|Title of Invention||
STEEL ALLOY FOR CUTTING DETAILS
|Abstract||The invention relates to a steel alloy of the following composition (in % by weight) C 0,40-0,60 Si 0,1-1,0 Mn 0,3-1,0 Cr 12-15 Mo 2,5-4,0 Ni 0-1,0 Co 0-4,0 N 0,15-0,20, with the balance Fe as well as normally occurring impurities, the hardness being > 56 HRC, which has been attained without deep freezing, as well as PRE > 25, defined as PRE = % Cr + 3,3 • % Mo + 16 • % N. Furthermore, the steel alloy comprises carbides, nitrides and/or carbonitrides the maximal diameter of which does not exceed 5 µm. This steel alloy has turned out surprisingly well suitable as edge material for a plurality of cutting operations.|
|Full Text||The present disclosure relates to a material for cutting details with high demands on,
among other things, corrosion resistance and hardness. Details of the material should be
possible to be made by photoetching, and in order to meet these demands, a very particular
combination of properties is required according to the discussion below.
When first studying what is required for a material to be suitable for cutting tools, the
hardness is ot great importance. A harder material resists plastic deformation better, which
is a common degradation mechanism for cutting edges - that they simply bend and/or are
deflected when stressed. Furthermore, a harder material will resist wear better and thereby
an edge will remain sharp longer, or in other words, have better edge durability. An addi-
tional advantage of a harder material is that the normally seen decreasing toughness gives
an improved burr breaking in mechanical grinding and polishing, whereby a sharper edge
may be obtained. An absolute minimum in hardness of a material intended for edges with
demands on edge durability and possibility of mechanical sharpening is judged to be 56
HRC (hardness on the Rockwell C-scale, which corresponds to approx. 615 HV 1 kg
measured as hardness in Vickers with the load of 1 kg).
A factor that further drastically affects the edge durability of a material is the presence of
hard particles (carbides, nitrides, and carbonitrides which henceforth are denominated
jointly as carbonitrides) in the material. An increasing volume fraction of carbonitrides
gives a material having better edge durability. However, there are limitations that have to
be taken into consideration - the possibility again of making a truly sharp edge by
mechanical machining or photoetching. In mechanical machining of edges having small
edge angles ( inclusions), of a diameter larger than 10 µm, cause tear outs and edge damage, the initial
sharpness of the edge drastically being deteriorated. For the manufacture of edges by
etching, the demands are even greater. In photoetching, which is suitable for the manu-
facture of complex details in thin materials, parts of the material surface are protected by a
protection film. On the areas that are unprotected, the etching medium (for instance a
mixture of HCl and FeCl3), which is sprayed on the surface, is allowed to make a chemical
processing. By virtue of the different electrochemical properties of the bulk mass and the
carbonitrides, the etching will be accelerated in the border between bulk mass and
earbonitride. This entails that the carbonitrides risk being etched out of the material. In
order for this phenomenon not to affect the finished product negatively, carbonitrides of a
diameter larger than 5 µm must not be present in the material. A usual cause of large
carbonitrides is alloying additives of very strong carbide formers, such as, for instance,
vanadium, and therefore this type of alloying elements should preferably be avoided.
Another cause of large carbonitrides is poor process control when casting and hot working
the materials. Large (0 > 10 µm) carbonitrides, above all angular primary carbides formed
in the casting, also limits the possibility of polishing the material shiny.
Upon corrosion attacks on martensitic stainless chromium steel, they are most often of the
type pitting corrosion. The three most important alloying elements to control this corrosion
type are chromium, molybdenum and nitrogen. A frequently used measurement of the
resistance against pitting corrosion is the PRE-value (Pitting Resistance Equivalent),
PRE =% Cr + 3,3● % Mo + 16 ● % N. Experience shows that the PRE-value according to
the expression above should be above 25 for martensitic chromium steels in order to attain
a sufficient corrosion resistance in an environment of chloride ions.
An additional demand on the material according to the present invention is that it, in a
cost-effective and quality-assured way, should be possible to harden by a continuous
process (strip widths up to 1000 mm and strip thicknesses down to 15 µm) including
furnace for the austenitizing, quenching for the conversion to martensite and finally a fur-
nace for tempering. In the austenitizing, the carbonitrides in the material are dissolved to a
certain extent and the contents of alloying elements increase in the matrix. In order for this
dissolution to occur evenly (enables good dimensional tolerances) and within a short time
(high productivity), it is required that the carbonitrides are small in size (Ø furthermore that the size distribution is even, which is controlled by an accurately con-
trolled production process. The production process for the material includes melting of
raw materials in an electric arc furnace alternatively a high frequency furnace. The content
of carbon in the material can be controlled by choice of raw materials or by carbon
elimination either in AOD (Argon Oxygen Decarburization), CLU (Creusot Loire
Uddeholm) or another refining process. As an alternative, the material may be remelted in
a secondary metallurgical process such as VIM (Vacuum Induction Melting), VAR
(Vacuum Arc Remelting), ESR (Electroslag Remelting), or the like. Casting may take
place in the traditional way into ingot or by continuous casting. A first strong reduction is
made in the warm state, and then the material is spheroidized. Next, cold rolling is carried
out in a plurality of steps including intermediate annealing operations. The material may
be delivered to customer either in cold-rolled, annealed, or hardened and tempered form.
The stainless martensitic chromium steel according to the discussion above has advantages
to austenitic materials for the manufacture of details by photochemical processing. These
advantages are, among other things, that the material after hardening has a very good
flatness and is almost strain free. The material also allows a good productivity for this type
In order to be able to meet the demands above and simultaneously produce a finished
material in strip form in a cost-effective way, a very accurate optimization is required of
above all alloying elements, but also process parameters. For the production cost to end up
on a reasonable level, it is required that the material can be produced by a normal (non-
pressurized) metallurgical process. This gives a practical limitation in the content of nitro-
gen of maximum 0,20 % by weight, in a well-controlled process. Therefore, the content of
nitrogen should be between 0,15-0,20 % by weight. The hardness of the material in the
hardened form is substantially determined by the content (carbon + nitrogen) in % by
weight, and in order to be able to attain a hardness of over 56 HRC without deep freezing,
with sufficient remaining volume fraction of carbonitrides for the edge durability, this sum
has to be greater than 0,55 % by weight, provided that high contents of carbonitride
formers such as chromium and molybdenum are pres??ent. This entails that the content of
carbon generally has to be above 0,40 % by weight and that the ratio of carbon to nitrogen
becomes greater than 2. With this relatively high content of carbon, the carbon activity has
to be limited for avoiding formation of primary carbides in the solidification, which is
provided by keeping the content of silicon low, i.e. within the interval of 0,1-1,0%,
preferably 0,1-0,80 %, most preferably 0,15-0,55 %, by weight. In the hardening, the
material is austenitized at 950-1150 °C, preferably 1000-1070 °C, and then quenched
(suitably in oil, between cooling clamps or by means of compressed air) to room
temperature. A tempering is made at about 200 °C in order to achieve a hardness > 56
HRC. With deep freezing to -80 °C before tempering, an additional hardness enhancement
of about 2 HRC can be attained.
Chromium has to be added to the material in a sufficient quantity in order to form a
corrosion-protecting oxide film on the material surface, but at high contents of chromium,
again the risk of the formation of large primary carbides arises, which has to be avoided.
Therefore, the content of chromium should be 12-15 %, preferably 13-15 %, most
preferably 14-15 %, by weight. Molybdenum is then added in sufficient quantity to give a
PRE > 25. A suitable content of Mo is 2,5-4,0 %, preferably 2,6-4,0 %, most preferably
2,6-3,0 %, by weight/At high additions of molybdenum and nitrogen, it is risked that the
hot working properties of the material are deteriorated, and in order to limit this risk, other
elements having similar effect should be kept on a minimal level - the content of copper
has to, for instance, be kept below 0,1 % by weight. Nickel and cobalt are expensive
alloying materials, which are stable in a normal metallurgical process, which means that
the contents are accumulated over time in steel making based on recycled steel. For
stainless steels, there are limitations as regards the content of nickel of max 1 % in order
for the material not to be classified as potential carcinogenic and allergenic according to
the Euro directive 99/45/EC, and therefore this content has been set as a maximum content
regarding nickel for the alloy according to the patent. Preferably, nickel is not added
actively in the material and the content of nickel is determined to max. 0,7 % in order to
avoid the austenite stabilization that otherwise would be the consequence. The alloy also
contains 0,1-1,0 %, preferably 0,4-0,8 %, most preferably 0,4-0,7 %, by weight, of Mn
which is another element that stabilizes the austenite. The maximum content of cobalt has
been set to 4 %, on one hand because of the expense and on the other hand to avoid too a
fast accumulation of cobalt in the processing of recycled steel depending on the element
normally being seen as an impurity in stainless steel, above all within the nuclear power
industry. Preferably, cobalt is not added actively in the material and the content of cobalt
is set to max 0,5 %, in spite of the increasing impact of the element on the martensite
formation temperature. Hence, an addition of cobalt may displace the phase transformation
upon cooling after hardening toward more martensite.
In consideration of today's standard materials, it is observed that it is very few that meet
the demand PRE > 25 in combination with HRC > 56. Adding the demand on carboni-
trides Ø 440 C meet, for instance, only the demand on hardness. In order to meet the demands on
PRE-value and carbonitrides according to the above, there are only austenitic and duplex
materials available, but in these, the hardness and the edge durability is, however,
Upon an examination of other patent specifications within this area, in particular the
following four have been noticed. DE-A-39 01 470 discloses a material suited for, among
other things, razor blades and knives. However, the patent teaches a pressurized
metallurgy in order to achieve contents of nitrogen above 0,20 % by weight, and thereby
maximally twice as high content of carbon as of nitrogen. Furthermore, two experimental
alloys are mentioned, both with hardness below 600 HV. The patent also teaches additions
of vanadium in low contents. Therefore, the material will not meet the above-mentioned
demand on hardness and avoidance of the alloy element vanadium, and furthermore, the
production cost will become very high. According to EP-A-638 658, vanadium is used in
order to achieve a strong secondary hardening upon tempering to high temperatures, which
may be an advantage, for instance if the material is to be coated or used at high
temperatures. However, this is objectionable if the material is to be etched into final form
or be used to produce very sharp edges, according to the above. The patent states 40 µm as
the largest allowable size of carbonitrides unlike the 5 µm stated as the maximum limit
according to the present invention. EP-A-750 687 states the maximum content (carbon +
nitrogen) to 0,55 % by weight, which according to the pres??ent invention is judged to be
a minimal content in order to achieve sufficient hardness. This is confirmed by the fact
that the aim what relates to hardness in the EP publication is HRC > 50, and that the
experimental alloy that achieves the highest hardness reaches 56,3 HRC (this is after
tempering for 1 h at only 180 °C). This limited hardness in combination with a small share
of remaining carbonitrides will cause inadequate edge durability for edge applications with
high demands. Foremost, the patent specification also focuses articles with extremely high
demands on corrosion resistance, why also copper has been added, and therefore the
hardness and the hot workability has been neglected. As regards the patent specification
US-A-6 235 237, which above all relates to steel edges for skis with demands on damping,
the combination of high content of chromium, low content of molybdenum and low
content of nitrogen gives a hardness below 50 HRC according to the examples in the
patent specification, and thereby inadequate edge durability for edge applications with
Thus, a first object of the present invention is to provide a new steel alloy, which
overcomes all the above-mentioned drawbacks of prior art.
In particular, the object of the present invention is to provide a steel alloy that has a
hardness of at least 56 HRC, has excellent corrosion resistance and can be machined by
means of photoetching.
In a surprising way for a person skilled in the art, these and additional objects have
succeeded to be attained by providing a steel alloy according to the following composition
(in % by weight):
the hardness > 56 HRC being attained without deep freezing, as well as PRE > 25, defined
as PRE = % Cr + 3,3 • % Mo + 16 • % N. The balance to achieve 100 % consists of iron
and normally occurring impurities due to the raw material and/or the manufacturing
process. Preferably, it is considered that the maximum size of carbides, nitrides and
carbonitrides is Ø able to dissolve the carbides, nitrides and carbonitrides during austenitisation.
Preferably, the steel alloy according to the present invention has the following
composition (in % by weight):
as well as the balance Fe and normally occurring impurities.
Even more preferred, the steel alloy according to the present invention has the following
composition (in % by weight):
as well as the balance Fe and normally occurring impurities.
Materials manufactured according to the present disclosure are especially suitable for the
use in applications such as, for instance, knives in the food industry having high demands
on hardness and edge durability in combination with corrosion resistance due to chloride
ion-containing environment as well as corrosive dishwashing detergents. Other areas are
cutting edges for dry and wet shaving, surgical edge applications as well as diving knives.
Additional fields of application for the new material are, for instance, doctor blades in the
printing industry as well as doctor blades (also known as coater blades) and creping blades
in the pulp industry.
Choice of way of manufacture of the material depends, among other things, on desired
material volume, maximum allowed production cost and demands on slag purity. Cus-
tomer requirements such as hardened and tempered, and cold-rolled, respectively, finish,
naturally has also an effect. However, the manufacture will always comprise a metallurgi-
cal process at normal atmospheric pressure (1 atm = 1 bar). The metallurgical process
comprises melting in an electric arc furnace or a high frequency furnace. The content of
carbon is adjusted either by the choice of alloying materials or by carbon elimination in
AOD or CLU or another refining process. The content of nitrogen is adjusted either by the
supply in the form of gas or by the use of nitrogenous alloying materials. Alternatively, the
material may be remelted in a secondary metallurgical process such as VIM, VAR, ESR or
the like. Casting may be effected into ingot or via continuous casting, and then hot
working follows down to strip form. After the hot working, the material is spheroidized
and then cold-rolled in a plurality of steps into desired thickness including intermediate
recrystallization annealing operations. Upon customer want of a hardened and tempered
delivery finish, this hardening takes place in a continuous strip process in the form of an
austenitizing in protective atmosphere, a quenching (for the phase transformation into
martensite), and finally a tempering to desired hardness. The material is then cut into
desired widths or is cut into planar lengths depending on the customer want. The final
product may be produced by any conventional process; for example, from hardened strip
material by photoetching and forming, or from cold-rolled strip material by
punching/cutting, forming, hardening, tempering and finally grinding. It is also
conceivable to sell the material in the wire, tube or ingot form.
Brief description of the figures
Figure 1 illustrates a general outline between three comparative examples with
regard to hardness/edge durability and corrosion resistance.
Figure 2 illustrates the result of a CPP test of Alloy 1 and two comparative
Figure 3 illustrates the hardness as a function of the tempering temperature for
Alloy 1 and three comparative examples.
Figure 4 illustrates the hardness and CPP corrosion resistance for Alloy 1 and two
Figure 5 shows a microphotograph of Alloy 1 according to the present disclosure
illustrating the microstructure of the composition.
Figure 6 shows a microphotograph of a comparative example illustrating the
microstructure of the composition.
Figure 7 illustrates a comparison between Alloy 1 and two comparative examples
with regard to the hardness levels and structures.
One melt of material of the present disclosure, Alloy 1, has been produced in ten
ton scale with CLU-metallurgy. The material has been ingot casted, hot rolled and
thereafter cold rolled with intermediate annealings down to suitable thickness for
evaluation. The melt of the present invention has the composition as indicated in Table 1,
Alloy 1. The material according to the present disclosure is compared with three grades:
Comparative examples 1-3. The nominal composition of the comparative examples 1-3 is
also given in Table 1.
A general outline between the comparative examples is illustrated in Figure 1,
showing the hardness versus corrosion resistance as well as the influence of the alloying
elements C, N, Cr and Mo.
For evaluation of corrosion resistance and cutting ability for the material according
to the present disclosure six knife blades were produced and evaluated according to ISO
8442.1 and ISO 8442.5. Three blades (A-C) were produced in material from Alloy 1,
hardened at 1055°C, deep frozen, and tempered at 275 °C to reach hardness between
58 - 58,5 HRC. Three blades (D-F) were also produced in the composition according to
Comparative example 1 hardened at 1080°C, deep frozen, and tempered at 230°C to reach
hardness between 58 - 58,5 HRC. All the blades were ground and finished in the same
equipment to produce comparable edges and finishes. The result from the test is shown in
the Table 2.
The corrosion test according to ISO 8442.1 shows that the material according to
the present disclosure passes the test whereas the Comparative example 1 fails in the test.
The results from the edge testing according to ISO 8442.5 are on the very same level for
the new grade and for Comparative example 1.
The corrosion properties of the material of the present disclosure were also
measured by anodic polarization/critical pitting potential (CPP) and compared with
Comparative example 1 and Comparative example 2. Samples were taken from Alloy 1
and from Comparative example 1 and Comparative example 2 respectively, all
compositions given in Table 1. The sample of Alloy 1 was hardened at 1035°C, the
samples of Comparative example 1 were hardened at 1080°C, and the samples of
Comparative example 2 were hardened at 1030°C, according to recommendation for each
alloy. The tempering for all grades was performed at 225°C. All surfaces of the samples
were finished with 600 grit wet grinding. The test solution was 0,1% NaCl, the test was
performed at 20°C, and the potential over the sample was increased with 75 mV/minute
with a start at -600 mV. Nitrogen gas was bubbled through the solution to reduce the
oxygen level. The criteria used for start of pitting was set to I > 10 µA/cm2. The result
from the test is shown in Figure 2.
Hardening tests were performed on material of Alloy 1 and compared with typical
data for Comparative example 1, Comparative example 2 and Comparative example 3.
The hardening of Alloy 1 was done at 1035°C and quenching to 20°C, but also at 1055°C
in combination with deep freezing at -70°C. The hardness is shown in Figure 3 as a
function of tempering temperature after tempering for 30 minutes.
It can be seen that for all normal tempering temperatures in the range 175-450°C
Alloy 1 has a higher hardness than Comparative example 1 and Comparative example 2.
For tempering temperatures above 225°C the hardness for Alloy 1 is also higher than for
Comparative example 3 because of an improved tempering resistance of Alloy 1 caused by
the high alloy contents of molybdenum and nitrogen, which makes it less sensitive to high
temperatures. This bonus of an improved tempering resistance of Alloy 1 can be highly
beneficial for example in products where surface coating by PVD or with PTFE is
In Figure 4, Alloy 1 is compared with Comparative example 1 and Comparative
example 2 with regard of corrosion resistance and hardness. All samples were tempered at
225°C and heat treated as described above. It is desired that the composition possesses a
high corrosion resistance as well as a high hardness. This is illustrated by an arrow in
Figure 4 showing the desired direction of the properties. It is easily seen that Alloy 1 of
the present invention combines an improved hardness as compared with Comparative
example 1 with an improved corrosion resistance as compared with Comparative example
The typical microstructure for material of Alloy 1 in the annealed condition is a
ferritic matrix with uniformly distributed secondary carbides, nitrides and carbonitrides.
Furthermore, the microstructure of Alloy 1 is free from primary carbides, nitrides or
carbonitrides with a diameter bigger than 5 µm. A typical structure of Alloy 1 is shown in
Figure 5, wherein the microphotograph is taken in light optical microscope at 1000 x
magnification after polishing and etching of a transverse cross section. Etching was done
in 4% Picric acid with a minor addition of hydrochloric acid. The average diameter of the
carbides, nitrides and/or carbonitrides was estimated to approximately 0,4 µm.
For edge applications where very keen edges are to be produced either by
mechanical methods or by etching the above structure free from primary carbides with a
diameter larger than 5 µm is necessary to avoid tear outs or etching defects on the edge.
As a comparison a microphotograph, taken under the same conditions, showing the typical
structure for Comparative example 3 is shown in Figure 6.
In Figure 7 the hardness levels and structures are compared for Alloy 1 of the
invention, Comparative example 1 and Comparative example 3.
Since the properties of the steel are highly dependent on the hardening conditions,
estimations outgoing from the basic chemical composition may be misleading.
Equilibrium calculations at a predetermined suitable hardening temperature using the
software ThermoCalc is one way to more accurately calculate the final properties and have
been performed for Alloys 2-6, Alloy 1, as well as Comparative examples 1-3. The
compositions of Alloys 2-6 are given in Table 3 and the results of the calculations are
shown in Table 4.
The database used has been TCFE3. Optimal hardening temperatures have been
selected and used in the modelling for the different grades. Outgoing from the austenite
phase composition at hardening temperature, values for PRE, Ms and weight percent of the
interstitials nitrogen and carbon have been calculated. Also the phase percentage of M23C6
carbide in equilibrium with the austenitic phase, which is an important factor for wear and
edge durability, has been calculated. For PRE, the previously discussed equation has been
used. Ms was calculated using Andrew's formula, as shown below:
Comparison between Alloy 1 and Comparative example 1 shows that the steel
according to the invention has significantly higher PRE-values but at the same time
comparable interstitial content and amount of carbide phase, which should result in a steel
with similar hardness and edge performance but significantly increased corrosion
resistance. Comparative example 2 is closer to Alloy 1 in PRE but the lower amount of
'interstitials in the matrix together with a lower amount of carbide phase predicts a lower
hardness and inferior edge properties. These data correspond to the actual measurements
in the previous example. The Ms-temperature for Alloy 1 is lower than both Comparative
example 2 and Comparative example 1 but in the same range as Comparative example 3,
which has a known good hardenability, but where the carbide content significantly higher
resulting in a coarser microstructure as shown before in Figure 6.
Alloys 2-6 are other possible embodiments of the composition according to the
present disclosure, that result in different properties even though the difference in
chemical composition is small. Alloys 2 and 4 have comparable values for PRE, interstitial
content and Ms, which results in similar corrosion resistance, hardness and hardenability
but with about twice the amount of M23C6 carbides in Alloy 4, the edge durability will be
higher in this grade. Highest amount of interstitials in the matrix and thus the highest
expected hardness is achieved in Alloy 3, which still has a sufficient hardenability due to
the addition of Cobalt. Alloy 5 has even higher amount of cobalt compared to Alloy 6,
which improves the hardenability even further without drastically changing the other
We Claim :
1. Steel alloy characterized in having the following composition (in % by weight)
with the balance Fe as well as normally occurring impurities, the hardness being > 56
HRC, which should be possible to be attained by hardening without deep freezing, as well
as PRE > 25, defined as PRE = % Cr + 3,3 • % Mo + 16• % N.
2. Steel alloy according to claim 1, wherein C = 0,42-0,60, preferably 0,42-0,50 % by
3. Steel alloy according to claim 1 or 2, wherein Si = 0,15-0,80, preferably 0,15-0,55 %
4. Steel alloy acccording to any one of claims 1-3, wherein Mn = 0,4-0,8, preferably 0,4-
0,7 % by weight.
5. Steel alloy according to any one of the preceding claims, wherein Cr = 13-15,
preferably 14-15 % by weight.
6. Steel alloy according to any one of the preceding claims, wherein Mo = 2,6—4,0,
preferably 2,6-3,0 % by weight.
7. Steel alloy according to any one of the preceding claims, the steel alloy comprising
carbides, nitrides and/or carbonitrides, wherein the maximal diameter of the carbides,
nitrides and/or carbonitrides does not exceed 5 µm.
8. Knife, such as a knife suitable for the food industry, carving knife and the like,
characterized in that it comprises a steel alloy according to any one of claims 1-7.
9. Cutting edges for either dry or wet shaving, char-acterized in that they comprise
a steel alloy according to any one of claims 1-7.
10. Cutting tool for surgical applications, such as for instance a scalpel,
characterized in that it comprises a steel alloy according to any one of claims 1-7.
11. Doctor blade or creping blade, characterized in that it comprises a steel alloy
according to any one of claims 1-7.
The invention relates to a steel alloy of the following composition (in % by weight)
with the balance Fe as well as normally occurring impurities, the hardness being > 56
HRC, which has been attained without deep freezing, as well as PRE > 25, defined as
PRE = % Cr + 3,3 • % Mo + 16 • % N. Furthermore, the steel alloy comprises carbides,
nitrides and/or carbonitrides the maximal diameter of which does not exceed 5 µm. This
steel alloy has turned out surprisingly well suitable as edge material for a plurality of
|Indian Patent Application Number||2982/KOLNP/2006|
|PG Journal Number||23/2012|
|Date of Filing||16-Oct-2006|
|Name of Patentee||SANDVIK INTELLECTUAL PROPERTY AB|
|Applicant Address||S-811 81 SANDVIKEN|
|PCT International Classification Number||C22C 38/22|
|PCT International Application Number||PCT/SE2005/000422|
|PCT International Filing date||2005-03-22|