Steel Alloy For Cutting Details
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.
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 of 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 additional 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 (<30°), experience shows that carbonitrides (also applying to slags and 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 manufacture 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 carbonitride. 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 (Ø>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 furnace 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<5 μm) and furthermore that the size distribution is even, which is controlled by an accurately controlled 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 of machining.
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 nitrogen 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 present. 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 carbonitrides Ø<5 μm, there are no standard materials that meet them. Materials of the type AISI 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, inadequate.
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 present 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 U.S. Pat. No. 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 high demands.
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<5 μm, in order to reduce the risk of edge related problems and to be 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. Customer requirements such as hardened and tempered, and cold-rolled, respectively, finish, naturally has also an effect. However, the manufacture will always comprise a metallurgical 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
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
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.
EXAMPLE 2 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
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
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 involved.
EXAMPLE 4 In
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
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
In
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:
Ms=539−423*C−30.4*Mn−12.1*Cr−17.7*Ni−7.5*Mo+(−423*N−7.5*Si+10*Co)
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
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 properties.
Claims
1. Steel alloy characterized in having 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 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% by weight.
3. Steel alloy according to claim 1, wherein Si=0.15-0.80% by weight.
4. Steel alloy according to claim 1, wherein Mn=0.4-0.8% by weight.
5. Steel alloy according to claim 1, wherein Cr=13-15% by weight.
6. Steel alloy according to claim 1, wherein Mo=2.6-4.0% by weight.
7. Steel alloy according to claim 1, wherein the steel alloy comprises carbides, nitrides and/or carbonitrides, wherein a maximal diameter of the carbides, nitrides and/or carbonitrides does not exceed 5 μm.
8. Knife comprising the steel alloy according to claim 1.
9. Cutting edges for either dry or wet shaving comprising the steel alloy according to claim 1.
10. Cutting tool for surgical applications comprising the steel alloy according to claim 1.
11. Doctor blade or creping blade comprising the steel alloy according to claim 1.
12. Steel alloy according to claim 2, wherein C=0.42-0.50% by weight.
13. Steel alloy according to claim 3, wherein Si=0.15-0.55% by weight.
14. Steel alloy according to claim 4, wherein Mn=0.4-0.7% by weight.
15. Steel alloy according to claim 5, wherein Cr=14-15% by weight.
16. Steel alloy according to claim 6, wherein Mo=2.6-3.0% by weight.
17. A steel alloy, comprising:
- a composition including (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 balance Fe as well as normally occurring impurities;
- a hardness>56 HRC; and
- a value for PRE>25, wherein PRE=% Cr+3.3•% Mo+16•% N.
18. The steel alloy of claim 17, wherein the hardness is attained by hardening without deep freezing.
Type: Application
Filed: Mar 22, 2005
Publication Date: Nov 29, 2007
Inventors: Jonas Nilsson (Sandviken), Andreas Rosberg (Gavle)
Application Number: 10/584,246
International Classification: C22C 38/44 (20060101); C22C 38/22 (20060101); C22C 38/52 (20060101); C22C 38/30 (20060101);
