Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ENGINEERING STEEL WITH A BAINITIC STRUCTURE, FORGED PARTS
PRODUCED THEREFROM AND METHOD FOR PRODUCING A FORGED PART
The invention relates to an engineering steel with high strength and a
structure comprising at
least 80 vol.-% bainite.
The invention also relates to a forged part, produced from such an engineering
steel.
Finally, the invention relates to a method for producing a forged part from an
engineering
steel according to the invention.
Where, in the following, details of alloys or other steel compositions are
provided in "%", in
each case these relate to weight, unless expressly indicated to the contrary.
All mechanical properties indicated in the present text of the steel according
to the invention
and any steels cited for comparison, unless indicated to the contrary, have
been determined
according to DIN EN ISO 6892-1.
As Dipl.-Ing. Christoph Keul et al. report in the article "Entwicklung eines
hochfesten duktilen
bainitischen (HDB) Stahls fUr hochbeanspruchte Schmiedebauteile" [Development
of a high-
strength, ductile, bainitic (HDB) steel for highly-stressed forged
components], appearing in
the Schmiede-Journal, issue of September 2010, published by lndustrieverband
Massivumformung e.V., particularly in the forging industry there is a demand
for steel
material concepts which offer the possibility of achieving high strength and
ductility and at the
same time short process chains for their production. The article also states
that in this regard
promise has been shown by materials with a bainitic structure, in which good
strength and
ductility properties are combined without the need for additional heat
treatment, characterised
by a tensile strength of more than 1,200 MPa, a yield strength of more than
850 MPa and an
elongation at rupture of more than 10 % with a notch impact energy of 27 J at
room
temperature. As an example of alloying concepts, offering such properties, the
article
presents a steel with (in wt.-%) 0.18% C, 1.53% Si, 1.47% Mn 0.007% S, 1.30%
Cr, 0.07
% Mo, 0.0020 % B, 0.027 % Nb, 0.026 % Ti, 0.0080 % N, retained iron and
unavoidable
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impurities and a steel with 0.22 % C, 1.47 % Si, 1.50 % Mn, 0.006 % S, 1.31 %
Cr, 0.09 %
Mo, 0.0025 % B, 0.035 % Nb, 0.026 % Ti, 0.0108 % N, retained iron and
unavoidable
impurities.
Another development which is similarly aimed at a steel for producing drop-
forged parts,
which without additional heat treatment possess high strength and at the same
time high
ductility, is described in EP 1 546 426 B1. The steel known from this patent
specification
contains (in wt.-%) 0.12 - 0.45 % C, 0.10- 1.00% Si, 0.50- 1.95% Mn, 0.005 -
0.060% S,
in each case 0.004 - 0.050 % Al and Ti, in each case up to 0.60 % Cr, Ni, Co,
W, Mo and
Cu, up to 0.01 % B, up to 0.050 % Nb, 0.10 - 0.40 % V, 0.015 - 0.04 % N and
retained iron
and unavoidable impurities, provided that the product of the V and N contents
of the steel is
0.0021 - 0.0120, that the S-content %S, the Al content %Al, the Nb content %Nb
and the Ti
content %Ti, meet the condition 1.6 x %S + 1.5 x %Al + 2.4 x %Nb + 1.2 x %Ti =
0.040 -
0.080 % and the Mn content %Mn, the Cr content %Cr, the Ni content %Ni, the Cu
content
%Cu and the Mo content %Mo meet the condition 1.2 x %Mn + 1.4 x %Cr + 1.0 x
%Ni + 1.1 x
%Cu + 1.8 x %Mo = 1.00 - 3.50 %.
It is considered essential here that the necessary improvement in ductility is
achieved by
reducing the carbon content in the steel. The essential loss of strength that
accompanies this
according to the prior art is balanced out by the usual alloying elements, the
contents of
which are coordinated such that strengthening through mixed crystal formation
results.
Furthermore, from DE 697 28 076 T2 (EP 0 787 812 B1) a process for producing a
forged
steel part is known, in which a steel with (in wt.-%) 0.1 - 0.4 % C, 1 -1.8%
Mn, 0.15- 1.7 %
Si, up to 1 % Ni, up to 1.2 %Cr, up to 0.3 % Mo, up to 0.3 % V, up to 0.35 %
Cu and in
each case optionally 0.005 - 0.06 % Al, 0.0005 - 0.01 % B, 0.005 - 0.03 % Ti,
0.005 % -
0.06 % Nb, 0.005 - 0.1 % S, up to 0.006 % calcium, up to 0.03 % Te, up to 0.05
% Se, up to
0.05 % Bi and retained iron and unavoidable impurities is cast to form a semi-
finished
product which is hot-forged in a conventional manner to produce a forged part.
The forged
part then undergoes heat treatment, comprising cooling at a rate of cooling Vr
of more than
0.5 C/s from a temperature at which the steel is austenitic, to a temperature
Tm of between
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Ms +100 C and Ms -20 C. The forged part is then maintained for at least two
minutes at a
temperature between the temperature Tm and a temperature Tf, for which Tf > Tm
-100 C
applies. In this way the intention is to obtain a steel component with a
substantially bainitic
structure, comprising at least 15 % lower bainite and preferably at least 20 %
bainite formed
between Tm and Tf.
Practical trials with steel materials of the kind described above have shown
that such bainitic
steels are unsuitable for components with major changes in cross section due
to their
tendency to warp and highly fluctuating mechanical characteristics.
Against this background, the problem for the invention was to provide a steel
having a high
strength, without the need to perform complex heat treatment processes, with a
low tendency
to warping and which as such is particularly suitable for the production by
forging techniques
of forged parts with major changes in cross section over their length.
The intention is similarly to indicate a forged part which has an optimum
combination of
properties without complex heat treatment processes.
Finally, the intention is to propose a method for producing a forged part
allowing, with simple
means, the creation of forged parts with an optimised combination of
properties.
With regard to the steel, the invention has solved the abovementioned problem
with the
engineering steel.
Wirth regard to the forged part, the solution according to the invention to
the abovementioned
problem consists of producing such a steel component from a steel according to
the invention.
Lastly, with regard to the method, the invention has solved the abovementioned
problem in
that in the production of a forged part the process steps described herein may
be carried out.
Advantageous forms of the invention will be explained in detail in the
following together with
the general inventive idea.
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. .
An engineering steel according to the invention has a yield strength of at
least 750 MPa and
a tensile strength of at least 950 MPa and an at least 80 vol.-% bainitic
structure, wherein the
remaining 20 vol.-% of the structure can be retained austenite, ferrite,
perlite or martensite.
Here, the steel according to the invention is characterised by a high
elongation at rupture A of
at least 10 %, in particular of at least 12 %, wherein it has been shown in
practice that steels
according to the invention routinely achieve an elongation at rupture A of at
least 15 %.
According to the invention, the engineering steel therefore comprises (in wt.-
%) up to 0.25 %
C, up to 1.5 % Si, in particular up to 1 % Si or up to 0.45 % Si, 0.20 ¨ 2.00
% Mn, up to 4.00
% Cr, 0.7 ¨ 3.0 % Mo, 0.004 - 0.020 % N, up to 0.40 % S, 0.001 - 0.035 % Al,
0.0005 -
0.0025% B, up to 0.015% Nb, up to 0.01 % Ti, up to 0.50% V, up to 1,5% Ni, .up
to 2,0%
Cu and retained iron and unavoidable impurities, wherein the Al content %Al,
the Nb content
%Nb, the Ti content %Ti, the V content %V and the N content %N of the
engineering steel in
each case meet the following condition:
%AI/27 + %Nb/45 + %Ti/48 + %V/25 > %N/3.75
The unavoidable impurities resulting from production include all elements
which with regard
to their properties of interest here are present in quantities that have no
effect on the alloying
process and due to the steel-making route or the respective starting material
selected (scrap)
find their way into the steel. The unavoidable impurities also include, in
particular, contents of
P of up to 0.0035 wt.- /0.
A steel according to the invention and the forged parts produced from this can
be
characterised by a particularly uniform distribution of properties even if,
due to variable
component dimensions, during cooling from the forging heat, considered across
the forged
part volume, localised highly different cooling conditions prevail. This
insensitivity to the
cooling conditions is achieved in that the engineering steel according to the
invention has a
homogenous, as far as possible exclusively bainitic structure with low
variation in hardness.
This homogenous microstructure at the same time has low internal stresses,
which has a
positive influence on the warping behaviour.
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Consequently, steel according to the invention is particularly suited to the
production of
forged components, in which sections with highly differing volumes and
diameters come up
against one another. Examples of such forged parts, for the manufacture of
which using
forging techniques the steel according to the invention is particularly
suited, are crankshafts,
piston rods and similar, intended in particular, for combustion engines.
Furthermore, parts in the area of the chassis and the wheel suspension with
highly different
cross sections can be reliably produced from the steel according to the
invention without
major post-processing through grinding while maintaining the predetermined
strength
characteristics.
As will be understood from the time-temperature diagram attached as Fig. 1 of
a steel
according to the invention, from a material technology point of view, this
means that with an
engineering steel according to the invention a particularly broad window can
be used for the
bainitic treatment, if the engineering steel according to the invention is
continuously cooled
from the forging heat. In so doing, the alloying of the engineering steel
according to the
invention is selected such that in the course of the cooling none of the
quantities of
martensite or ferrite and/or perlite influencing its properties result in the
structure.
Engineering steel according to the invention is thus characterised in that it
has a
predominantly, that is to say up to at least 80 vol.-%, bainitic structure,
wherein the content of
non-bainitic structural components in steels according to the invention is
typically minimised
to such an extent that the steel according to the invention has a completely
bainitic structure
in the technical sense.
Here, with the engineering steel according to the invention an almost constant
hardness
occurs in the bainite as far as possible independently of the speed of
cooling. The constant
hardness is a consequence of the almost complete transformation of what was
previously
austenite into bainite, preferably in a bainitic transformation stage.
Limiting the content of C to a maximum of 0.25 wt.-% means on the one hand
that an
engineering steel according to the invention despite its maximised strength
has good
elongation and ductility properties. In a steel according to the invention,
the low C content
also contributes to accelerating the bainite transformation so that the
development of
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undesired structural components is avoided.
At the same time, however, a certain quantity of carbon in the engineering
steel according to
the invention can also contribute to the strength. To this end, contents of at
least 0.09 wt.-%
C in the steel can be envisaged. An optimised effect of the presence of C in
the steel
according to the invention can thus be achieved in that the C content is
adjusted to 0.09 -
0.25 wt.-%.
The Si content of a steel according to the invention is limited to 1.5 wt.-%,
in particular 1 wt.-
% or 0.75 wt.-%, to allow the bainite transformation to take place as early as
possible. To be
particularly sure of achieving this effect, the Si content can also be limited
to a maximum of
0.45 wt.-%.
Mo is present in the engineering steel according to the invention in contents
of 0.6 ¨ 3.0 wt.-
%, to delay the transformation of the structure into ferrite or perlite. This
effect occurs in
particular if at least 0.7 wt.-%, in particular more than 0.70 wt.-% Mo, is
present in the steel.
For contents of more than 3.0 wt.-% no further economically viable increase in
the positive
effect of Mo occurs in the steel according to the invention. Apart from this,
above 3.0 wt.-%
Mo there is a danger of formation of a molybdenum-rich carbide phase, which
can negatively
influence ductility properties. Optimum effects of Mo in the steel according
to the invention
can be expected if the Mo content is at least 0.7 wt.-%. Here, Mo contents of
a maximum of
2.0 wt.% have proven particularly effective.
Manganese is present in contents of 0.20 ¨ 2.00 wt.-% in the steel according
to the invention,
in order to adjust the tensile strength and yield strength. A minimum content
of 0.20 wt.-% Mn
is necessary to achieve an increase in strength. If it is intended to achieve
this effect with
particular reliability, then an Mn content of at least 0.35 wt.-% may be
provided for.
Excessively high Mn contents lead to delays in bainite transformation and thus
to a
predominantly martensitic transformation. The Mn content is therefore limited
to a maximum
of 2.00 wt.-%, in particular 1.5 wt.-%. Negative influences from the presence
of Mn can be
particularly reliably avoided by limiting the Mn content in the steel
according to the invention
to a maximum of 1.1 wt.-%.
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. .
The sulphur content of a steel according to the invention can be up to 0.4 wt.-
%, in particular
max. 0.1 wt.-% or max. 0.05 wt.-%, to support the machinability of the steel.
Fine adjustment of the alloying techniques with regard to the mechanical
properties and the
microstructure of an engineering steel according to the invention takes place
according to the
alloying concept according to the invention through combined micro-alloying of
the elements
of boron in contents of 0.0005 - 0.0025 wt.-%, nitrogen in contents of 0.004 -
0.020 wt.-%, in
particular at least 0.006 wt.-% N or up to 0.0150 wt.-% N, aluminium in
contents of 0.001 -
0.035 wt.-% and Niob in contents of up to 0.015 wt.-%, titanium in contents of
up to 0.01 wt.-
% and vanadium in contents of up to 0.10 wt.-%.
Here, the contents of %Al, %Nb, %Ti, %V and %N and Al, Nb, Ti, V and N are
linked
together by the condition
%AI/27 + %Nb/45 + %Ti/48 + %V/25> %N/3,75
such that the nitrogen contained in the engineering steel through the
respective contents
present of Al and the necessary contents of Nb, Ti and V also added, is
completely bound
and boron can thus have a transformation-delaying effect. At the same time,
the contents of
microelements provided according to the invention and balanced with one
another and the N
content contribute to an increase in the fine grain stability and strength.
The binding according to the invention of N also allows the boron to be
effective as a
dissolved element in the matrix and suppresses the formation of ferrite and/or
perlite.
To allow advantage to be taken reliably of the microalloying elements and of
aluminium, it
may be expedient to set the Al content to at least 0.004 wt.-%, the Ti content
to at least 0.001
wt.-%, the V content to at least 0.02 wt.-% or the Nb content to at least
0.003 wt.-%. Here,
the microalloying elements V, Ti and Nb, on the one hand, and Al, on the
other, can in each
case be present in combination with one or more elements from the group "Al,
V, Ti, Nb" or
alone in quantities above the stated minimum contents.
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. .
For contents of up to 0.008 wt.-% Ti, of up to 0.01 wt.-% Nb, of up to 0.075
wt.-% V or of up
to 0.020 wt.-% Al, the actions of these elements in the construction steel
according to the
invention can be used to particularly good effect. At the same time, the
nitrides or
carbonitrides formed lead to an increase in the strength and contribute to the
fine grain
stability. Here also, the stated upper limits of the contents of Ti, Nb, V or
Al in each case
alone or in combination with one another can be adhered to in order to achieve
the optimum
effect of the alloying element concerned.
Optionally present contents of Cr of up to 4.00 wt.-%, in particular up to 3
wt.-% or up to 2.5
wt.-%, contribute to the durability and the corrosion-resistance of the steel
according to the
invention. To this end, by way of example, at least 0.5 wt.-% or at least 0.8
wt.-% Cr can be
provided.
Similarly optionally present contents of Ni of up to 1.5 wt.-% can likewise
contribute to the
hardenability of the steel.
The alloying elements reaching the steel according to the invention via the
starting material
or intentionally added alloying elements also include Cu, the content of
which, in order to
avoid negative influences in the steel according to the invention is limited
to a maximum of
2.0 wt.-%. A positive effect of the optional presence of copper in the
alloying of an
engineering steel according to the invention consists of the formation of the
finest retained
austenite films and the associated significant raising of the level of
ductility. This effect can be
achieved in that at least 0.3 wt.-% Cu, in particular more than 0.3 wt.-% Cu,
is present in the
engineering steel according to the invention. By limiting the Cu content to a
maximum of 0.9
wt.-%, an optimised positive effect of the copper content can be achieved.
If a steel according to the invention is heated to heat temperatures typical
for hot working of
at least 100 C above the respective Ac3 temperature, in particular a heat
temperature of
more than 900 C for the hot working, then hot worked and finally cooled in a
regulated or
unregulated fashion under stationary or moving air to a temperature of less
than 200 C, in
particular to room temperature, then over an extremely broad range of cooling
speeds
following transformation a uniform bainitic structure results. The Ac3
temperature of the steel
can be determined in a known manner on the basis of its respective
composition. The upper
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. .
limit to the range of the heating temperature is typically 1,300 C, in
particular 1,250 C or
1,200 C.
As a dimension for the range of cooling speeds, the t8/5 time can be used
here, thus the time
taken by the respective hot-worked part to cool from 800 C to 500 C. For the
cooling of
components produced from the steel according to the invention, this t8/5 time
is intended to
be 10 ¨ 1,000 s.
The cooling time selected in each specific case should be selected based on
the respective
heating temperature. The influence of the heating temperature can be
understood from the
time-temperature diagram attached as Fig. 2, in which for the heating
temperatures 900 C
(unbroken line), 1,100 C (dashed line) and 1,300 C (dotted line) the
respective position of
the respective bainite range is shown across the cooling time. Accordingly, at
low heating
temperatures of 900 C shorter t8/5 times should be selected, to achieve the
desired bainitic
structure, whereas at higher heating temperatures the cooling can be slower. A
high certainty
that during cooling of steel according to the invention the bainite range will
be reached
independently of the respective heating temperature exists for steels
according to the
invention at heating temperatures in the range of 900 ¨ 1,300 C and
accordingly if the t8/5
time is 100 - 800 s.
The alloying concept according to the invention therefore allows high hot-
working
temperatures of more than 1,150 C, as a result of which the forming forces
during the hot-
working can be reduced without an undesired grain growth occurring.
The method according to the invention for producing forged parts with a yield
strength of at
least 750 MPa and a tensile strength of at least 950 MPa and an at least 80
vol.-% bainitic
structure, which can contain in total up to 20 vol.-% of retained austenite,
ferrite, perlite or
martensite, comprises the following process steps:
a) providing a semi-finished product for forging, comprising an engineering
steel with a
composition according to the invention as explained above;
b) heating the semi-finished product for forging to a forging temperature
of at least
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100 C above the Ac3 temperature of the respective engineering steel, wherein
the Ac3
temperature is determined in a conventional manner as a function of the
respective
composition of the engineering steel;
C)
forging the semi-finished product for forging heated to the forging
temperature into the
forged part;
cooling the forged part from the forging heat to a temperature of below 500 C,
wherein the t8/5-time for cooling is 10 -1,000 s
To reduce the forming forces, in the course of the method according to the
invention with
regard to minimisation of the necessary forging forces, it may prove
advantageous if the
respective semi-finished product representing the starting point of the
forging is heated to a
forging temperature of more than 1,150 C.
A further adjustment of the mechanical properties, in particular the strength
and ductility of
the components hot-worked, in particular forged, from steel according to the
invention, can
take place by means of tempering treatment, during which the respective part
is maintained
for a duration of 0.5 - 2 h in the temperature range 180 - 375 C.
In practice, with the steel according to the invention, tensile strengths of
at least 950 MPa, a
yield strength of at least 750 MPa, and an elongation at rupture A of at least
15 %, wherein it
has been shown in practice that even higher elongation values A of at least 17
%, can be
reliably achieved. This combination of features in forged parts comprising
steel according to
the invention is present in particular if they are created in the manner
according to the
invention.
In the following the invention is explained in more detail using exemplary
embodiments.
Steel melts El - E6 according to the invention and a comparison melt V1 with
the
compositions shown in Table 1 were smelted and cast into semi-finished
products, which
involved blocks, as normally made available for further processing using
forging techniques.
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The semi-finished products are heated for forging deformation to a heat
temperature Tw,
then in a conventional manner hot worked using drop forging to produce forged
parts and
then cooled to room temperature in the air. With some of the forged parts
obtained, a
tempering treatment is then performed.
Table 2 shows the heating temperatures Tw applied in the examples, the t8/5
time necessary
in each case for passing through the critical temperature range of 800 - 500
C, the
temperature and duration of the tempering treatment, where this was actually
carried out, and
the proportion of bainite in the structure, the tensile strength Rm, the yield
strength Re, the
extension A and the notch impact energy W of the forged part obtained after
forging.
The examples show that when the specifications according to the invention are
met, forged
parts can be produced which allow the operating parameters set during their
creation to be
varied over a wide range and in so doing to obtain hot-worked components with
optimised
mechanical properties.
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0
0
0
01
,1
co
0
IA
IA
IA
IA
Steel C Si Mn Cr Mo N S Al B Nb Ti V Ni Cu p
(1) (2) (1)>(2)
El
0.13 0.4 0.55 2.37 1,04 -0.0069 0.003 0.015 0.0012
0.003 0.002 , 0.03 0.24 0.19 0.019 0.006864 0.00184 YES
E2
0.17 0.25 0.72 2.05 0.71 0.0100 0.005 0.020 0.0012
0.021 0.001 0.10 0.24 0.23 0.021 0.005228 0.002667 YES
E3
0.17 0.24 0.90 1.72 0.74 0.0082 0.003 0.031 0.0008
0.007 0.001 0.03 0.22 0.62 0.017 0.002525 0.002187 YES
E4 0.23 0.27 0.43 1.23 0.77 0.0076 0.034 0.017 0.0013 0.003 0.001 0.04 0.17
0.21 0.017 0.0023170.002027 YES
E5
0.16 0.73-1.49 0.94 0.78 0.0077 0.004 0.027 -0.0013
0.003 0.001 0.06 0.21 0.17 0.016 0.003488 0.002050 YES
E6
0.19 0.67 Ø89 1.47 0.79 0.0092 0.005 0.035 0.0012
0.003 0.001 0.03 0.22 0.13 0.020 0.0025840.002453 YES
Ni -V1 0.24 0.10 1.50 2.00 0.03 0.0100 0.002 0.023 =
0.020 0.015 0.02 0.40 0.50 0.018 0.002409 0.002667 NO
Data in wt.-%, retained iron and unavoidable impurities
(1): %AI/27 + %Nb/45 + %Ti/48 + %V/25
(2): %N/3,75
Table 1
0
0
0
01
,1
0
IA
Tw t8/5 Tempering Proportion of Rm
Re A
Steel treatment bainite in
According to the
structure
invention?
[ C] [s] [ C], [h] [vol.-%] [MPa]
[MPa] [%]
El 1050 320 None >97% 965 763 22
YES
E2 1080 580 None >97% 1225 972 17
YES
E3 1080 640 None >97% 1174 840 25
YES
E4 1150 500 300 C, 1.5h >97% 1192
1034 24 YES
E5 950 100 None >97% 1353 1112
24 YES
E6 950 200 None >97% 1367 1167
22 YES
V1 1075 500 None 75%(Remainder1352 897 8
NO
MS)
Table 2
