickness after
flexible rolling or to the rolling pressure during the flexible
rolling. For this, the coating is formed varyingly thick.
[0005] From DE 10 2009 051 673 B3 a method for manufacturing steel
strips with a cathodic corrosion protection layer is known. For
this, the steel strip is hot rolled, subsequently cold rolled and
is electrolytically galvanized. After the electrolytic
galvanization, the steel strip is heat treated in a bell-type
annealing furnace at temperatures from 250.degree. C. to
350.degree. C. for a time of 4 to 48 hours, whereby a zinc-iron
layer is produced.
[0006] From DE 10 2007 019 196 A1 a method for producing flexibly
rolled strip material with a cathodic corrosion layer is known. The
method comprises the steps of providing a rolled strip as a hot or
cold strip with a cathodic corrosion layer, and flexibly cold
rolling of the coated rolled strip with a rolling gap adjustable
during the rolling process.
[0007] From DE 601 19 826 T2 a method for achieving a workpiece
with very high mechanical properties is known, which starting from
a steel sheet strip is formed by means of deep-drawing. The
workpiece is hot rolled and coated with a metallic alloy made from
zinc. For this, the sheet is cut to size, heated up to a
temperature of 800.degree. C. to 1200.degree. C. and subsequently a
hot deep drawing process is carried out. Then, the sheet excesses,
necessary for the deep drawing process, are removed by means of
cutting.
SUMMARY OF THE INVENTION
[0008] The present invention is based on the object to provide a
method for manufacturing coated steel sheets from a flexibly rolled
strip material, which offers an especially good corrosion
protection.
[0009] A first solution consists of a method for manufacturing a
product from a flexibly rolled strip material comprising the steps:
providing a strip material made from sheet steel, flexible rolling
of the strip material, wherein a variable thickness is produced
along the length of the strip material, electrolytic coating with a
metallic coating material, which contains at least 93% by mass of
zinc, wherein the electrolytic coating is carried out after the
flexible rolling, heat treatment at temperatures above 350.degree.
C. and below a solidus line of the coating material, wherein the
heat treatment is carried out after the electrolytic coating,
working a blank from the flexibly rolled strip material, and cold
or hot forming of the blank.
[0010] A second solution is a method for manufacturing a product
from a flexibly rolled strip material comprising the steps:
providing a strip material from sheet steel, flexible rolling of
the strip material, wherein a variable thickness is produced along
the length of the strip material, electrolytic coating with a
metallic coating material, which at least contains zinc and iron,
working a blank from the flexibly rolled strip material and cold or
hot forming of the blank.
[0011] An advantage of the two above named methods is, that the
electrolytic coating is carried out after the flexible rolling.
Thus, it is achieved, that the deposited coating has a constant
thickness along the length of the flexibly rolled strip material.
Insofar also the areas of the strip material, which are stronger
rolled-out, have a layer thickness, which reliably protects against
corrosion. Altogether the process time for manufacturing the
product can be shortened and less coating material is necessary,
which again has an advantageous effect on the manufacturing
costs.
[0012] A flexibly rolled product is, in connection with the present
invention, understood to be a steel strip with varying thickness as
well as a rectangular blank or a form-cut (profile cut),
respectively, which is produced from a flexibly rolled steel strip
by means of mechanical cutting or laser cutting. As strip material
for the flexible rolling, a hot strip or cold strip can be used,
wherein these terms should be understood in the sense of the
technical terminology. A hot strip is here seen to be a rolling
steel finished product (steel strip), which is produced by means of
rolling after preliminary heating. A cold strip is here meant to be
a cold rolled steel strip (flat steel), at which the last thickness
reduction is carried out by means of rolling without a preceding
heating. The strip material which is provided for being rolled can
also be referred to as band material.
[0013] In both of the above named solutions it is understood, that
between the individual method steps, further steps could be
interposed. For example, after the flexible rolling, a strip
straightening can be provided. The working of the blanks from the
strip material can be carried out before or after the electrolytic
coating. Conceptually, "working a blank from a strip" is supposed
to include, that the sheet blank can be stamped from the strip
material, which means an edge remains at the strip, which is not
further used, as well as, that a simple cutting of the strip
material into partial pieces can be carried out, especially by
means of a cutting process. Working a blank from a strip can also
be referred to as producing a blank from a strip.
[0014] In the first solution, a coating consisting at least of 93%
by mass of zinc is deposited on the strip material, wherein the
proportion of zinc may especially be larger than 95% by mass, 97%
by mass, or 99% by mass and can even be 100% (pure zinc coating).
For the electrolytic coating, anodes made from pure zinc or from
zinc and other alloy elements, are used, which during feeding of
current deposit metal ions on the electrolyte. The zinc ions and
possible ions of further alloy elements are deposited as atoms on
the strip material, which is connected as a cathode, and form a
coating. In a deposition of a coating with a high proportion of
zinc of more than 93% by mass, as it is provided in the first
solution, the following heat treatment leads in an advantageous
manner to an alloy formation between the deposited zinc and the
iron contained in the strip material, so that altogether a
zinc-iron coating is produced.
[0015] In the second solution from the start a zinc-iron-alloy
layer is produced by means of electrolytic deposition. The
proportions of zinc and iron are preferably selected such, that at
least one of the following conditions is valid: the alloy layer
contains at least 5% by mass of iron, the alloy layer contains at a
maximum 80% by mass of iron, the alloy layer contains at a minimum
20% by mass of zinc and/or the alloy layer contains at a maximum
95% by mass of zinc. It is especially advantageous when the
proportions of zinc and iron are selected such that in the
deposited state, at least partially .delta.1-phase, especially
.delta.1-phase and .GAMMA.-phase, or only intermetallic
.GAMMA.-phase is present. This is, for example, achieved with an
iron proportion of 10% by mass to 30% by mass or a zinc proportion
of 70% by mass to 90% by mass, wherein the addition of further
alloy elements is not excluded. In this embodiment, a subsequent
heat treatment is not necessary, as the coating itself already
contains zinc and iron. The zinc and iron atoms are arranged at a
distance of few nanometers from each other so that especially short
diffusion paths are produced. It can however be understood that
also with an electrolytic deposition of a zinc-iron alloy, the
named heat treatment can be carried out. By means of the short
diffusion paths, a very short heat treatment is sufficient, for
example by means of induction heating. Altogether by means of the
named method process a shortening of the process time can be
achieved in an advantageous manner.
[0016] The method according to the second solution can be carried
out according to a first possibility without heat treatment after
the electrolytic coating and before forming. According to a second
possibility of the second solution, a heat treatment at a
temperature range above 350.degree. C. and below the melting
temperature of the coating material (solidus line) can be provided
as a further step after the electrolytic coating. The solidus line
marks in the finite state diagram for the coating material that
line, below which only solid phase is present. Above the solidus
line the coating material is at least present partially as
melt.
[0017] With progressing heating time, the iron proportion in the
coating increases, as iron atoms diffuse from the base material
into the coating material. Because of the increasing iron
proportion in the coating, the heat treatment temperature can then
be increased, without reaching the solidus line or exceeding it.
This is possible with suitable process control up to a temperature
of 781.degree. C. The possibility of the temperature increase
during the heat treatment is obviously also valid for the first
solution. The temperature can be step-wise or continuously
increased with increasing iron proportion.
[0018] The liquidus line marks in the finite state diagram for the
coating material that line, below which a two-phase or multi-phase
range, solid-liquid, is present. Above the liquidus line, the
coating material is in the liquid form. The lower limit of the
two-phase range is characterized as the solidus line. The
temperature of the solidus line depends on the proportional
composition of the alloy. For pure zinc, the solidus line lies at
419.5.degree. C., for a zinc-iron alloy it is maximal 782.degree.
C., insofar as still parts of F-phase are present. With a
corresponding proportion of iron it is, thus, possible, to
electrolytically coat the flexibly rolled strip material in a full
hard (hard as rolled) condition and subsequently to carry out a
heat treatment at a relatively high temperature of more than
500.degree. C. up to maximal 782.degree. C., without that a liquid
phase is produced.
[0019] A heat treatment in a temperature range of 500.degree. C. up
to 782.degree. C. is, furthermore, suitable to carry out a
re-crystallization annealing, so that the produced material is
especially suited for an indirect hot forming. An otherwise
necessary re-crystallization annealing can, thus, be omitted after
the flexible rolling and before the coating. For example, in the
first named solution with the use of pure zinc (coating material
100% zinc), the heat treatment process can be started at an
annealing temperature of 380.degree. C. and, with increasing iron
proportions due to diffusion processes, can then be step-wise
increased up to a temperature of maximal 781.degree. C.
[0020] For both solutions it applies, that the coating material can
also contain further alloy elements, like for example manganese,
chromium, silicon or molybdenum. Independent of the type and number
of alloy elements, a feature of the invention is the temperature
control for the purpose of forming the zinc-iron alloy layer. The
respective alloy temperature is selected such, that the solidus
line of the coating material in the composition, currently present
during the process, is reached or exceeded at no point of time of
the alloy formation of the binary zinc-iron phase diagram or of a
layer structure, containing more than two alloy elements,
respectively. The alloy is thus formed by solid phase
diffusion.
[0021] During the heat treatment, a diffusion of iron from the
coated material into the metallic coating takes place. In this
case, zinc of the coating converts into a zinc-iron alloy, which
offers a cathodic corrosion protection. The stated temperature
range above 350.degree. C. and below the solidus line is especially
advantageous, as the diffusion takes place relatively quickly.
Because of the iron content, the affinity to solder cracking of the
coating is reduced, so that the fatigue limit of the component is
increased.
[0022] The phase conversion can be achieved, as mentioned above,
according to a first possibility by means of inductive heating.
This process method is especially suitable in an electrolytic
deposition of zinc and iron, as here short diffusion paths are
present, so that a short heat treatment can lead already to the
required phase conversion. According to a second possibility, the
heat treatment can be carried out by annealing in a bell-type
annealing furnace. This annealing is especially suitable for the
electrolytic deposition of pure zinc. Preferably, during the
annealing in an annealing furnace a holding time of 10 to 80 hours,
preferably 30 to 60 hours, is provided, so that sufficient time is
available, so that by means of diffusion a zinc-iron alloy is
produced. The holding time (dwell time) characterizes preferably
the whole time, in which the blanks or the strip material is heat
treated, and can also comprise a heating-, holding, and cooling
phase. A further possibility is the conductive heating, but other
technically possible heat treatment methods are obviously not
excluded.
[0023] As a further method step it can be provided before the
electrolytic coating that the strip material is coated with an
intermediate layer. As intermediate layer, especially a nickel or
aluminum containing layer can be used. These are layers, which
contain at least partially nickel or aluminum, which also includes
a pure nickel layer or a pure aluminum layer. The nickel layer
forms an additional protection of the surface and improves the
adhesion of the coating, subsequently deposited and containing
zinc. The nickel coating can be formed, for example, by
electrolytic deposition or deposition without a current from an
external source. It is obvious, that other materials are not
excluded for the intermediate layer. For example, also a coating
containing manganese or chromium can be used. Manganese and
chromium have both a cubic lattice and have a good solubility in
iron, which has advantageous effects on the alloy behavior.
[0024] According to a possible embodiment, the strip material can
be provided with a scaling protection after the electrolytic
coating. This is especially applicable, when the austenitization
for a later hot forming is not carried out in an inert gas
atmosphere. Scaling are mainly oxidic corrosion products, produced
during the reaction of metallic materials in air or other oxygen
containing gases at a high temperature. The deposition of the
scaling protection layer can be carried out by spraying or rolling,
respectively coating. Besides the protection against oxidation, a
further advantage of the scaling protection layer is, that the
surface has a high quality. Especially, before a later vanishing of
the sheet, no cleaning treatment like shot-blasting is necessary.
Furthermore, because of the scaling protection, the friction value
is positively influenced during the hot forming as well as the heat
absorption behavior. A further advantage of the scaling protection
is, that the adhesion of the cathodic corrosion protection layer
arranged below is improved. Furthermore, a widening of the
temperature-time window in the frame of the austenitization is
possible, for example by means of alloy formation of the scaling
protection material with the below arranged layer. The scaling
protection can be deposited before or after the heat treatment
carried out below the solidus line.
[0025] At a suitable position of the process, blanks or form cuts
are produced from the flexibly rolled strip material, which can be
carried out by means of mechanically cutting or by means of laser
cutting. Blanks are understood to be especially rectangular sheet
plates, which are cut from the strip material. Form cuts means in
particular sheet elements, cut from the strip material, which outer
contour is already adapted to the form of the final product.
Predominantly the term blanks is used uniformly for rectangular
blanks as well as for form cuts. The manufacture of blanks can be
carried out before or after the electrolytic coating and if
necessary before or after the deposition of a scaling
protection.
[0026] According to a possible process embodiment which is valid
for both solutions, the sheet blanks are hot formed. Hot forming
means forming processes in which the workpieces are heated up to a
temperature in the range of the hot forming, before being formed.
The heating is carried out in a suitable heating device, for
example in a furnace. The hot forming can be carried out according
to a first possibility as an indirect process, which comprises the
partial steps cold pre-forming of the blanks to a pre-formed
component, subsequent heating at least of partial areas of the cold
pre-formed component up to an austenitization temperature, as well
as subsequent hot forming for producing the final contour of the
product. Austenitization temperature is understood to be a
temperature range, in which at least a partial austenitization
(structural conditions in the two-phase area of ferrite and
austenite) is present. Furthermore, it is also possible, to
austenitize only partial areas of the blank, to enable for example
a partial hardening. The hot forming can be carried out according
to a second possibility also as a direct process, which is
characterized in that at least partial areas of the blank are
directly heated to austenitization temperature and are subsequently
hot formed to the required final contour in one step. An earlier
(cold) pre-forming does not take place here. Also during the direct
process, a partial hardening can be achieved by means of
austenitization of partial areas. For both processes it is valid,
that a hardening of partial areas of the component is also possible
by means of varyingly tempered tools or by using several tool
materials, which enable different cooling velocities. In the latter
case, the whole blank or the whole component can be completely
austenitized.
[0027] According to a process embodiment, which is valid for both
hot forming processes, the coating material at the point of time of
initiating the hot forming is preferably in the solid state, i.e.
the temperature has cooled down to an area below the solidus line
of the coating material. After the hot forming, the iron content in
the boundary layer is below 80% by mass, preferably below 60% by
mass, especially preferred below 30% by mass.
[0028] According to an alternative process embodiment, which is in
principle valid for both above named solutions, the sheet blanks
can also be cold formed. Cold forming are forming processes, in
which the blank is not specifically heated before forming. The
forming thus takes place at room temperature, the blanks are heated
by the dissipation of the fed energy. Cold forming is especially
used as a process for forming soft car body steels.
[0029] The solution of the above named object is further a sheet
blank made from flexibly rolled sheet steel, which is
electrolytically coated after the flexible rolling with a metallic
coating and is hot formed after the coating. Thus, the above named
advantages of a constant layer thickness along the length of the
flexibly rolled strip or of the blanks produced therefrom is
achieved. The blanks are produced according to one or more of the
above named method steps, so that concerning the steps and the
advantages connected therewith it is referred to the above
description.
[0030] Following, preferred embodiments are described by using the
figures. It shows
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 a method according to the invention as a flow chart
schematically in a first embodiment,
[0032] FIG. 2 a method according to the invention as a flow chart
schematically in a second embodiment,
[0033] FIG. 3 a method according to the invention as a flow chart
schematically in a third embodiment, and
[0034] FIG. 4 a zinc-iron-phase diagram.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] FIG. 1 shows a method according to the invention for
manufacturing a product from a flexibly rolled strip material 2
according to a first processing embodiment. In the method step V1,
the strip material 2, which is wound onto a coil 3 in the starting
condition, is worked by rolling, more particularly by means of
flexible rolling. For this, the strip material 2, which before the
flexible rolling has a more substantially constant sheet thickness
along the length, is rolled by rolls 4, 5 such, that a variable
sheet thickness is produced in longitudinal direction of the
rolling direction. During rolling, the process is monitored and
controlled, wherein the data, determined from a sheet thickness
measurement 6, are used as input signals for adjusting the rolls 4,
5. After the flexible rolling, the strip material 2 has varying
thicknesses in rolling direction. The strip material 2 is wound
again to a coil 3 after the flexible rolling, so that it can be
transferred to the next method step.
[0036] After the flexible rolling, the strip material 2 is smoothed
in the method step V2, which is carried out in a strip
straightening device 7. The method step of smoothing is optional
and can be omitted.
[0037] After the flexible rolling (V1) or the smoothing (V2),
respectively, the strip material 2 is provided with an
anticorrosive coating in the method step V3. For this, the strip
material 2 passes through an electrolytic strip coating device 8.
It is visible, that the strip coating is produced in through-feed
method, this means, that the strip material 2 is wound from the
coil 3, passes through the coating device 8 and after the coating
process is again wound to a coil 3. This method process is
especially advantageous, as the handling expenditure is small for
depositing the anticorrosive coating onto the strip material 2 and
the process velocity is high. The strip coating device 8 comprises
an immersion tank 9, which is filled with an electrolytic liquid
10, through which the strip material 2 runs. Guiding of the strip
material 2 is achieved by means of sets of rolls 11, 12.
[0038] The electrolytic coating is achieved in the present method
embodiment with a metallic coating material, which contains at
least 93% by mass zinc. Because of the high zinc content, an
especially good resistance to corrosion is achieved. It is
understandable, that the zinc proportion could also be higher, for
example larger than 95% by mass, especially larger than 97% by mass
and can even be 100% by mass (pure zinc). For the coating process
for example anodes made from zinc can be used, which release during
a current feed zinc ions to the electrolyte. The zinc ions are
deposited as zinc atoms and form a zinc layer on the strip material
2, which is connected as a cathode. Alternatively, also inert
anodes and a zinc electrolyte can be used.
[0039] Besides the mentioned zinc proportion, the coating can still
contain further alloying elements, as for example aluminum,
chromium, manganese, molybdenum, silicon. The proportion of the
added alloying elements, if necessary, are less than 7% by mass.
Manganese has a good solubility in iron, which has an advantageous
effect on the alloy formation during heating.
[0040] After the electrolytic coating (V3), the strip material 2,
wound to a coil 3, is heat treated in method step V4. The heat
treatment can be carried out in principle in any technically
suitable manner, for example in an annealing furnace such as a
bell-type annealing furnace or also by means of inductive heating,
to only name two methods for example. In the present case the heat
treatment is shown in a furnace 13.
[0041] The heat treatment is carried out at temperatures larger
than 350.degree. C. and below the solidus line of the coating
material. The temperature profile of the solidus line depends on
the proportional composition of the alloy. At the temperature
within the stated range, a diffusion of iron is triggered into the
zinc layer, so that with progressing holding time of the heat
source a diffusion layer is produced.
[0042] During the heat treatment a diffusing of iron from the to be
coated strip material into the metallic coating takes place. In
this case, zinc of the coating converts into a zinc-iron alloy,
which offers a cathodic corrosion protection system. Because the
temperature range is above 350.degree. C. and below the solidus
line, the diffusion takes place relatively quickly. The holding
time for the heat treatment in an annealing furnace is preferably
10 to 80 hours, preferably 30 to 60 hours, so that sufficient time
is available, so that a zinc-iron alloy is formed by diffusion.
[0043] A further effect of the heat treatment is, that hardenings
of the material, produced during the rolling, are reduced or
disappear, so that the rolled strip material 2 takes up again a
higher ductility and elasticity. The strip material can be easier
further processed in the following method steps, wherein
furthermore the material properties of the to be manufactured
finished product can be positively influenced.
[0044] After the heat treatment (V4) the individual sheet blanks 20
are worked from the strip material 2 in the next method step V5.
The working of the sheet blanks 20 from the strip material 2 takes
place preferably by means of stamping or cutting. Depending on the
shape of the to be manufactured sheet blanks 20, these can be
stamped from the strip material 2 as a shape cut, wherein a strip
of the strip material remains, which is not further used, or the
strip material 2 can simply be cut to length into partial pieces. A
sheet blank 20, worked from the strip material 2, which also could
be characterized as three-dimensional sheet blanks (3D-TRB), is
shown schematically.
[0045] After producing the blanks 20 from the strip material 2, a
forming of the blanks 20 to the required finished product takes
place in method step V5. According to a first possibility the
blanks 20 are hot formed or according to a second possibility cold
formed.
[0046] The hot forming can be carried out as a direct or indirect
process. During the direct process, the blanks 20 are heated to the
austenitization temperature before the forming, which heating can
for example be done by means of induction or in a furnace.
Austenitization temperature is, in this case, a temperature range,
in which at least a partial austenitization (structure in the
binary phase region ferrite and austenite) are present. However,
also partial areas of the blanks can be austenitized, to enable for
example a partial hardening. After the heating to the
austenitization temperature, the heated blank is formed in a
shape-giving tool 14 (forming tool) and at the same time is cooled
with a high cooling velocity, wherein the component 20 receives its
final profile and is hardened at the same time.
[0047] During the indirect hot forming, the blanks 20 are
pre-formed before the austenitization. The pre-forming takes place
in the cold condition of the blank, i.e. without prior heating.
During the pre-forming the component receives its profile, which
however still does not correspond to the final shape, however this
is approximated. Then, after the pre-forming an austenitization and
hot forming takes place, like during the direct process, wherein
the component receives its final shape and is hardened.
[0048] The steel material should, insofar as a hot forming (direct
or indirect) is provided, contain a proportion of carbon of at
least 0.1% by mass up to 0.35% by mass.
[0049] Alternatively to the hot forming as a shape giving process,
the blanks can also be cold formed. The cold forming is especially
suitable for soft body steels or components, which do not have
special requirements in view of strength. During the cold forming,
the blanks are formed at room temperature.
[0050] A special feature of the method according to the invention
is, that the electrolytic coating (V3) is carried out after the
flexible rolling (V1). The coating deposited on the strip material
2 has a constant thickness along the length, i.e. independent of
the respective thickness of the strip material 2. Also the areas,
which have been rolled more intensely to a smaller thickness, have
a sufficient thick coating, which protects reliably against
corrosion. A further special feature is the step of heat treatment
after the electrolytic coating at a temperature range between
350.degree. C. and below the solidus line of the coating material.
Because of the heat treatment, zinc diffuses from the coating into
the base material and iron from the base material into the coating.
With increasing iron proportion in the coating, the temperature
during the heat treatment can slowly be increased because of the
displacement of the solidus line towards higher temperatures. A
zinc-iron alloy is produced as coating, which withstands also
higher temperatures of a subsequent hot forming process if needed
and offers a reliable corrosion protection.
[0051] It is understood, that the method sequence according to the
invention can also be modified. For example, between the named
steps, also intermediate steps, not shown here separately, can be
provided. For example, the strip material can be provided with an
intermediate layer, especially a nickel, aluminum, or manganese
layer, before the electrolytic coating. This intermediate layer
forms an additional protection of the surface and improves the
adhesion capability of the subsequently deposited coating
containing zinc. It can also be provided, that the strip material
or the blanks manufactured therefrom, are provided with a scale
protection after the electrolytic coating (V3) and before or after
the heat treatment (V4). This is especially advisable, when the
austenitization for a later heat forming does not take place in an
inert atmosphere. The deposition of the scale protection can be
carried out by means of spraying or calendar coating. Besides the
protection against oxidization, a further advantage of the scale
protection layer is, that the surface has a high quality.
Furthermore, the friction coefficient during the hot forming as
well as the heat absorption behavior is positively influenced by
the scale protection. A further advantage of the scale protection
is, that the adhesion of the cathodic anti-corrosion layer arranged
below is improved. Furthermore, a widening of the
temperature-time-window during the austenitization is possible, for
example by means of alloy formation of the scale protection
material with the layer arranged below. An example for this is
aluminum fins in a scale protection lacquer.
[0052] Further it is understood, that the processing according to
the invention can also be modified concerning the sequence of the
carried out steps. For example, the working of blanks can also be
carried out at another point, for example before the electrolytic
coating or if necessary before or after the deposition of a scale
protection.
[0053] FIG. 2 shows a method according to the invention for
manufacturing a sheet blank from a strip material 2 according to a
second processing embodiment. This corresponds in wide parts to the
method of FIG. 1, so that in view of the similarities it is
referred to the above description. At the same time, the same or
modified components or steps are provided with the same reference
numerals as in FIG. 1. In the following particularly the
differences of the present methods are described.
[0054] The method steps V1 (rolling), V2 (strip straightening), V5
(stamping) and V6 (forming) are identical to the corresponding
method steps V1, V2, V5 and V6 of FIG. 1.
[0055] A first difference of the present embodiment to the method
of FIG. 1 is the method step V3 of the electrolytic coating. In the
present method processing of FIG. 2, the strip material is coated
with a metallic coating material, which contains at least zinc and
iron. The zinc-iron-alloy layer is produced by the electrolytic
deposition of a zinc-iron-layer. The proportions of zinc and iron
are in this case selected according to an advantageous method
processing such, that the alloy layer contains at least 5 mass
percent and/or at a maximum 80 mass percent, or that the alloy
layer contains at least 20 mass percent and/or at a maximum 95 mass
percent of zinc.
[0056] Especially advantageous is, when the proportions of zinc and
iron are selected such, that in the deposited state at least
partially .delta.1, especially .delta.1-phase and .GAMMA.-phase, or
only intermetallic .GAMMA.-phase is present. For this, for example
a proportion of iron in the coating can be selected between 10% by
mass and 30% by mass, or a proportion of zinc of 70% by mass to 90%
by mass. With these proportions at least partially an intermetallic
phase is formed in the deposited state.
[0057] It is advantageous for carrying-out a direct hot forming,
when the content of .GAMMA.-phase is relative high and the content
of .delta.1-phase is as small as possible. To prevent solder
fissuring or cracking, the melting temperature of the coating for
the hot forming should be relative high. With the increase of the
proportion of iron and thus with increasing proportion of
.GAMMA.-phase, the solidus line is displaced in the binary phase
diagram of zinc-iron (see FIG. 4) towards higher temperatures.
[0058] After the electrolytic coating (V3) blanks are worked from
the strip material 2 in method step V5, wherein it is obvious, that
the blanks could also be cut-out in a modified method processing
before the coating.
[0059] A further feature of the present method sequence of FIG. 2
is, that between the step of coating (V3) and the step of forming
(V6) no interconnected heat treatment is carried out below the
solidus temperature. The method of FIG. 2 is thus time-wise
especially short.
[0060] The subsequently carried-out step of forming corresponds to
that of FIG. 1, so that concerning this it is referred to the above
description. The blank 20 can be cold or hot formed (directly or
indirectly).
[0061] It is understood, that also in the present method sequence
modifications, especially additional intermediate steps or
subsequent method steps, can be carried out. It is, concerning
this, referred to the above description for preventing
repetitions.
[0062] FIG. 3 shows a method according to the invention for
manufacturing a sheet blank from a strip material 2 according to a
third method processing embodiment. This corresponds essentially to
a combination of the methods of FIGS. 1 and 2, so that concerning
the similarities it is referred to the above description. At the
same time, the same or modified components or steps are provided
with the same reference numerals.
[0063] Steps V1 (rolling), V2 (strip straightening), V3
(electrolytic coating), V5 (stamping) and V6 (forming) are
identical to the corresponding method steps of FIG. 2. The only
difference to the method of FIG. 2 is, that after the electrolytic
coating (V3) a heat treatment is carried out in method step V4, as
in the method of FIG. 1.
[0064] As in the method processing of FIG. 1, also in the present
method processing of FIG. 4, the special feature is the temperature
control for forming a zinc-iron-alloy layer. The respective alloy
temperature is selected during the heat treatment (V4) such, that
at no point of time of the formation of the alloy, the solidus line
of the binary zinc-iron-phase diagram (compare with FIG. 4) or the
solidus line of a layer structure, consisting of more than two
alloy elements, is reached or exceeded.
[0065] An example for such a layer structure would be for example a
ternary alloy from zinc, iron and manganese, wherein the manganese
stems from the steel substrate and reaches by means of the
diffusion during the above named heating into the electrolytically
deposited zinc layer or zinc-iron-alloy layer and does not form
part of an electrolytic deposition. Instead of manganese it is also
possible, that for example chromium or aluminum or silicon or
molybdenum diffuses into the electrolytically deposited layer. It
is understood, that for the coating also steel alloy elements can
be provided, which have not been named up to now and which are
suitable, to diffuse by the above named heating process into the
electrolytic deposited layer.
[0066] Also in the present method sequence modifications,
especially additional intermediate steps or subsequent method
steps, can be carried out. Concerning this, for preventing
repetitions it is referred to the above description.
[0067] FIG. 4 shows the phase diagram for zinc-iron. On the x-axis,
the proportions of iron (Fe) and zinc (Zn) are shown, respectively.
In this case, on the left edge, a material with 100% by mass iron
and 0% by mass zinc is present, while at the right edge, inversely
0% by mass iron and 100% by mass zinc is present. Between the
edges, respectively, the percentaged composition, which is stated
on the x-axis, is found. S characterizes the molten mass, .alpha.
and .gamma. are iron-zinc-mixed crystal systems (rich in iron),
.zeta. and .delta. or .delta.1 and .GAMMA. are intermetallic
phases, and .eta. is a zinc-iron mixed crystal (rich in zinc).
[0068] In the following, by means of the zinc-iron phase diagram,
different possibilities of the electrolytic deposition according to
one of the methods according to the invention are exemplary
described.
[0069] During the deposition of a pure zinc layer, as it can be
produced in a method processing of FIG. 1, at the beginning an
alloying temperature above 350.degree. C. and below the melting
temperature (solidus line) of 419.5.degree. C. is selected, for
example 400.degree. C. At this temperature, a diffusion of iron
into the zinc layer takes place, so that with continuing holding
time during the heat treatment (V4) a diffusion layer is formed,
for example a .delta.-phase. The further temperature processing is
such, that the respective temperature is always below the solidus
line of the binary zinc-iron-phase diagram.
[0070] During an electrolytic deposition of a coating, which
already contains iron in the zinc layer, as it can be produced in a
method processing of FIG. 3, the starting temperature can be
selected above the melting temperature of pure zinc. For example,
in a composition of the electrolytic deposited layer of 85% by mass
zinc and 15% by mass iron, a starting temperature of 600.degree. C.
can be selected. This temperature lies in fact above the melting
temperature of zinc, however below the solidus line of the
two-phase-range .GAMMA.+.delta.1.
[0071] For an electrolytic deposition of a zinc-iron layer, which
consists of 60% by mass of zinc and of 40% by mass iron, a starting
temperature smaller than 782.degree. C. is possible. An increase
above this temperature is only then possible, when the layer is
enriched during a following heat treatment so far with iron, that
only an austenitic iron mixed crystal would be present (for example
70% by mass iron and 850.degree. C.).
[0072] The type of heat treatment is, as above described, not
prescribed. For example, it can be an inductive heating or a
heating in an annealing furnace or a heating by means of contact
with a hot body, for example a thick steel plate, which delivers
its heat to the blank or the profile cut.
[0073] In a special embodiment of the invention, an electrolytic
zinc-iron alloy with an iron proportion of 8% by mass to 12% by
mass is provided. In this case, it is a composition, as it is used
for steels with a so-called "galvannealed" coating. The advantage
of this composition is that the elements zinc and iron have a
distance in the range of nanometers so that a drawn-out diffusion
treatment can be waived. Rather, by means of a short heat treatment
in the method step V4, an intermetallic .delta.1-phase can be
produced from an electrolytic deposited zinc-iron alloy with an
iron proportion of 8% by mass to 12% by mass. Such a composition
can be used for the cold forming as well as for the hot
forming.
[0074] In a further special embodiment of the invention, an
electrolytic zinc-iron alloy is deposited, which stoichiometric
composition corresponds to the .GAMMA.-phase. Alternatively, this
composition can also be reached by a deposition of a zinc-iron
layer with a low iron proportion and a subsequent heat treatment,
at which end the .GAMMA.-phase is present. This layer starts only
to melt at a temperature of 782.degree. C., so that this layer is
especially suitable for the hot forming, as in this case the
formation of a melting phase can be restricted or can be prevented
by means of stabilizing the layer with elements from the steel
substrate as manganese (ternary system iron-zinc-manganese).
[0075] In a further embodiment, which is also provided for the hot
forming (V6), a layer is electrolytically deposited, which itself
is not present in a molten state even during the heating to the
maximum austenitizing temperature for the hot forming (for example
at 900.degree. C.). Such a coating would for example have a
composition of 20 weight percent of zinc and 80 weight percent of
iron. In this case, it is an iron based alloy of the binary
iron-zinc system.
[0076] Altogether with the method according to the invention
products with a reliable cathodic corrosion protection can be
manufactured, which are especially suitable for a hot forming
process. By means of the at least as far as possible prevention of
the formation of a liquid phase in the coating during the process,
the susceptibility to cracking of the solder of the product is
minimized in an advantageous manner.
REFERENCE NUMERALS LIST
[0077] 2 strip material [0078] 3 coil [0079] 4 rolls [0080] 5 rolls
[0081] 6 thickness control [0082] 7 smoothing device [0083] 8
coating device [0084] 9 immersion tank [0085] 10 electrolyte [0086]
11 set of rolls [0087] 12 set of rolls [0088] 13 furnace [0089] 14
molding tool [0090] 20 blank [0091] V1-V6 method steps
* * * * *