U.S. patent application number 14/896749 was filed with the patent office on 2016-05-05 for producing a product from a flexible 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, Jurgen Butzkamm, Wolfgang Eberlein, Christoph Hahn, Thomas Muhr, Christoph Schneider, Hubertus Steffens.
Application Number | 20160122889 14/896749 |
Document ID | / |
Family ID | 51022834 |
Filed Date | 2016-05-05 |
United States Patent
Application |
20160122889 |
Kind Code |
A1 |
Muhr; Thomas ; et
al. |
May 5, 2016 |
PRODUCING A PRODUCT FROM A FLEXIBLE ROLLED STRIP MATERIAL
Abstract
A product is made from a rolled strip material with the steps:
rolling of a strip material from a sheet metal; working of a blank
out of the rolled strip material; forming of the blank to a formed
part; cleaning the formed part such that an amount of maximal 0.7
ppm of diffusible hydrogen is introduced into the formed part by
cleaning, and coating the formed part with a metal coating material
for producing a corrosion protection coating, wherein the step of
coating is carried out in an immersion bath with an electrolyte
solution, wherein between the formed part and the electrolyte
solution a flow is generated.
Inventors: |
Muhr; Thomas; (Attendorn,
DE) ; Schneider; Christoph; (Lennestadt-Elspe,
DE) ; Brecht; Jorg Dieter; (Olpe, DE) ; Hahn;
Christoph; (Attendorn, DE) ; Butzkamm; Jurgen;
(Olpe-Thieringhausen, DE) ; Steffens; Hubertus;
(Drolshagen, DE) ; Eberlein; Wolfgang; (Wilnsdorf,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MUHR UND BENDER KG |
Attendorn |
|
DE |
|
|
Assignee: |
Muhr und Bender KG
Attendom
DE
|
Family ID: |
51022834 |
Appl. No.: |
14/896749 |
Filed: |
June 17, 2014 |
PCT Filed: |
June 17, 2014 |
PCT NO: |
PCT/EP2014/062693 |
371 Date: |
December 8, 2015 |
Current U.S.
Class: |
428/659 ;
148/320; 148/537; 205/102; 205/208; 205/210 |
Current CPC
Class: |
C21D 9/0068 20130101;
C25D 3/22 20130101; C22C 38/14 20130101; C21D 6/00 20130101; C25D
5/18 20130101; C22C 38/12 20130101; C25D 5/36 20130101; C21D 8/005
20130101; C25D 5/00 20130101; C25D 3/565 20130101; C25D 7/0614
20130101; C25D 5/50 20130101; B21B 2205/02 20130101; C25D 5/08
20130101; C22C 38/04 20130101; C23C 2/06 20130101; C25D 21/10
20130101 |
International
Class: |
C25D 3/22 20060101
C25D003/22; C21D 6/00 20060101 C21D006/00; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C25D 5/00 20060101
C25D005/00; C23C 2/06 20060101 C23C002/06; C25D 5/36 20060101
C25D005/36; C25D 5/50 20060101 C25D005/50; C25D 5/18 20060101
C25D005/18; C21D 8/00 20060101 C21D008/00; C21D 9/00 20060101
C21D009/00; C22C 38/04 20060101 C22C038/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 17, 2013 |
DE |
10 2013 010 025.9 |
Claims
1.-16. (canceled)
17. A method for producing a product from a rolled strip material,
comprising: rolling a strip material from sheet metal; working a
blank from the rolled strip material; forming the blank to a formed
part; cleaning the formed part such that an amount of at most 0.7
parts per million (ppm) of diffusible hydrogen is introduced into
the formed part by cleaning; and coating at least a portion of the
formed part with a metal coating material for producing a corrosion
protection coating, wherein said coating is carried out in an
immersion bath with an electrolytic solution, wherein a flow is
generated between the formed part and the electrolytic
solution.
18. The method of claim 17, wherein said cleaning is carried out by
pickling.
19. The method of claim 17, wherein said cleaning is carried out
mechanically.
20. The method of claim 19, wherein said cleaning is carried out by
blasting or brushing.
21. The method of claim 17, wherein the rolling of the strip
material is a flexible rolling, wherein a variable thickness is
produced along the length of the strip material.
22. The method of claim 21, wherein the flexible rolling is carried
out such that at least two portions are produced with different
thicknesses, wherein a first thickness is smaller than a second
thickness and the ratio of the first thickness to the second
thickness is less than 0.8.
23. The method of claim 17, wherein the forming is a hot-forming
with simultaneous hardening, the forming further comprising:
heating at least one partial area of the blank to an austenitizing
temperature; and hot-forming the blank with quick cooling, wherein
the at least one heated partial area is hardened.
24. The method of claim 17, wherein the forming is a cold-forming,
wherein the cold-formed part is hardened before the coating.
25. The method of claim 17, wherein the coating is carried out with
a coating material, which has a mass proportion of zinc of at least
50%.
26. The method of claim 17, wherein the coating is carried out
continuously.
27. The method of claim 17, wherein the coating is carried out such
that the electrolytic solution is subjected to a pulsed
current.
28. The method of claim 17, wherein the coating comprises hot-dip
galvanizing, wherein the formed part is dipped into an immersion
bath of molten coating material with a temperature of at least
350.degree. C. and at most the AC1-temperature of the steel
material.
29. The method of claim 17, further comprising, after said coating,
heat treating the coated formed part at a temperature of at least
200.degree. C. and at a temperature of at most the AC1-temperature
of the steel material.
30. The method of claim 17, wherein the steel material includes
manganese and at least one of the micro-alloying elements niobium
and titanium, wherein the total proportion of said micro-alloying
elements amounts to 1000 ppm of the total mass at most.
31. The method of claim 17, wherein areas of different ductility
are produced in the formed part.
32. A product made from a flexible rolled metal sheet, produced by:
rolling a strip material from sheet metal; working a blank from the
rolled strip material; forming the blank to a formed part; cleaning
the formed part such that an amount of at most 0.7 parts per
million (ppm) of diffusible hydrogen is introduced into the formed
part by cleaning; and coating at least a portion of the formed part
with a metal coating material for producing a corrosion protection
coating, wherein said coating is carried out in an immersion bath
with an electrolytic solution, wherein a flow is generated between
the formed part and the electrolytic solution.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a National Stage of, and claims priority
to, Patent Cooperation Treaty Application No. PCT/EP2014/062693,
filed on Jun. 17, 2014, which claims priority to German Patent
Application No. 10 2013 010 025.9, filed on Jun. 17, 2013, each of
which application are hereby incorporated herein by reference in
their entireties.
DESCRIPTION
[0002] The present disclosure relates to a method for producing a
product from a rolled strip material, as well as a product made
from a rolled strip material, especially as a structural component
for a motor vehicle.
[0003] From DE 10 2004 037 206 A1 a car body for a motor vehicle is
known which is made from single elements. For this, single elements
made from a flexible rolled steel sheet with a sheet thickness
variable along one direction, are used, in which the distribution
width of the specific loading across the individual elements is
reduced by the selection of the sheet thickness distribution. Such
sheet elements with variable thickness are also called Tailor
Rolled Blanks (TRB).
[0004] The trend present in the motor vehicle industry towards a
light weight design and passenger protection leads to an increased
use of high strength and super high strength car body steels. In
the course of this development, multiphase steels and martensite
phase steels especially are used. The latter steels are generally
worked to structural components by an indirect or direct
hot-forming method.
[0005] Generally, structural components for motor vehicles are
provided with a coating which shall protect the sheet steel against
corrosion. However, the implementation of a reliable corrosion
protection is difficult especially with regard to hot-formed steel
materials. Various coatings and coating methods are known, which
differ from each other by applying the coating before or after the
hot-forming.
[0006] From EP 2 327 805 A1 a method and a production facility for
producing a sheet metal part with a corrosion protection coating
are known. The method comprises the steps: forming a starting
material to a formed sheet metal part, electrolytic coating the
formed sheet metal part for producing the corrosion protection
coating and subsequent heat treatment of the coated formed sheet
metal part.
[0007] From EP 2 412 848 A1 a similar method is known, wherein a
zinc-nickel-coating is applied as a corrosion protection coating on
the formed sheet metal part. Hereby, initially a thin nickel layer
is deposited at the beginning of the coating process, which shall
prevent a subsequent hydrogen embrittlement of the sheet steel
material.
[0008] A method for coating steel components is galvanic zinc
coating (electrolytrogalvanising) for example. During galvanising,
the workpiece is dipped into a zinc electrolyte. Electrodes of zinc
act as "sacrificial anode", because of their metal, which is less
noble compared to the workpiece. The workpiece to be galvanised
acts as a cathode; that is why the coating is also referred to as a
cathodic corrosion protection.
[0009] Further known coating methods are hot-dip galvanising, zinc
spraying by thermal spraying, flame spraying, high velocity flame
spraying, arc spraying or plasma spraying, sherardizing,
galvanising, electrostatic deposition of metal powder on the
component surface or further deposition methods from the gas phase
(CVD).
[0010] With regard to the industrial scale coating methods for
ultra high strength structural components it is a problem that
corrosion protection of coatings applied before hot-forming
negatively changes the features of the component and of the coating
due to the temperatures acting on the coating system before and
during the hot-forming. This can lead to a solder cracking and
micro cracks in the component, which has a negative effect on the
material properties of the workpiece. Coating systems and methods
like flame spraying and sherardizing, which are applied after the
hot-forming, have the great disadvantage that the layer thickness
shows large fluctuations and the methods are in total very
cumbersome.
[0011] Regarding the full surface batch galvanising of components
from the liquid phase (hot-dip galvanising), the galvanising
temperature of above 420.degree. C. reduces the strength of the
component. With regard to the electrolytical hot-dip galvanising
the danger exists, that hydrogen is introduced into the component
by the preceding cleaning process and the galvanic coating process.
The introduced hydrogen can lead to a material failure in the high
strengths of the components.
[0012] Therefore, disclosed herein is a method for producing a
product from rolled strip material, which offers a particular good
corrosion protection.
[0013] A method for producing a product from a rolled strip
material includes the following steps: rolling of a strip material
from sheet metal; working a blank from the rolled strip material;
forming the blank to a formed part; cleaning the formed part such
that an amount of at most 0.7 ppm of diffusible hydrogen is
introduced into the formed part by cleaning; and coating the formed
part with a metal coating material for producing a corrosion
protection coating.
[0014] Advantageously, during the cleaning process no diffusible
hydrogen is introduced into the material or at most only in very
small amounts. In this manner, unwished hydrogen embrittlements of
the steel material can be prevented or at least can be reduced. An
advantage of the batch coating, i.e., coating the already cut
blanks respectively the formed parts produced therefrom, is that
the coating is not negatively influenced by subsequent
heat-treatment processes. This again has advantageous effects on
the quality of the coating and, thus, on the corrosion resistance
of the produced formed part.
[0015] Preferably, cleaning is carried out such that the proportion
of diffusible hydrogen measured directly before and after the
cleaning processes is less than 0.7 ppm (parts per million),
especially less than 0.3 ppm, preferably less than 0.1 ppm, or even
less than 0.05 ppm. Directly before and after the cleaning can
include a time frame of respectively up to 10 minutes before or
after, within which the content of diffusible hydrogen is measured
in the material.
[0016] For producing the product, preferably a hardenable, in
particular a manganese containing steel material is used. This can
contain still further micro-alloying elements, like for example
niobium and/or titanium, wherein the proportion of the mass of
these micro-alloying elements is preferably at most 1000 ppm of the
total mass. Further micro-alloying elements, like boron and/or
vanadium, can be added in small proportions of mass. Examples for a
useable steel material are 17MnB5, 22MnB5, 26MnB5 or 34MnB5. The
starting material (strip material) has preferably a tensile
strength of at least 450 MPa and/or at most 850 MPa. The finished
formed part can have a final tensile strength of at least 1100 MPa,
preferably of at least 1300 MPa, especially preferred of even 1500
MPa, at least in partial areas.
[0017] According to a possible embodiment, rolling is conducted as
flexible rolling, wherein a variable thickness is produced along
the length of the strip material. By flexible rolling a rolling
process is meant, wherein a steel strip with a uniform thickness
along the length is rolled to a strip material with a variable
thickness along the length. The starting thickness before the
flexible rolling can be up to 8 mm. As a strip material for the
flexible rolling, a hot (rolled) strip or cold (rolled) strip can
be used, wherein these terms have to be understood in the sense of
the technical terminology. A hot (rolled) strip is a rolled steel
product (steel strip), which is produced by rolling subsequently to
previous heating. A cold (rolled) strip is a cold rolled steel
strip (flat steel), in which the last thickness reduction is
carried out by rolling without previous heating. After the flexible
rolling, the strip material can have, for example, a thickness of
up to 6.0 mm at the thickest point.
[0018] Preferably the flexible rolling is carried out such that at
least two portions with different thickness are produced, wherein
the ratio of a first thickness of a thinner first portion to a
second thickness of a second portion is smaller than 0.8, in
particular smaller than 0.7, preferably smaller than 0.6. It is to
be understood, however, that depending on the requirements of the
finished product any deliberate number of portions with different
thickness can be produced in principle. The thickness is adjusted
along the length in particular such, that the loadings of the
component are at least substantially uniform, respectively that
loading peaks are prevented or are at least reduced.
[0019] The step of working out conceptually means any type of
producing blanks or form cuts from the strip material. This can be
done by mechanical cutting, like punching or cutting, or by laser
cutting. Blanks refer to in particular rectangular sheet panels,
which are separated from the strip material. Form cuts are
understood to be sheet elements worked from the strip material,
which outer contour is already adapted to the form of the final
product. During the production of the form cuts or blanks, an edge
can remain on the strip material, which is not further processed,
wherein also a simple cutting of the strip material into pieces can
be carried out, in which no edge would remain. In this disclosure,
the term "blank" is used for form cuts as well as for rectangular
blanks.
[0020] It will be recognized that further steps can additionally be
carried out before, during, or after the above named method steps.
For example, a heat treatment of the strip material can be carried
out before the flexible rolling. After the flexible rolling, a
strip straightening can be provided. Furthermore, a pre-treatment,
like rinsing and/or pickling (surface activation), of the
workpieces can be provided before the coating process. After the
coating process, a further heat treatment can be carried out.
[0021] According to a possible embodiment it is provided that the
cleaning of the formed part is carried out mechanically. Cleaning
in this context means any treatment in which unwished
contaminations present after the forming process are removed
mechanically from the surface. An advantage of the mechanical
cleaning is that no unwished hydrogen is introduced into the
workpiece. Preferably, the formed part is blasted or brushed.
Shot-blasting, blasting with corundum or with dry ice (CO2) can be
used as a method for blasting. For the shot-blasting, steel balls
with a preferred ball diameter of 0.7 to 0.9 mm can be used. By the
blasting process a rougher surface is produced than in the
unblasted condition, which is advantageous with regard to the
adhesion properties of a subsequently applied coating. However, in
principle it is also possible that the cleaning of the formed part
is carried out in a different manner, so that a proportion of
maximal 0.7 ppm, in particular 0.1 ppm, especially maximal 0.05
ppm, of diffusible hydrogen (H) is introduced into the formed part
by the cleaning process. For example, cleaning can be carried out
by pickling as an alternative method step. A first method variant
is anodic pickling, wherein the formed parts are dipped into an
immersion bath, wherein removal of scale and other impurities takes
place under the influence of direct current. According to an
alternative method variant the removal of scale and other
impurities can take place purely chemical, for example by an
inhibited pickling.
[0022] According to an embodiment, the forming of the workpiece
comprises a hot-forming. Hot-forming here means that before the
forming process the workpieces are heated above the austenitizing
temperature and at least partial regions are hardened during the
forming process. The heating is carried out in a suitable heating
device, for example, in an oven. The hot-forming can be carried out
according to a first possibility as an indirect process, which
comprises the sub-steps cold-preforming of the blanks to a
pre-formed component, subsequent heating at least partial regions
of the cold preformed component up to the austenitizing temperature
as well as subsequent hot-forming for producing the final contour
of the product. The austenitizing temperature is understood as a
temperature range, in which at least a partial austenitization
(structure in the two-phase range ferrite and austenite) is
present. Furthermore, it is also possible, to austenitize only
partial regions of the blanks to enable, for example, a partial
hardening. The hot-forming can also be carried out according to a
second possibility as a direct process, which is characterised in
that at least partial regions of the blanks are directly heated to
the austenitizing temperature and are then hot-formed to the
required final contour and hardened in one step. An earlier (cold)
pre-forming is not carried out in this case. Also in the direct
process, a partial hardening can be achieved by austenitization of
partial regions. For both processes it applies that a hardening of
partial regions of the components is possible also by differently
tempered tools, or by the use of several tool materials, which
enable different cooling velocities. In the latter, the whole blank
or the whole component can be completely austenitized.
[0023] According to an an embodiment, the sheet blanks can also be
cold-formed. Cold-forming is understood as forming processes, in
which the blanks are not heated in a targeted manner before the
forming process. The forming thus takes place at room temperature;
the blanks are heated by the dissipation of the introduced energy.
Cold-forming is used in particular as a process for forming soft
car body steels. Subsequently to the cold-forming, the formed parts
can optionally be hardened.
[0024] During and after the forming process, a heat treatment can
be provided as an integrated or separate method step, wherein
regions with different ductility are produced in the workpiece.
Ductility is the formability of the steel material without damage
or crack forming. The ductility can be evaluated for example by the
elongation at break or the contraction at fracture in a tensile
test. An increased ductility in partial regions leads in an
advantageous manner to a reduced edge cracking susceptibility and
an improvement of the weldability of the material in said
regions.
[0025] The ductility can in particular be selected such, that one
or more first regions of the formed part have a larger yield
strength of at least 800 MPa and/or that one or more second regions
have a lower yield strength of maximal 800 MPa. The strength in the
first region can be at most 1100 MPa and/or in the second region at
least 1100 MPa.
[0026] For producing areas of different ductility, different
embodiments are possible. According to a first possibility, a
temperature gradient can be produced in the workpiece during the
heat treatment carried out before the forming process. After the
heat treatment, which for example can be carried out in an oven,
areas with higher or with lower temperature are then present. The
subsequent forming process leads then to an increased ductility,
respectively a lower strength in the regions with higher
temperature, while in the regions of lower temperature a lower
ductility and higher strength, respectively, is produced.
Alternatively, a temperature gradient can be produced in the
workpiece also during the transfer processes between the heat
treatment and the forming process, for example such that partial
regions of the workpiece, completely heat treated beforehand, are
cooled before insertion into the forming tool. According to a
further possibility, the ductility can also be adjusted during the
forming process, for example by differently tempering partial
regions of the tool. For this, the forming tool can have respective
means, such as channels through which a cooling medium flows. In
the cooled areas of the tool, a higher strength and lower ductility
is produced in the formed part; correspondingly the hotter areas of
the forming tool cause the formation of lower strengths and higher
ductility. According to a further possibility, the regions of high
ductility can be produced during the coating process, in particular
by hot-dip galvanising. In this case, the high temperature of the
liquid coating material leads to a softening in the coated regions,
and thus to a higher ductility.
[0027] A heat treatment step can be carried out as an integrated or
a separate method step before, during, or after the forming
process, wherein surface areas with lower hardness than in the core
area are produced in the workpiece. This can be achieved by
targeted surface decarburisation, in which a depletion of alloying
components is caused in the starting material across the thickness,
i.e., the proportion of alloying components like carbon or
manganese is larger in a core area of the strip material than in
the surface area. Preferably, the depleted area has a hardness,
which is reduced by at least 50 HV.sub.0.1 compared to the core
area. The depletion of the alloying elements can, for example, be
achieved by a heat treatment during a galvannealing treatment or by
heating above the AC1-temperature. The extent of the surface
decarburisation is determined by the process parameters in the
oven. Especially the atmosphere in the oven, i.e. the gas mixture
in the oven, or also the temperature count thereto.
[0028] The coating process is in particular carried out with a
coating material that has a proportion of at least 50 mass percent
zinc, preferably at least 90 mass percent zinc, wherein the zinc
content can also be 100 percent (pure zinc coating).
[0029] According to an embodiment, the coating can be applied
galvanically (electrolytically). For this, anodes are used from a
coating material, i.e., from pure zinc or from zinc and other
alloying elements, which, when energised, release metal ions to the
electrolyte. Alternatively, also form-stable anodes can be used; in
this case, the coating material is already dissolved in the
electrolyte. The zinc ions and if applicable, ions of the further
alloying elements are deposited as atoms on the formed part, which
is connected as a cathode, and form the coating. The coating
process is carried out by dipping the workpiece into an immersion
bath with an electrolyte solution, preferably in a continuous
process, wherein between the formed part and the electrolyte
solution a flow is generated. By the flow provided between the
formed part and the electrolyte solution, an electrolyte
impoverishment is avoided and thus an unwanted hydrogen
introduction into the workpiece is avoided. The flow can generally
be achieved by moving the formed part relative to the electrolyte
and/or by moving the electrolyte relative to the formed part. By
using a continuous process, i.e., by continuous movement of the
workpiece, a good reproducibility of the coating process can be
achieved as well as a uniform coating across the surface of the
workpiece. However, thus also temporal pauses, in which the advance
is temporarily stopped, should be comprised in a specific extent as
they may be present, for example, in a chain conveyor system. The
flow can be produced such that the formed parts are moved through
an immersion bath by a device, i.e., the formed parts move relative
to the immersion bath and to the electrolyte solution.
Alternatively or in addition, a flow of the electrolyte solution
can be produced by an appropriate design of the coating facility.
For this, the coating facility can be provided with pumps, which
agitate the electrolyte solution to a flow movement relative to the
workpiece. Preferably, the electrolyte solution is ejected by jets
onto the formed parts, which can be done under a jet angle of
90.degree. up to .+-.45.degree. in relation to the surface of the
workpiece. Generally, an inhomogeneous distribution of the current
density can be present in an electrolyte solution. Thus, the flow
of the electrolyte solution can be adjusted relative to the
workpiece such, that a homogeneous distribution of the current
density is generated.
[0030] According to an embodiment, the coating process can be
carried out such that the formed part to be coated is subjected to
a pulsed current in at least one step. Alternatively or in
addition, the formed part can also be subjected to an unpulsed
current. More specifically, the step of coating by an electrolyte
solution can especially comprise following sub-steps: in a first
workstation, the electrolytic solution is subjected to a pulsed
current for coating the formed part; in a following second
workstation, the electrolytic solution is subjected to an unpulsed
current for coating the formed part. It is understood that a
reverse order for treatment with pulsed and unpulsed current is
also possible. By a pulsed current feed of a pair of anodes in the
first step, a nanocrystalline layer structure is achieved, which,
for example, can have a layer thickness of 1 to 2 micrometers.
Thus, the coating has a particular fine granulation close to the
workpiece, so that the formation of rust is prevented.
[0031] According to an embodiment, the coating process can also
comprise hot-dip galvanising, wherein the formed part is dipped
into a dipping bath of molten coating material with a temperature
of at least 350.degree. C., preferably at least 420.degree. C.
and/or the AC1 -temperature of the steel material at most,
preferably at a maximum of 600.degree. C. In this manner, the
coated regions are softened because of the introduced heat, so that
here the material obtains a higher ductility than the uncoated
areas. The coating material is preferably configured as described
above, i.e., it has a proportion of at least 50 percent by mass of
zinc, if necessary with additional alloying elements. Further
possible coating methods are flame spraying or chemical vapour
deposition (CVD).
[0032] As a further method step before or after the coating
process, a heat treatment of the coated formed part can
additionally be carried out at a temperature of more than
210.degree. C., in particular more than 220.degree. C., preferably
more than 230.degree. C. The maximum temperature for the heat
treatment is preferably up to the AC1-temperature of the steel
material, in particular not more than 400.degree. C. The heat
treatment, which also can be referred to as effusion annealing,
reduces internal stresses in the workpiece, respectively stress
peaks in the hardened component, respectively increases the failure
strain. At the same time, the hydrogen effusion is accelerated by
the selected temperature, so that in total a lower hydrogen
embrittlement is achieved in the finished product. The heat
treatment can be carried out in a time frame of few seconds up to 3
hours. Furthermore, the heat treatment can take place after the
coating process or between the individual coating process steps. A
heat treatment following the coating process accelerates in an
advantageous manner the drying of the formed parts and when using
high strength steels, the material properties concerning the
ductility and the elongation at fracture are improved by
annealing.
[0033] The solution for the above named object is further a
product, which is produced from a flexible rolled sheet steel
according to the above named method. Thus, said advantages of a
uniform layer thickness of the corrosion protection coating across
the coated surface of the formed part as well as a lower risk of
the hydrogen embrittlement are achieved. The formed part can be
produced according to one or more of the above-mentioned method
steps, so that concerning the steps and the advantages connected
thereto it is referred to the above description.
[0034] Overall, a formed part is produced, which by its sheet
thicknesses and the applied corrosion protection system is
advantageously adapted to the requirements concerning light weight
design, crash properties and life time (corrosion protection). The
formed part can be any car body component of a motor vehicle, for
example a structural component like an A-, B- or C-pillar.
[0035] Following, a preferred embodiment is described by means of
the drawings.
[0036] FIG. 1 shows a method for producing a product from a
flexible rolled strip material schematically as a process
diagram.
[0037] FIG. 2A shows a coating schematically as a detail in a side
view.
[0038] FIG. 2B shows a coating schematically in a cross-sectional
view according to section line A-A of FIG. 2A.
[0039] FIGS. 1 and 2 are jointly described in the following. In the
method step V1, the strip material, which is wound to a coil in the
starting condition, is processed in a rolling manner, i.e., in
particular by flexible rolling. For this, the strip material that
has a substantially uniform sheet thickness along the length before
flexible rolling, is rolled by rollers such that it receives a
variable sheet thickness along the rolling direction. During
rolling, the process is monitored and controlled, wherein the data,
determined from a sheet thickness measurement, are used as input
signal for controlling the rolling process. After the flexible
rolling the strip material has different thicknesses in rolling
direction. The strip material is again wound to a coil after the
flexible rolling, so that it can be transported to the next process
step.
[0040] The material for the strip material is a hardenable steel
material, like, for example, 17MnB5, 22MnB5, 26MnB5 or 34MnB5. The
starting martial has preferably a tensile strength of at least 450
MPa and at at most 850 MPa. It can be provided for specific
components, that the starting material has a depletion of alloying
components across the thickness, i.e., the proportion of alloying
components like carbon or manganese is larger in a core region of
the strip material than in the surface region. Preferably, the
depleted region has a hardness reduced by at least 50 HV.sub.0.1
compared to the core region. The depletion of the alloying elements
can be achieved by a heat treatment during a
Galvannealing-treatment or by heating above the
AC1-temperature.
[0041] After the flexible rolling, the strip material can be
smoothed in a strip straightening device. The method step of
smoothing is optional and can also be omitted.
[0042] After the flexible rolling (V1) and smoothing, respectively,
individual sheet blanks are worked out of the strip material during
the next method step V2. The working of the sheet blanks from the
strip material takes place preferably by punching or cutting.
Depending on the shape of the sheet blanks to be produced, a sheet
blank can be punched from the strip material as a contour cut
wherein an edge of the strip material remains that is not further
used, or the strip material can simply be cut into partial
pieces.
[0043] After producing the blanks from the strip material, forming
of the blanks to the required end product can then be carried out.
According to a first possibility, the blanks are hot-formed or
according to a second possibility can be cold-formed.
[0044] The hot-forming can be carried out as a direct or indirect
process. In the direct process, the blanks are heated to the
austenitizing temperature (method step V3) before forming, which
can for example be carried out by induction or in an oven. The
austenitizing temperature is to be understood as a temperature
range, at which at least a partial austenitization (a structure in
the two-phase range ferrite and austenite) is present. However,
also only partial areas of the blank can be austenitized, to enable
for example a partial hardening.
[0045] After heating to the austenitizing temperature, the heated
blank is formed in a forming tool and is cooled at the same time
with a high cooling velocity, wherein the component receives its
final contour and is hardened at the same time. This process, which
is called hot-forming, is represented as method step V4. A special
form of the hot-forming is the press-hardening, which is carried
out at high pressures.
[0046] For indirect hot-forming, the blank undergoes a pre-forming
before the austenitization. The pre-forming is carried out in a
cold condition of the blank, which means without previous heating.
During the pre-forming, the component receives its profile, which
does not yet correspond to the final shape, however is approximated
to it. After the pre-forming, an austenitization and hot-forming
then is carried out, as during the direct process, wherein the
component receives its final contour and is hardened.
[0047] During the forming, areas with different ductility and/or
areas with different strength can be produced in the workpiece.
[0048] The steel material should, if a hot-forming (direct or
indirect) is provided, contain a proportion of carbon of at least
0.1 percent by mass up to 0.35 percent by mass. Independent of the
type of hot-forming, the complete workpiece or only partial areas
can be hardened. When the hot-forming is carried out such that only
partial areas are hardened, the formed part has areas with reduced
strength with at the same time increased tensile strength. By means
of applying a coating in a later method step only in these soft
zones, the danger of hydrogen embrittlement is reduced here.
[0049] Alternatively to the hot-forming as a forming process, the
blanks can also be cold-formed. The cold-forming is especially
suitable for soft car body steels or components, which have
essentially strengths of less than 1200 MPa. For cold-forming, the
blanks are formed at room temperature.
[0050] After the forming (method step V4), the formed parts are
undergoing a cleaning process in method step V5. The cleaning of
the formed parts is carried out such that an amount of maximal 0.7
ppm, in particular of up to 0.3 ppm, preferably up to 0.1 ppm, or
where appropriate up to 0.05 ppm, of diffusible hydrogen (H) is
introduced into the formed part. For this, a preferably mechanical
cleaning process or a pickling process is provided, during which
unwished contaminations are removed mechanically, respectively
electrochemically when using pickling, from the surface of the
formed part. For mechanical cleaning in particular shot-blasting or
brushing can be used for cleaning the formed parts, wherein the
shot-blasting is preferably carried out with steel balls with a
particle size of approximately 0.7 mm to 0.9 mm. Because of the
shot-blasting, the surface of the formed part receives a rough
surface, by which a good adhesion of a later applied coating is
achieved. As an alternative, a pickling process can be used.
[0051] After the step of cleaning, in the next method step (V6), a
cutting of the formed part to the final contour can be carried out,
for example by a laser, or an oiling of the formed part as
anti-corrosion protection can be carried out for a following
interim storage. If, however, the workpiece can directly be further
processed, an oiling is sensibly not carried out.
[0052] After the intermediate step (V6), the formed parts are
provided with a corrosion protection. For this, the formed parts
run through an electrolytic coating facility, which comprises
several workstations.
[0053] During a method step (V7), the formed parts are initially
rinsed. After the rinsing, the formed parts are pickled during the
method step (V8). For this, the formed parts are freed of unwished
oxides by means of dipping into a diluted acid.
[0054] After pickling, the formed parts are provided with a
corrosion protection layer in the method step V9. For the coating
process, coating material with a proportion of preferably at least
50 percent by mass of zinc, especially at least 90 percent by mass
of zinc, is used, wherein also a pure zinc coating may be
considered. The coating material can also contain further alloying
elements.
[0055] The step of coating can be carried out galvanically by means
of an electrolyte solution, into which the formed parts are dipped.
Preferably, the step of coating is carried out in an immersion bath
with an electrolyte solution, wherein a flow is generated between
the formed part and the electrolyte solution. A corresponding
coating device is shown schematically in FIGS. 2A and 2B. Formed
parts 12 are visible, which are moved in feed direction R relative
to the dimensionally stable anodes 13 and jet bars 14 with
respectively several jets 15. The formed parts 12 can, for example,
be structural components of the car body of a motor vehicle, like
A-, B- or C-pillars or other car body parts. The anodes 13 are
shaped in the form of gratings, so that they can be passed through
by the electrolyte solution exiting from the jet devices 14. Jet
devices 14 are arranged on both sides of the immersion bath,
between which the formed parts 12, 12' are moved along. The
electrolyte flow is drawn schematically as 16. It is directed
towards the formed parts 12, 12' and serves for a uniform
distribution of the current density in the electrolyte solution and
thus, serves for an even layer structure on the surface of the
formed parts 12, 12'.
[0056] It is advantageous for a good reproducibility of the method
if the coating is continuously carried out, wherein a flow is
produced between the formed parts 12, 12' and the electrolyte
solution. The flow is mainly produced by moving the formed parts
12, 12' through an immersion bath, wherein the electrolyte solution
can alternatively or additionally be set in a flow movement
relative to the formed parts by pumps. For the electrolytic coating
process, anodes 13 are used made of the coating material, i.e.,
made from pure zinc or from zinc and other alloying elements,
which, when a current is applied to them, release metal ions into
the electrolyte, or dimensionally stable anodes are used which are
made of specifically coated conductive material (Releasing station
9). The zinc ions and where applicable ions of the further alloying
elements are deposited as atoms on the formed part 12, 12', which
is connected as a cathode, and form the corrosion protection
coating.
[0057] It is advantageous if the electrolyte solution is subjected
to a pulsed current at least in one sub-step for producing the
coating. For example, in a first partial step (V91) a pulsed
current can be used for the coating process. Thus, an especially
fine grained layer with a thickness of for example 1 up to 2
micrometers, is formed on the surface of the workpiece.
Followingly, in a second partial step (V92), the electrolyte
solution, respectively the anodes, are fed with an unpulsed current
for coating the formed part, till the corrosion protection layer
has reached the complete thickness of for example 7 to 8
micrometers. The coating facility can in practice be formed such
that an elongated immersion bath is provided, through which the
individual formed parts 12, 12' are continuously moved in
longitudinal direction R. Hereby, in a first portion of the
immersion bath, a first arrangement of anodes 13 can be provided,
which are fed by a pulsed current, while the workpiece is moved
passing the same. In a second portion, following the first portion
in feed direction R of the workpiece the anodes 13 provided there
are subjected to an unpulsed current, while the workpieces 12, 12'
move passing the same.
[0058] Here the galvanic coating process of the formed parts is
described by using an electrolyte solution. It is however obvious
that the method step V9 of the coating process can alternatively be
carried out by means of hot-dip galvanising, flame spraying or
chemical vapour deposition (CVD), too.
[0059] Independently of the type of the coating process, the formed
parts can be coated completely or only partially. When only partial
portions of the formed parts are coated, the expenditure and, thus,
the costs can be reduced, and also a following welding process, if
necessary, for connecting the formed part to other components can
be simplified. Furthermore, hydrogen can easily be effused in the
uncoated areas, so that the risk of a hydrogen embrittlement is
reduced. In this regard it is favourable if the formed parts are
only coated with the corrosion protection coating in the
corrosion-endangered areas. These are for example areas, which
increasingly get wet in motor vehicles and are, thus, also called
wet areas.
[0060] After the coating process the formed parts are optionally
rinsed in the method step V10.
[0061] After the rinsing process (V10), the formed parts can be
heat treated in the method step V11. The heat treatment can, in
principle, be carried out in any technically suitable manner, for
example in a batch annealing process or also by an inductive
heating, to only name two methods for example. The heat treatment
can be carried out at a temperature of more than 210.degree. C.,
preferably more than 220.degree. C., where applicable more than
230.degree. C. The maximum temperature for the heat treatment is
preferably less than the AC1-temperature of the steel material,
especially at most 400.degree. C.
[0062] By means of the heat treatment, which can also be called
effusion annealing, internal stresses in the workpiece or stress
peaks in the hardened component are reduced, respectively the
failure strain is increased. At the same time, the hydrogen
effusion is accelerated by the selected temperature, so that as a
whole a lower hydrogen embrittlement is achieved. The heat
treatment can be carried out in a time frame of few seconds up to 3
hours, where appropriate also above 3 hours, in particular 6 to 8
hours. The heat treatment following the coating process accelerates
the drying of the components and when using high-strength steels,
the material properties concerning ductility and failure strain are
improved by means of annealing.
[0063] A characteristic of the disclosed method is that the
electrolytic coating (V9) is carried out after the flexible rolling
(V1), after the cutting of the blanks (V2) and after the forming
(V4). The coating deposited on the formed parts has a uniform
thickness, i.e., independent of the respective thickness of the
workpiece. Also the stronger rolled areas have a sufficiently thick
coating, which protects reliably against corrosion. A further
characteristic is the step of the preferably mechanical cleaning
(V5), respectively of cleaning with anodic or inhibited prickle,
whereby the introduction of unwished hydrogen into the workpiece
and thus the hydrogen embrittlement is prevented. By means of the
upstream or downstream heat treatment in a temperature range
between preferably 230.degree. C. and 400.degree. C., internal
stresses in the workpiece are reduced and the hydrogen effusion is
accelerated, which also leads to a lower hydrogen embrittlement of
the material.
[0064] It is understood, that the method according to the present
disclosure can also be modified. For example, also intermediate
steps, not specifically shown here, can be provided between the
mentioned steps. For example, the formed parts can be provided with
an intermediate layer before the electrolytic coating process,
especially with a nickel-, aluminium- or manganese layer. This
intermediate layer forms an additional protection of the surface
and improves the adhesion of the followingly applied coating
containing zinc.
* * * * *