U.S. patent application number 14/903022 was filed with the patent office on 2016-05-26 for wear-resistant, partially uncoated steel parts and methods of producing same.
This patent application is currently assigned to THYSSENKRUPP STEEL EUROPE AG. The applicant listed for this patent is THYSSENKRUPP STEEL EUROPE AG. Invention is credited to Janko BANIK, Sascha SIKORA, Thiemo WUTTKE.
Application Number | 20160145705 14/903022 |
Document ID | / |
Family ID | 51059430 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160145705 |
Kind Code |
A1 |
SIKORA; Sascha ; et
al. |
May 26, 2016 |
WEAR-RESISTANT, PARTIALLY UNCOATED STEEL PARTS AND METHODS OF
PRODUCING SAME
Abstract
A wear-resistant steel part may be formed by hot forming and/or
hardening a hardenable steel grade semifinished part. The steel
part may be used, for example, as a processing, conveying, and/or
crushing mechanism in agricultural machines, conveying machines,
mining machines, or building machines. The semifinished part may be
heated to a temperature above an Ac1 transformation temperature and
then subsequently hot formed and/or hardened. The steel part may be
particularly suitable for use with abrasive materials. To that end,
the steel part may have at least one region that has been hardened
to a depth of not more than 100 microns by surface hardening before
the semifinished part is hot formed or hardened."
Inventors: |
SIKORA; Sascha; (Lunen,
DE) ; BANIK; Janko; (Altena, DE) ; WUTTKE;
Thiemo; (Recklinghausen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THYSSENKRUPP STEEL EUROPE AG |
Duisburg |
|
DE |
|
|
Assignee: |
THYSSENKRUPP STEEL EUROPE
AG
Duisburg
DE
|
Family ID: |
51059430 |
Appl. No.: |
14/903022 |
Filed: |
June 24, 2014 |
PCT Filed: |
June 24, 2014 |
PCT NO: |
PCT/EP2014/063259 |
371 Date: |
January 5, 2016 |
Current U.S.
Class: |
148/226 ;
148/319 |
Current CPC
Class: |
C23C 8/80 20130101; C21D
1/06 20130101; C21D 8/005 20130101; C21D 7/13 20130101; C21D 1/18
20130101; C21D 9/18 20130101; C21D 2221/02 20130101; C23C 8/22
20130101; C23C 8/26 20130101; C21D 6/00 20130101; C21D 9/22
20130101; C21D 1/673 20130101; C21D 9/0068 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C21D 1/06 20060101 C21D001/06; C21D 8/00 20060101
C21D008/00; C21D 6/00 20060101 C21D006/00; C23C 8/22 20060101
C23C008/22; C23C 8/26 20060101 C23C008/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2013 |
DE |
10 2013 107 100.7 |
Claims
1.-12. (canceled)
13. A method for producing a wear-resistant, uncoated steel part
for a processing, conveying, and/or crushing mechanism of an
agricultural machine, a conveying machine, a mining machine, or a
building machine from a semifinished part comprising a hardenable
steel grade, the method comprising: heating at least regions of the
semifinished part to a temperature above an Ac1 transformation
temperature; at least partially subjecting the semifinished part to
surface hardening by hardening a surface region to a depth of not
more than 100 microns by heat treating the semifinished part in a
heat treatment atmosphere comprising up to 25% by volume of
H.sub.2, 0.1-10% by volume of NH3, H2O and balance N2 and
impurities at a hold temperature of 600-900 degrees Celsius; and at
least one of hot forming or hardening the semifinished part.
14. The method of claim 13 wherein hardening the surface region
comprises nitriding or carburizing.
15. The method of claim 13 wherein hardening the surface region
comprises maintaining the semifinished part at the holding
temperature for 30 to 120 seconds.
16. The method of claim 13 wherein the surface hardening is
performed in a continuous hardening furnace.
17. The method of claim 13 wherein the semifinished part to be
surface hardened comprises a manganese-boron steel or a TRIP
steel.
18. A wear-resistant, uncoated steel part formed of a hardenable
steel grade and produced by hot forming and/or hardening a
semifinished part, wherein the wear-resistant, uncoated steel part
comprises a surface region that has been hardened to a depth of not
more than 100 microns by surface hardening by nitriding prior to
hot forming and/or hardening the semifinished part, wherein the
wear-resistant, uncoated steel part is configured for use as a
processing, conveying, and/or crushing mechanism in agricultural
machines, conveying machines, mining machines, or building
machines, wherein at least regions of the wear-resistant, uncoated
steel part that are to be subjected to abrasive forces have been
surface hardened.
19. The wear-resistant, uncoated steel part of claim 18 wherein
after hot forming and/or hardening the steel part has at least the
hardness of a base material of the steel part located under the
surface region.
20. The wear-resistant, uncoated steel part of claim 18 wherein the
steel part comprises at least one of a manganese-boron steel, a
dual-phase steel, or a TRIP steel.
21. The wear-resistant, uncoated steel part of claim 18 wherein the
surface region that has been hardened has before hot forming and/or
hardening at least in some regions a hardness of 400-700 HV.
Description
[0001] The invention relates to a wear-resistant, at least partly
uncoated steel part consisting of a hardenable steel grade which
has been produced from a semifinished part by hot forming and/or
hardening. In addition, the invention relates to a process for
producing a wear-resistant, at least partly uncoated processing,
conveying and/or crushing means of agricultural machines, conveying
machines, mining machines or building machines from a semifinished
part, in which the semifinished part is heated to a temperature
above the Ac1 transformation temperature and is subsequently hot
formed and/or hardened.
[0002] Wear-resistant, at least partly uncoated steel parts which
have to have high strengths and at the same time are subjected to
abrasive forces are required, for example, for the production of
agricultural machines, in particular plows, and also for buckets of
a dredge or conveying screws for abrasive materials, for example
the conveying screw of a concrete mixer. In order to achieve the
necessary high strengths in the abovementioned applications, the
parts are preferably subjected to hot forming in which the
semifinished parts from which the steel parts are produced are
firstly heated to a temperature above the Ac1 transformation
temperature point, so that transformation hardening of the
microstructure is effected by hot forming and subsequent hardening,
i.e. rapid cooling, and a material having a martensitic
microstructure is formed. The martensitic microstructure has a
significantly greater hardness but also a significantly greater
mechanical strength, for example tensile strength. Corresponding
steel parts are known, for example, from the German patent DE 10
2010 050 499 B3. The German patent describes a process for
producing dredger buckets, concrete mixer conveying screws,
conveying screw blades or other transport blades of conveying
plants, in which the components are hot formed and press
hardened.
[0003] However, it has been found that the components produced in
this way have problems in respect of the wear resistance despite
the hardening process during production, especially on contact with
abrasive materials.
[0004] The German first publication DE 10 2010 017 354 A1 is
concerned with the problem of hot forming of zinc-plated flat steel
products to produce high-strength or very high-strength steel
components. When the melting point of the metal of the protective
coating is exceeded, there is a risk of "liquid metal
embrittlement" which is caused by penetration of the molten metal
of the coating into the notches or cracks arising in forming of the
flat steel product. The liquid metal which has penetrated into the
steel substrate deposits at grain boundaries and there reduces the
maximum tensile or compressive stress which can be withstood. As a
solution, the patent publication offers nitriding of the outer
layer regions, so as to produce finely structured outer layer
regions.
[0005] The present invention is, in contrast, concerned with the
problem that hot-formed and/or hardened steel parts do not have the
desired wear resistance in the uncoated regions and are therefore
not optimally suited for use as conveying means, for example on
contact with abrasive materials. It is therefore an object of the
present invention to propose at least partly uncoated steel parts
having improved suitability for use with abrasive materials. In
addition an inexpensive production process for corresponding steel
parts should be proposed.
[0006] The object indicated is achieved, for a steel part, the
steel part at least partially having a surface region which has
been hardened to a depth of not more than 100 .mu.m, preferably to
a depth of up to 40 .mu.m, by surface hardening before hot forming
and/or hardening.
[0007] It has been found that the heating of the semifinished parts
for production of the steel parts to a temperature above the Ac1
transformation temperature or above the Ac3 temperature before hot
forming and/or hardening leads to decarburization of regions close
to the surface, so that the carbon content of these regions is
significantly lower than the carbon content of the base material.
As a result, the region close to the surface up to a depth of 100
.mu.m, in particular the region up to a depth of 40 .mu.m, cannot
be hardened to the required degree during hot forming and/or
hardening. However, it has been found that at least partial surface
hardening of the uncoated regions of the semifinished parts before
hot forming and/or hardening to give the steel part leads to both
the surface region and the base material having very high hardness
despite the decarburization of the regions close to the surface as
a result of the high temperatures during hot forming or hardening.
This provides a steel part of which at least partially has a
surface region which has been hardened to a depth of preferably 100
.mu.m or in the region down to a depth of 40 .mu.m and is therefore
significantly more wear resistant than the at least partly uncoated
steel parts known hitherto.
[0008] In a first embodiment, the hardened surface region of the
steel part is hardened by carburization or nitriding. Both
processes offer the opportunity of hardening regions close to the
surface of the steel part in a targeted manner before hot forming
or hardening. In addition, nitriding has the advantage that the
hardness is not reduced during hot forming. In the case of
carburization, the carbon content in the surface regions is
increased but decreases again due to hot forming.
[0009] In a further embodiment, after hot forming and/or hardening
the hardened surface region of the steel part preferably has at
least the hardness of the base material of the steel part located
under the surface region.
[0010] The wear resistance of the steel part can preferably also be
improved by the hardness of the surface region of the steel part
being greater than the hardness of the base material. It has been
found that, in particular, the hardness of the surface regions is
responsible for the wear resistance of the steel part on contact
with highly abrasive materials, so that a very wear-resistant steel
part can be produced even when using a somewhat softer base
material.
[0011] Consequently, the steel part is, according to a further
embodiment of the steel part, configured for use as processing,
conveying and/or crushing means in agricultural machines, conveying
machines, mining machines or building machines, with at least the
regions of the steel part which are subjected to abrasive forces
being surface-hardened.
[0012] In addition, manganese-boron steels, dual-phase steels or
TRIP steels, in which particularly pronounced martensite formation
or transformation of residual austenitic components into martensite
makes an increase in the hardnesses possible, are also particularly
advantageous.
[0013] In a further embodiment of the steel part, the surface
region of the steel part which has been hardened before hot forming
and/or hardening has, at least in regions, a hardness of from 400
to 700 HV. These values are generally achieved only by very
high-strength steel grades after hot forming or hardening in the
base material. The surface hardening before hot forming or
hardening offers, in particular, the opportunity of providing the
starting material for production of the steel components on a
coil.
[0014] According to further teaching of the present invention, the
abovementioned object is achieved by a process for producing a
wear-resistant, at least partly uncoated steel part for processing,
conveying and/or crushing means of agricultural machines, conveying
machines, mining machines or building machines from a semifinished
part, in which the semifinished part is heated, at least in
regions, to a temperature above the Ac1 transformation temperature
and is subsequently hot formed and/or hardened, in that the
semifinished part at least partially is subjected to surface
hardening in which a surface region is hardened to a depth of not
more than 100 .mu.m before hot forming and/or hardening. Preference
is given to hardening a surface region having a depth of up to 40
.mu.m, in which decarburization processes usually take place during
hot forming. The depth of the surface region which is to be
hardened is controlled by the duration of the hardening treatment.
It has been found, in particular, that despite heating to a
temperature above the Ac1 transformation temperature point, the
surface-hardened regions of the steel part remain stable in respect
of the surface hardness, so that high surface hardness can be
achieved after hot forming and/or hardening. This leads to the
steel parts of processing, conveying and/or crushing means of
agricultural machines, conveying machines, mining machines or
building machines which are in contact with abrasive materials
displaying reduced wear.
[0015] The hardening of the surface regions before hot forming or
before hardening makes it possible to carry out the surface
hardening on coilable materials, i.e. on steel strip, so that
particularly economical production of wear-resistant, at least
partly uncoated steel parts from semifinished parts is made
possible. In a preferred embodiment of the process, hardening of
the surface region is effected by nitriding or by carburization.
Both processes make it possible to provide a higher hardness in the
surface region, which after hot forming and/or after hardening make
a higher wear resistance of the surface of the hot-formed or
hardened steel part possible.
[0016] The surface hardening is, in a further embodiment,
particularly preferably carried out by a heat treatment in a heat
treatment atmosphere comprising up to 25% by volume of H.sub.2,
0.1-10% by volume of NH.sub.3, H.sub.2O and a balance N.sub.2 and
also unavoidable impurities at a holding temperature of from
600.degree. C. to 900.degree. C. The dew point of the heat
treatment atmosphere is preferably in the range from -50.degree. C.
to -5.degree. C., so that the effect of atmospheric moisture on the
hardening process is reduced. In addition, preference is given to a
maximum of 10% by volume of H.sub.2 and a maximum of 5% by volume
of NH.sub.3 being permitted and the dew point being set to a dew
point temperature of from -40.degree. C. to -15.degree. C. at a
temperature of from 680 to 840.degree. C. The latter process
parameters gave improved and more uniform surface hardening.
[0017] The depth of the surface hardening can be set via the time
for which the holding temperature is maintained. The time for which
the semifinished part has the holding temperature during surface
hardening is preferably set to from 5 s to 600 s, preferably from
30 s to 120 s.
[0018] The surface hardening is preferably carried out in a
continuous hardening furnace, so that, for example, a strip-like
semifinished part, i.e. a coilable semifinished part, is also
surface-hardened and can be fed to the further hot forming and/or
press hardening steps. However, surface hardening in a chamber
furnace is also conceivable.
[0019] As indicated above, semifinished parts such as
manganese-boron steels, dual-phase steels and TRIP steels firstly
display a particularly high strength increase during hot forming or
during hardening and secondly provide the opportunity of bringing
the regions close to the surface to identical hardness in the range
from 400 to 700 HV by nitriding. As a result, steel parts which are
very wear-resistant and have particularly high strengths can be
produced inexpensively.
[0020] In the following, the invention will be illustrated with the
aid of examples in conjunction with the drawing. In the
drawing,
[0021] FIG. 1 schematically shows an example of the process for
producing a wear-resistant, at least partly uncoated steel
part,
[0022] FIG. 2 shows the layer structure of the semifinished part or
steel part treated as per the example in FIG. 1 in a schematic
illustration,
[0023] FIGS. 3, 4 shows examples of a steel part for agricultural
machines and conveying machines and
[0024] FIG. 5 shows a graph of the hardness profile as a function
of the distance from the surface for two examples and a comparative
example.
[0025] FIG. 1 firstly shows, very schematically, an example of the
production of a wear-resistant, at least partly uncoated steel part
in a schematic illustration. The semifinished part 1, which
consists of a steel, for example a manganese-boron steel,
dual-phase steel or TRIP steel, is firstly fed to surface hardening
2. If a strip-like semifinished part is reeled off a coil 1a and
fed to surface hardening 2, it is, for example, advantageous to
carry out surface hardening, for example in the case of nitriding,
in a continuous hardening furnace at the end of which, for example,
the strip-like semifinished part 1, now provided with a hardened
surface, can be wound up on a coil (not shown). The
surface-hardened strip-like semifinished part is cut to length and
fed to hot forming and/or hardening 3, so that process step 3 can
produce a formed, at least partly uncoated steel part 4 which is
suitable for processing, conveying and/or crushing means of
agricultural machines, conveying machines, mining machines or
building machines. Firstly, the steel part 4 produced in this way
characterizes high strength values owing to the hot forming and/or
hardening step. Secondly, the surface region of the steel part also
has an increased hardening due to the nitriding of the surface
which has taken place before hot forming and/or before hardening.
As indicated above, the process of the invention enables the
decarburization of the surface regions, which takes place to a
depth of 100 .mu.m, to be countered by the surface region being
surface-hardened to a depth of 100 .mu.m or in a region down to a
depth of 40 .mu.m. The surface hardening is preferably carried out
by nitriding. However, carburization of the surface region is also
conceivable.
[0026] The surface hardening in process step 2 is preferably
carried out by means of a heat treatment in a heat treatment
atmosphere comprising up to 25% by volume of H.sub.2, 0.1-10% by
volume of NH.sub.3, H.sub.2O and balance N.sub.2 and also
unavoidable impurities at a holding temperature of from 600.degree.
C. to 900.degree. C. Reduction of the hydrogen concentration to a
maximum of 10% by volume or limiting of the NH.sub.3 concentration
to a maximum of 5% by volume also leads to a further improvement of
the nitriding result.
[0027] The depth of the surface hardening can be set via the
duration of the surface hardening, for example at a holding
temperature of from 5 s to 600 s. The surface is preferably
nitrided at a holding temperature of from 30 s to 120 s, with the
temperature being from 680.degree. C. to 840.degree. C. Carrying
out the surface hardening before hot forming or hardening has the
advantage that a heat treatment process can be carried out
significantly more efficiently using a, for example, strip-like
semifinished part in a continuous hardening furnace or a plate in a
continuous hardening furnace than when using formed steel parts
which have different shapes and different geometries. The quality
of the surface hardening can likewise be ensured more easily by the
use of strip-like semifinished parts or semifinished parts
configured as a blank.
[0028] FIG. 2 then schematically shows a cross section of the
semifinished part at three different points in time during the
process. At first, the semifinished part 1 has a more or less
homogeneous, for example ferritic microstructure 1a corresponding
to the production process, which is determined by the combination
of production process and steel composition. As a result of the
surface hardening, the surface region lb is hardened by inward
diffusion of nitrogen in the case of nitriding or carbon in the
case of carburization, with the microstructure changing there. The
thickness of the surface region 1b depends on the duration of the
heat treatment. The surface region is usually up to a maximum of
100 .mu.m in which the hardness of the semifinished part is
altered. A preferred region, which is a compromise between
sufficient surface hardening and duration of the heat treatment for
surface hardening, has a thickness of from 20 to 40 .mu.m. The
duration of surface hardening, for example in nitriding, is then
preferably from 30 s to 120 s. The microstructure of the material
1a remaining underneath the surface region 1b remains essentially
unchanged during the heat treatment.
[0029] In the hot forming step, the microstructure of the base
material 1a is then firstly converted into austenite and, by means
of hardening, later partially into martensite. In this way, high
hardness and good mechanical strengths are achieved in the base
material 1c. The surface region 1b remains unchanged except for
carburization of these layers. As a result of nitriding, the
surface region can continue to remain hardened. In the case of
targeted carburization of the surface region 1b instead of
nitriding, decarburization can be countered, so that an increase in
the hardness is also achievable here. The formed steel part 4 thus
has a hardened region 1b and also a region 1c which has been
hardened by the hot forming and hardening.
[0030] FIGS. 3 and 4 show typical fields of application for the
wear-resistant, at least partially uncoated steel part in the form
of a conveying screw 5 in FIG. 3 and a plowshare 6 for agricultural
plows in FIG. 4. Both components are typical representatives of
processing, conveying and/or crushing means which are used in
agricultural machines, conveying machines, mining machines or
building machines, for example concrete mixers, and are exposed to
highly abrasive materials. The use of hot formed and/or press
hardened steel parts has hitherto not been very advantageous
because of the increased susceptibility to wear. Due to the surface
hardening of the region which is decarburized during hot forming
and/or hardening, the hot forming steels gain an enlarged range of
uses.
TABLE-US-00001 TABLE 1 Measurement of HV 0.01 Sample A Sample B
depth .mu.m (1% NH.sub.3) (4% NH.sub.3) 5 460 546 10 404 490 15 436
447 20 333 415 25 409 394 30 479 453 35 453 479 40 436 485 45 492
466
[0031] Table 1 shows measurements of the hardness of samples A and
B which consist of a steel of grade 22MnB5. The samples A and B
were subjected to surface nitriding in a heat treatment atmosphere
comprising 1% by volume of NH.sub.3 or 4% by volume of NH.sub.3 at
760.degree. C. and 90 s in each case. The surface nitriding was
carried out at inter-critical temperatures (T>Ac1) since
austenite can dissolve more nitrogen than ferrite. The samples were
subsequently hot formed and hardened. Polished sections were made
from the hot formed or hardened steel parts and the hardness HV
0.01 (DIN EN ISO 6507-1) was measured at a distance of 5 .mu.m from
the surface. The microhardness measurement on the samples as a
function of the content of NH.sub.3 in the heat treatment
atmosphere had a greater hardness at a higher NH.sub.3 content of
the heat treatment atmosphere at the same heat treatment
parameters, i.e. hold time and hold temperature.
[0032] The hardness of sample A firstly decreases from the value of
460 HV measured at the surface to a value of 333 HV at a depth of
20 .mu.m. The hardness then increases again to a value of about 492
HV, which indicates that the decarburization of the base material
ceases here. The uppermost region, in particular, from 5 to 15
.mu.m was significantly hardened by the surface hardening. It can
be seen from sample B that the surface hardening is more
pronounced, both in terms of the amplitude and the depth of
hardening, at an increased NH.sub.3 content. This can be attributed
to greater diffusion of nitrogen into the surface of the steel part
taking place due to the higher NH.sub.3 concentration in the heat
treatment atmosphere. The values for sample B start at 546 at a
depth of 5 .mu.m and decrease to a value of 394 at a depth of 25
.mu.m. The values subsequently increase again to about 466 at a
depth of 45 .mu.m. It can clearly be seen that the surface is
harder than the base material at a depth of 45 .mu.m.
[0033] A similar picture is shown by the measurements on two
further examples shown in FIG. 5 compared to a comparative example.
The comparative example illustrated by a dotted line displays a
reduced hardness below 400 HV 1 (DIN EN ISO 6507-1) in the region
of 5 to 35 .mu.m. The reduction in the hardness compared to the
base material, which is in the range from 450 HV 1 to 500 HV 1, is
explained by decarburization during hot forming. The two
comparative examples with two different nitriding variants, once
again 1% strength NH.sub.3 heat treatment atmosphere or 4% strength
NH.sub.3 heat treatment atmosphere, differ especially in this
region close to the surface, since hardness of above 500 could be
measured here. In this way, it is possible, in the case of
wear-resistant, at least partly uncoated steel parts, to provide
not only the particularly high tensile strength values of the hot
formed and/or hardened steel parts but also a high wear resistance
due to greater surface hardness in the range from, for example, 500
to 700 HV.
* * * * *