U.S. patent application number 13/703707 was filed with the patent office on 2013-08-15 for method for producing a hot-formed and hardened steel component coated with a metallic anti-corrosion coating from a sheet steel product.
This patent application is currently assigned to THYSSENKRUPP STEEL EUROPE AG. The applicant listed for this patent is Horst Berndsen, Frank Friedel, Jens Kondrattuk, Patrik Kuhn, Volker Marx, Manfred Meurer, Martin Norden. Invention is credited to Horst Berndsen, Frank Friedel, Jens Kondrattuk, Patrik Kuhn, Volker Marx, Manfred Meurer, Martin Norden.
Application Number | 20130206284 13/703707 |
Document ID | / |
Family ID | 44626931 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130206284 |
Kind Code |
A1 |
Norden; Martin ; et
al. |
August 15, 2013 |
Method for Producing a Hot-Formed and Hardened Steel Component
Coated with a Metallic Anti-Corrosion Coating from a Sheet Steel
Product
Abstract
A method for producing a steel component with a metallic
anti-corrosion coating from a sheet steel product comprising at
least 0.4% by weight Mn is disclosed. The sheet steel product is
annealed in a continuous furnace under an annealing atmosphere
containing up to 25% by volume H.sub.2, 0.1% to 10% by volume
NH.sub.3, H.sub.2O, N.sub.2, and process-related impurities as the
remainder, at a dew point between -50.degree. C. and -5.degree. C.
at a temperature of 400 to 1100.degree. C. for 5 to 600 s. The
annealed sheet steel product has a 5 to 200 .mu.m thick nitration
layer with a particle size finer than the particle size of the
inner core layer. Once coated with a metallic protective layer, a
blank is separated from the annealed sheet steel product, heated to
an austenitising temperature of 780 to 950.degree. C., hot-formed,
and cooled so that a hardened structure forms.
Inventors: |
Norden; Martin; (Mobile,
AL) ; Kondrattuk; Jens; (Buchs SG, CH) ;
Meurer; Manfred; (Rheinberg, DE) ; Kuhn; Patrik;
(Dortmund, DE) ; Marx; Volker; (Duisburg, DE)
; Berndsen; Horst; (Duisburg, DE) ; Friedel;
Frank; (Dortmund, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Norden; Martin
Kondrattuk; Jens
Meurer; Manfred
Kuhn; Patrik
Marx; Volker
Berndsen; Horst
Friedel; Frank |
Mobile
Buchs SG
Rheinberg
Dortmund
Duisburg
Duisburg
Dortmund |
AL |
US
CH
DE
DE
DE
DE
DE |
|
|
Assignee: |
THYSSENKRUPP STEEL EUROPE
AG
Duisburg
DE
|
Family ID: |
44626931 |
Appl. No.: |
13/703707 |
Filed: |
June 14, 2011 |
PCT Filed: |
June 14, 2011 |
PCT NO: |
PCT/EP11/59808 |
371 Date: |
February 27, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2011/059808 |
Jun 14, 2011 |
|
|
|
13703707 |
|
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Current U.S.
Class: |
148/217 ;
148/226 |
Current CPC
Class: |
C21D 8/0457 20130101;
C23C 28/3225 20130101; C23C 2/06 20130101; C23C 28/322 20130101;
C23C 2/02 20130101; C23C 2/28 20130101; C23C 22/82 20130101; C23C
28/36 20130101; C23C 2/12 20130101; C23C 28/34 20130101; C23C
28/321 20130101; C21D 9/561 20130101; C21D 9/48 20130101; C21D 1/74
20130101; C21D 1/673 20130101; C23C 28/00 20130101 |
Class at
Publication: |
148/217 ;
148/226 |
International
Class: |
C23C 28/00 20060101
C23C028/00; C23C 22/82 20060101 C23C022/82 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2010 |
DE |
102010017354.1 |
Claims
1. A method for producing a steel component that is coated with a
metallic anti-corrosion coating from a sheet steel product having
an Mn-content of at least 0.4% by weight, comprising the following
working steps: providing the sheet steel product; annealing the
sheet steel product in a continuous furnace, under an annealing
atmosphere containing up to 25% by volume H.sub.2, 0.1 to 10% by
volume NH.sub.3, H.sub.2O and N.sub.2 as the remainder as well as
process-related inevitable impurities and having a dew point
ranging between -50.degree. C. and -5.degree. C., at a holding
temperature of 400 to 1100.degree. C., for a holding period of 5 to
600 s, so that the sheet steel product obtained after the annealing
treatment has a 5 to 200 .mu.m thick nitration layer, which adjoins
its free surface and the particle size of which is finer than the
particle size of the inner core layer of the sheet steel product
covered by the edge layer; coating the annealed sheet steel product
with a metallic protective layer; separating a blank from the sheet
steel product; optionally preforming the blank; heating the blank
to an austenitising temperature of 780 to 950.degree. C.,
hot-forming the heated blank to form the steel component,
accelerated cooling of the steel component in such a way that a
hardened structure forms in the sheet steel product.
2. The method according to claim 1, characterised in that the
H.sub.2-content of the annealing atmosphere is at most 10% by
volume.
3. The method according to claim 1, wherein the NH.sub.3-content of
the annealing atmosphere is at most 5% by volume.
4. The method according to claim 1, wherein the dew point of the
annealing atmosphere is -40.degree. C. to -15.degree. C.
5. The method according to claim 1, wherein the holding temperature
of the annealing is 680 to 840.degree. C.
6. The method according to claim 1, wherein the holding period of
the annealing is 30 to 120 s.
7. The method according to claim 1, wherein the characteristic
particle size of the nitration layer of the annealed sheet steel
product, determined in accordance with DIN EN ISO 643 before the
blank is heated and hot-formed, is smaller by at least 2 than the
characteristic particle size of the basic material.
8. The method according to claim 1, wherein the coating of the
sheet steel product with the metallic protective layer takes place
by means of hot-dip coating, which is completed in a work sequence
carried out continuously following the annealing treatment.
9. The method according to claim 8, wherein an oxidation of the
surface of the sheet steel product is carried out before the
hot-dip coating.
10. The method according to claim 8, wherein the sheet steel
product is continuously diffusion-annealed after the hot-dip
coating.
11. The method according to claim 1, wherein the coating of the
sheet steel product with the metallic, metallic-organic or
metallic-inorganic protective layer takes place by electrolytic
coating or a physical vapour or chemical vapour deposition.
12. The method according to claim 1, wherein the metallic
protective layer is a Zn, an Al, a Zn--Al, a Zn--Mg, a Zn--Ni, an
Al--Mg, an Al--Si, a Zn--Al--Mg or a Zn--Al--Mg--Si coating.
13. The method according to claim 1, wherein the austenitising
temperature adjusted during the heating is 860 to 950.degree.
C.
14. The method according to claim 1, wherein the hot-forming and
the cooling of the component obtained by the hot-forming are
carried out in one step.
15. The method according to claim 1, wherein the component obtained
is subjected to a blasting treatment.
Description
[0001] The invention relates to a method for producing a hot-formed
and hardened steel component coated with a metallic anti-corrosion
coating from a sheet steel product, which has an Mn-content of at
least 0.4% by weight.
[0002] As reported in the Article "Potenziale fur den
Karosserieleichtbau" (Potential for lightweight body construction),
published in the Exhibition Newspaper of ThyssenKrupp Automotiv AG
for the 61.sup.st International Car Exhibition in Frankfurt, from
the 15.sup.th to 25.sup.th Sep. 2005, hot-forming hardening is
applied in practice in particular to the production of
high-strength body components from boron-alloyed steels. A typical
example of a steel of the type in question here is the steel known
by the designation 22MnB5, which can be found in the Key to Steel
2004 under the material number 1.5528.
[0003] A steel that is comparable with the steel 22MnB5 is known
from JP 2006104526A. This known steel contains, apart from Fe and
inevitable impurities (in % by weight), 0.05 to 0.55% C, max. 2%
Si, 0.1 to 3% Mn, max. 0.1% P and max. 0.03% S. To improve the
hardenability, contents of 0.0002 to 0.005% B and 0.001 to 0.1% Ti
can be added to the steel. The respective Ti-content is used here
to set the nitrogen present in the steel. The boron contained in
the steel can thus develop its strength-increasing effect as
completely as possible.
[0004] According to JP 2006104526 A, metal sheets, which are then
preheated to a temperature above the Ac.sub.3 temperature,
typically ranging from 850 to 950.degree. C., are firstly
manufactured from the steel composed in this manner. During the
subsequent rapid cooling starting from this temperature range and
taking place in the pressing tool, the martensitic structure
ensuring the high strengths aimed for is formed in the component
press-formed from the respective sheet metal blank. It is
advantageous here that the sheet metal parts heated to the
temperature level mentioned can be formed into components that are
formed in a complex manner at relatively low forming forces. This
also applies, in particular, to sheet metal parts of the type which
are manufactured from high-strength steel and provided with an
anti-corrosion coating.
[0005] The hot forming of zinc-plated sheet steel products into
high-strength or very high-strength steel components presents a
particular difficulty. If a steel sheet provided with a metallic
anti-corrosion coating has to be heated for the hot-forming and a
possible subsequent hardening or a hardening carried out in
combination with the hot-forming, to a temperature, which is above
the melting temperature of the metal of the protective coating,
there is a risk of so-called "liquid metal embrittlement". This
embrittlement of the steel occurs when molten liquid metal of the
coating penetrates into the notches being formed on the surface of
the respective sheet steel product during forming. The liquid metal
reaching the steel substrate settles there at the particle
boundaries and thus reduces the maximum absorbable tensile and
compressive stresses.
[0006] The risk of liquid metal embrittlement in sheet steel
products produced from higher-strength and high-strength
Mn-containing steels proves to be particularly critical. These
steels only have a limited ductility and as a result tend to form
cracks close to the surface and close to the particle boundary as a
result during their forming.
[0007] It is generally known from DE-OS 18 13 808 that the
corrosion and oxidation resistance of a steel sheet can be improved
by a nitriding treatment, by means of which an edge layer that is
2.5 to 19 .mu.m in thickness and close to the surface is produced
with a nitrogen content that is elevated relative to the core
region of the steel sheet. The nitration layer has good
adhesion.
[0008] It is furthermore known from DE 691 07 931 T2 that in a
region close to the surface of sheet steel products consisting of
low-carbon steels and intended for the construction of motor
vehicle bodies, higher C- or N-contents can be produced by a
carbonising or nitriding treatment in order to improve the
processability of the relevant sheet steel products.
[0009] These measures in the prior art are not in connection with
higher-strength or high-strength steels, which have Mn-contents of
at least 0.4% by weight, typical Mn-contents of the steels
processed according to the invention being in the range from 0.4 to
0.6% by weight, in particular 0.6 to 3.0% by weight.
[0010] The C-content of the sheet steel products processed
according to the invention is typically more than 0.06% by weight
and less than 0.8% by weight, in particular less than 0.45% by
weight.
[0011] Examples of the steels processed according to the invention,
to adjust their respective properties, may contain up to 0.2% by
weight Ti, up to 0.005% by weight B, up to 0.5% by weight Cr, up to
0.1% by weight V or up to 0.03% by weight Nb.
[0012] The nitriding, or the inner nitration, assume the presence
of nitrogen capable of diffusion. This prerequisite is satisfied
when the nitrogen is present in statu nascendi.
[0013] The nitration generally takes place by annealing the
respective sheet steel products in an ammonia-containing
H.sub.2-N.sub.2 annealing gas atmosphere. Ammonia and nitrogen are
available there as nitrogen dispensers. Ammonia gas splits into
nitrogen and hydrogen at atmospheric pressure and temperatures
above 400.degree. C. while doubling its volume. The dissociation of
ammonia gas can be described by the following reaction
equation:
2NH.sub.3->2[N]+3H.sub.2
[0014] Against the background of the prior art described above, the
object of the invention was to disclose a method, which, while
minimising the risk of the development of metal-induced cracks,
economically allows a high-strength steel component to be
produced.
[0015] This object was achieved according to the invention in that
when producing a high-strength steel component, the working steps
disclosed in claim 1 are carried out.
[0016] Advantageous configurations of the invention are disclosed
in the claims that depend on the respective independent claims and
will be described in detail below as will the general inventive
idea.
[0017] The method according to the invention for producing a steel
component coated with a metallic anti-corrosion coating is based on
the idea of carrying out a nitriding treatment on the sheet steel
product before it is hot-formed, a finely structured edge layer
being produced in the sheet steel product by means of said
nitriding treatment. On the one hand, this edge layer improves the
forming properties of the surface-finished steel product for the
hot-forming.
[0018] On the other hand, the edge region of the sheet steel
product nitrided in the manner according to the invention proves to
be surprisingly helpful in avoiding metal embrittlement of the fine
steel sheet during the hot-forming. The nitration zone thus brings
about a significant increase in the particle boundary
surfaces/phase boundary surfaces during the hot-forming process,
which counteracts the crack failure of the material as a
consequence of metal material of the coating penetrating into the
structure of the steel substrate. Moreover, an unusually high iron
diffusion is adjusted in the coating. As a result, the coating
becomes thermally more stable, in particular when processing
coatings based on zinc.
[0019] In order to utilise the positive influences of the edge
layer nitriding carried out according to the invention that is
summarised above, the method according to the invention comprises
the following working steps: [0020] A sheet steel product made of a
steel having an Mn-content of at least 0.4% by weight is provided.
If a sheet steel product is mentioned here this then means, in
general, steel sheets, bands, blanks or the like. A sheet steel
product of this type may be processed in the hot-rolled or
cold-rolled state in the manner according to the invention. It is
also conceivable to combine different steel blanks to form a sheet
steel product then processed in a manner according to the
invention, one of the steel blanks consisting of a steel of the
type disclosed in claim 1. [0021] The sheet steel product is
annealed in a continuous furnace under an annealing atmosphere,
which contains up to 25% by volume H.sub.2, 0.1 to 10% by volume
NH.sub.3, H.sub.2O and N.sub.2 as the remainder as well as
process-related inevitable impurities and which has a dew point of
between -50.degree. C. and -5.degree. C. The holding temperature,
at which the sheet steel product is held for a holding period of 5
to 600 s, in this case is 400 to 1100.degree. C. As a result, owing
to this nitriding-annealing treatment, a 5 to 200 .mu.m thick
ductile nitration layer adjoining its free surface is present on
the sheet steel product, the particle size of which nitration layer
is finer than the particle size of the inner core layer covered by
the edge layer and formed by the basic material of the sheet steel
product. [0022] After the production of the nitration layer, the
sheet steel product annealed in the manner disclosed above is
coated with a metallic protective layer. The invention utilises the
recognition here that the risk of a liquid metal embrittlement can
be minimised in that by a targeted modification of the region of
the sheet steel product close to the surface, the temperature range
susceptible to liquid metal embrittlement can be displaced in such
a way that it does not coincide with the temperature interval
typical for the hot-forming. [0023] Blanks are separated from the
sheet steel product coated with the metallic protective layer.
[0024] If the forming takes place in two or more stages, the blank
may optionally be preformed at this point. The preforming can go so
far here that after the preforming, the shape of the blank
virtually completely corresponds to the shape of the finished
component. Typically, the preforming takes place with a cold or
semi-hot blank heated below the austenitising temperature. With a
one-stage forming carried out only by hot-forming, the preforming
is dispensed with. [0025] For the hot-forming, the blank is heated
to an austenitising temperature of 780 to 950.degree. C. [0026] The
hot-forming of the heated blank into the finished steel component
then takes place. [0027] The steel component obtained is then
subjected to a cooling, in which, starting from the austenitising
temperature, accelerated cooling takes place. The cooling of the
steel component takes place here in such a way that a hardened
structure forms in the sheet steel product.
[0028] The hot-forming and the hardening may take place "in one
stage". In this case, the hot-forming and the hardening are carried
out in one step together in a tool. On the other hand, in the
two-stage process, the working steps "forming" and "producing the
heat treatment or hardened structure" are carried out separately
from one another.
[0029] Surprisingly, when applying the annealing conditions
predetermined according to the invention, it is possible to achieve
the desired nitriding depth even with very short conditioning
times. Thus, the method according to the invention is
distinguished, in particular, in that it can be carried out in a
particularly economical manner using a continuous furnace. This
makes it possible to incorporate the method according to the
invention in continuous production processes, which assume high
belt speeds, such as is the case, for example, in hot-dip
galvanising plants, in which steel bands are heat treated and are
hot-dip coated with the anti-corrosion coating in a continuous
run.
[0030] Iron surfaces present in the reaction chamber catalytically
promote the dissociation. A part of the nitrogen atoms released at
the moment of disintegration may diffuse into the iron
material.
[0031] Nitrogen transfer takes place in a plurality of part steps:
[0032] Transportation to the workpiece surface [0033] Adsorption on
the surface [0034] Penetration of the surface (absorption) [0035]
Diffusion into the workpiece interior
[0036] Because of the increased nitrogen solubility in the
austenite, it is expedient to carry out the annealing
intercritically, i.e. in the two-phase area .alpha./.gamma.-Fe.
Independently of whether the subsequent coating is carried out with
the metallic protective layer continuously or piece-wise, the
result of the nitriding treatment can accordingly be optimised
under the conditions generally provided in practice in a
particularly economical and environmentally compatible manner in
that at least one of the following conditions is adhered to: [0037]
the H.sub.2-content of the annealing atmosphere is at most 10% by
volume, [0038] the NH.sub.3-content of the annealing atmosphere is
at most 5% by volume, [0039] the dew point of the annealing
atmosphere is -40.degree. C. to -15.degree. C., [0040] the holding
temperature of the annealing is 680 to 840.degree. C., [0041] the
holding period of the annealing is 30 to 120 s.
[0042] It is decisive for the success of the invention that during
the annealing treatment according to the invention, a nitration
edge layer is adjusted, the particle size of which is significantly
finer than the particle size of the core layer of the sheet steel
product that is not nitrided during the annealing. Practical tests
have shown that according to DIN EN ISO 643, the characteristic
particle size of the nitration layer is smaller by at least 2 than
the characteristic particle size of the basic material (core layer)
of the annealed sheet steel product before the heating and
hot-forming of the blank.
[0043] During the method according to the invention, a nitrided
edge layer is produced in a targeted manner. The thickness of this
finely structured, optionally only partly recrystallized nitration
layer is determined by the nitration hardness depth determined
according to DIN 50190-3. Accordingly the nitration hardness depth
is the spacing from the surface to the point of the steel
substrate, at which the hardness corresponds to the core
hardness+50 HV. In this manner, a hardness is adjusted in the
nitrided edge layer region of the sheet steel product close to the
surface, which is at least 25% higher than the hardness of the core
region, i.e. Hv(nitrided)/Hv(core region).gtoreq.1.25.
[0044] Typically, in a sheet steel product processed according to
the invention, the thickness of the nitrided edge region after the
annealing treatment is >5 .mu.m and <200 .mu.m.
[0045] A configuration of the invention that is particularly
advantageous in practice is characterised in that the coating of
the sheet steel product with the metallic protective layer takes
place by means of a hot-dip coating, which is completed in a work
sequence carried out continuously following the annealing
treatment. In this case, the annealing treatment carried out
according to the invention is carried out at the same time as the
surface conditioning for the downstream surface finishing by means
of a heterogeneous annealing gas-metal reaction.
[0046] It is particularly advantageous here to use the method
according to the invention in a hot-dip coating line, as the
annealing treatment in this case may comprise the edge nitriding,
surface conditioning and recrystallisation of the basic material
and the hot-dip galvanising can then be carried out in a continuous
method sequence in-line following the annealing treatment. In this
case, it is basically conceivable to flood the furnace section
through which the sheet steel product runs with NH.sub.3-containing
gas over its entire length. In order to not subject all the
components of the continuous furnace to the nitriding atmosphere,
it may also, however, be advantageous to separate a portion of the
furnace section from the other portions of the furnace and to only
load this separated portion with the NH.sub.3-containing
atmosphere.
[0047] In order, in the case of a hot dip coating of the annealed
sheet steel product carried out, in particular, as a hot-dip
galvanising, to ensure optimum adhesion of the coating on the steel
substrate, before the hot-dip coating, an oxidation of the surface
of the sheet steel product can be carried out.
[0048] In the course of the surface finishing of a sheet steel
product produced according to the invention, preferably carried out
by hot-dip coating, coating systems known per se can be applied to
the steel substrate, which are based on Zn, Al, Zn--Al, Zn--Mg,
Zn--Ni, Zn--Fe, Al--Mg, Al--Si, Zn--Al--Mg or Zn--Al--Mg--Si.
Following the hot-dip coating, further heat treatment steps can be
carried out in order to configure the metallic protective coating
in a specific way. If necessary, a diffusion annealing, for example
a galvannealing treatment, may also take place continuously after
the hot-dip coating.
[0049] Alternatively or in addition to the hot-dip finishing taking
place in-line, a sheet steel product, on which a finely structured
nitration layer has been formed in a continuous annealing in the
manner according to the invention, may receive a metallic, a
metallic-inorganic or a metallic-organic coating, in that it is
coated electrolytically, for example with a Zn, ZnNi or a ZnFe
coating, by means of physical vapour or chemical vapour deposition
or by means of another metal-organic or metal-inorganic coating
method.
[0050] In order to further optimise the mechanical properties, an
ageing treatment carried out in a conventional manner may follow
the annealing treatment according to the invention.
[0051] Components that have been hot-formed from a sheet steel
product treated according to the invention and then hardened have
tensile strengths of 800 to 2000 MPa, in particular 900 to 2000
MPa.
[0052] The nitration layer produced according to the invention
allows the sheet steel product according to the invention to be
heated without problems to an austenitising temperature, in which
the sheet steel product has a substantially completely austenitic
structure. Even at a temperature as high as this, the risk of
embrittlement is minimised in a sheet steel product produced
according to the invention even when the sheet steel product is
provided with a metallic coating, the melt temperature of which is
less than or equal to the heating temperature. The fineness of the
particles of the edge layer produced by the nitriding according to
the invention prevents a crack formation and thus ensures that no
metal of the coating can penetrate into the core region or basic
material of the steel substrate.
[0053] Owing to the production according to the invention of a
finely structured, nitrided nitration layer, in the heat forming
process preferably carried out directly, i.e. without prior
preforming of the blank, solid metal embrittlement occurring from a
metallic coating, in particular a zinc coating, otherwise resulting
from diffusion of the coating metal onto the particle boundaries,
is therefore prevented. Likewise, the procedure according to the
invention, as a result of the coating configuration being produced
from the nitriding and advantageous with regard to the Fe/coating
metal ratio, prevents the occurrence of solder cracks and thus
counteracts the liquid metal embrittlement.
[0054] The invention will be described in more detail below with
the aid of embodiments, in which:
[0055] FIG. 1 shows a vertical microsection of a nitrided-annealed
steel sample according to the invention;
[0056] FIG. 2 shows a vertical microsection of a non-annealed,
rolled comparative sample;
[0057] FIG. 3 shows GDOES depth profiles of the nitrogen content of
the samples shown in FIGS. 1 and 2;
[0058] FIG. 4 shows a vertical microsection of the tensile area of
a steel component formed from the steel sample according to FIG.
1;
[0059] FIG. 5 shows a vertical microsection of the tensile area of
a steel component formed from the rolled steel sample according to
FIG. 2.
[0060] To check the effects achieved by the method according to the
invention, respective rolled cold band samples of a multi-phase
steel "MP" and of a steel "WU" conventionally used for hot-forming
have been produced. The compositions of the steels MP and WU are
given in Table 1.
[0061] Two samples manufactured from the steels MP and WU have been
subjected to an annealing treatment according to the invention in a
continuous furnace for an edge layer nitriding. The annealing
parameters applied here are given in Table 2.
[0062] For comparison, two further samples manufactured from the
steels MP and WU have been subjected in the continuous furnace to a
conventional annealing, such as is generally carried out to prepare
a hot-dip zinc-plating.
[0063] FIG. 1 shows the microsection of the sample treated by
annealing according to the invention and produced from the steel
WU. It can clearly be seen that a finely structured structural
region (nitration layer "N") close to the surface has been adjusted
as a consequence of the procedure according to the invention.
[0064] The microsection of the rolled sample also produced from the
steel WU, on the other hand, shows no such nitration layer (FIG.
2).
[0065] GDOES measurements of the nitrogen content have additionally
been carried out on the samples which were rolled or treated by
annealing according to the invention and consisted of the steel WU.
The GDOES measuring method ("GDOES"=Glow Discharge Optical Emission
Spectrometer) is a standard method to rapidly detect a
concentration profile of coatings. For example, it is described in
the VDI-dictionary "Werkstofftechnik" (Materials Technology),
published by Hubert Grafen, VDI-Verlag GmbH, Dusseldorf 1993.
[0066] The result of the GDOES measurements is summarised in FIG.
3, the dashed line showing the nitrogen distribution of the rolled
sampled and the solid line showing the nitrogen distribution of the
sample treated according to the invention.
[0067] FIG. 3 also clearly shows that the sample treated according
to the invention has a pronounced nitrided nitration layer N, the
thickness of which is about 20 .mu.m.
[0068] It was possible to show with the aid of micro hardness
measurements that the nitration region N nitrided in the sample
that was heat treated according to the invention and produced from
the steel WU has a micro hardness of 340 HV and the non-nitrided
core region (basic material) K has a hardness of 180 HV. The ratio
Hv.sub.N/Hv.sub.K of the hardness Hv.sub.N of the nitrided
nitration layer N to the hardness Hv.sub.K of the core region K was
therefore about 1.9 and therefore significantly above the value of
1.25 predetermined according to the invention for this ratio.
[0069] Following the annealment, a surface finishing of the samples
took place, in which zinc was electrolytically deposited with a
layer thickness of 10 .mu.m on the samples.
[0070] Subsequently, the samples consisting of the steel WU were
formed and press-hardened by means of the so-called one-stage or
direct hot-forming method to form a steel component. For this
purpose, the samples were heated over an austenitising period of 6
minutes at an austenitising temperature of 880.degree. C. and then
hot-formed in a hot-press forming tool to form a component for a
car body.
[0071] After the hot-forming, the components obtained were cooled
in a manner known per se so rapidly that a hardened structure
formed.
[0072] A comparison of FIGS. 4 and 5 makes it clear that no crack
formation of any kind in the region of the tensile area occurred in
the component produced in the manner according to the invention,
while clear intercrystalline crack formation is to be noted in the
component produced in the conventional manner.
[0073] For the zinc-plated and formed samples treated by annealing
and produced from the steel MP, comparable results could be shown
for the samples treated by annealing according to the invention and
conventionally.
[0074] The method according to the invention therefore improves the
forming properties of surface-finished sheet steel products for
hot-forming. For this purpose, by means of a targeted gas-metal
reaction during the annealing process before the surface finishing,
in a continuous process or piece-wise, an edge nitriding is
produced, as a result of which a finely structured
nitrogen-containing nitration layer N is adjusted. This nitrided
edge layer N, on the one hand, increases the Fe diffusion in the
coating and prevents the transportation of the "coating metal"
embrittlement producer, i.e. in particular zinc, onto the particle
boundaries during the annealing process carried out before the
hot-forming.
[0075] As a result, components are thus obtained, in which the
steel substrate is substantially completely crack-free.
TABLE-US-00001 TABLE 1 Remainder iron and inevitable impurities C
Mn P Si V Al Cr Ti B Nb Steel [% by weight] MP 0.22 1.7 0.02 0.1
0.002 1.7 0.06 0.1 0.005 0.001 WU 0.22 1.22 0.017 0.25 0.005 0.025
0.13 0.03 0.005 0.003
TABLE-US-00002 TABLE 2 Working step According to the invention
Annealing treatment Heating rate 10 K/s Holding temperature
800.degree. C. Holding period 60 s Annealing atmosphere 4% NH.sub.3
96% N.sub.2 Dew point -30.degree. C. Cooling rate to room
temperature 20 K/s
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