MXPA06006064A - Martensitic chromium-nitrogen steel and its use - Google Patents

Abstract

A steel material having a good resistance to corrosion, consisting of an alloy containing in%by weight:max 0.12 C 0.5-1.5 N 12-18 Cr max 0.5 Mn max 0.5 Ni 1-5 (Mo + W/2) max 1.5 (V + Nb/2 + Ti) 0.1-0.5 Si from traces and up to max 2.0 Co from traces and up to max 0.1 S balance iron and essentially only impurities at normal contents.

Description

WO 2005/054531 Al IIIIIS For two-letter codes and other abbreviations, refer to the "Guidance on Codes and Abbreviations" appearing at the beginning of the regular issue of the PCT Gazette.

MARTENSITIC STEEL OF CHROME-NITROGEN AND ITS USE Field of the Invention The invention relates to a steel material intended to be used for knives and tools, the use of which requires a high -hardness combined with an adequate resistance to corrosion. The invention relates in particular to a steel material which also has a suitable abrasive durability, in the hardened and tempered condition of the steel. The invention also relates to the use of steel material for knives and tools, in particular to machine knives and hand knives in the food industry, such as chopper knives for cutting and chopping trace animals and frozen fish, meat grinders and remove the hide, vibrating and circular knives for cutting machines. Other fields of use are knives for machines in the pharmaceutical industry and knives for cutting soft and moist crepe paper. Other conceivable applications are tools for molding plastics and injection worms for plastics, tools for cutting paper-based laminated packaging products for food and beverages. Another conceivable field of application is as a material for ball bearings. REF .: 172309 Background of the Invention In the food industry, high demands on the corrosion resistance and hardness for the tools used are presented. Cooking tools are commonly exposed to pitting corrosion because they come in contact with water that contains chlorine. The demands on abrasive wear resistance for these tools are also presented. Among the known materials having these properties, mention may be made of a group of nitrided martensitic steels, whose composition and properties are described in DE 3901 470 Cl. In the present patent application, these steels are collectively indicated A. Another related commercial steel corresponds to the composition of Wekrstoff No. 1.41 23 and indicated B. From EP 0 810 294 a number of alloy compositions are also known which have good corrosion properties, high strength and adequate ductility. These steels are collectively indicated C. Yet another material that has adequate corrosion resistance is described in EP 1 236 809 and is indicated D. The composition of the materials mentioned above is shown in the following table.

Table 1 It is common for the four prior steels to have adequate corrosion properties but that they lack adequate hardness and wear resistance, at least within certain of the fields of application mentioned above. The steels number 1 and 2 reach a hardness within the scale of 57-59 HRC. BRIEF DESCRIPTION OF THE INVENTION The object of the invention is to offer a steel material having an optimum properties profile for the fields of application mentioned above. In consequence, the steel material must fulfill mainly some or all of the following criteria: • Excellent resistance to corrosion, particularly a good resistance to pitting corrosion because the material is used for knives and tools, especially machine knives and hand knives in the food industry, as well as when it is used for plastic molding tools and injection worms for plastics, and tools for cutting paper-based packaging laminates for food and beverages. Another conceivable field of application is for ball bearings. • A high hardness in its hardened condition, so that in this way it does not deform to a high mechanical stress. A hardness of 58-65 HRC, preferably 60-64 HRC and most preferably around 62-63 HRC, in its hardened and tempered condition. • High strength (strength) to be suitable as steel for knives and other applications that place high demands on flexibility and highly sharp edges. • Proper wear resistance for the aforementioned fields of use, for example, wear resistance comparable to that of steels of the AISI 440C, AISI 618, 19C27, 13C26, 12C27, W 1.4034 or similar type. • A hardness of 230-240 HB in a soft tempered condition. Other desired parameters are: • Adequate workability • Adequate dimensional stability • High resistance to fatigue • Adequate ductility / resistance • Adequate compressive strength • Versatility that makes steel useful for several different fields of application . The invention is characterized by what is described in the appended claims, in order to achieve the desired properties. Considering the individual alloying materials, the following applies. Carbon can exist in the steel in a comparatively low quantity, in order to avoid the precipitation of chromium carbides in the contours of the grain. It is known that the carbides in the contours of the grain result in an increased risk of intercrystalline corrosion, the so-called intergranular corrosion. From this aspect, it is therefore desirable that the carbon content be kept as low as possible. From this aspect, carbon in principle is not desired at all in steel, but a carbon content of up to about -0.12% can be allowed without appreciable deterioration in the ability of the material to withstand intergranular corrosion. However, carbon contributes positively to the hardness of the material, which means that steel can be adequately allowed to contain a small amount of carbon. The scale of carbon content that is most preferred depends on the specific application for steel, which is mainly for knives and tools, especially knives for hand knives and machines in the food industry, and the specific application must in turn, according to with an aspect of the invention, having a great significance in the choice of the most suitable nitrogen content. Therefore, considering the most preferred carbon content scale, reference is made to the following discussion regarding the nitrogen content of the steel. In order to obtain corrosion properties among other things, a comparatively large amount of nitrogen has been added to the steel. Nitrogen contributes to a uniform distribution of chromium in austenite, and contributes to a better resistance to corrosion by effectively preventing the precipitation of grain contours in favor of particle precipitation. M2N nitride secondary very small and uniformly distributed, where M mainly represents chromium but also molybdenum. Nitrogen also helps to achieve adequate hardness in the material, despite the low carbon content. The hardness enhancing effect of nitrogen probably depends on the precipitation of M2N carbides described above. In addition to chromium and molybdenum the metals iron, niobium and vanadium form small particles of nitride. Moreover, the elements nitrogen, carbon, chromium and molybdenum contribute to the hardness of the martensite, by hardening in solution. Consequently and preferably, the steel contains 0.80-0.95% nitrogen. The nominal nitrogen content is around 0.9%. For the invention, an adequate carbon-nitrogen ratio in laboratory tests has been shown to be a nitrogen-to-carbon ratio of about 9: 1. The total amount of carbon in the steel, that is, the carbon that dissolves in the steel matrix plus the carbon that is bound in the carbides, should not exceed 0.12%, preferably 0.11% when very much and adequately being on the scale from 0.02-0.10%. Suitably, the average steel composition comprises about 0.08% carbon. So, an adequate nitrogen content is around 0.9%, but in laboratory loads that have been produced for development purposes, both carbon and nitrogen contents have varied and it is clear from the tests below, that desirable properties can be achieved with a nitrogen content on the scale of 0.5-1.5, preferably 0.7-1.2 and adequately 0.8-1.0 %. This leads to the ratio between nitrogen and carbon in the steel being in the range of 4: 1 - 75: 1, suitably 6: 1 - 50: 1 and preferably around 9: 1.

Silicon is included as a waste from steelmaking, and exists in a content of at least 0.1%. Silicon increases the activity of the carbon in the steel, and consequently can contribute to an adequate hardness of the steel without causing problems of cracking. However, silicon is a strong ferrite former and reduces the scale of the hardening temperature, and consequently it can not exist in contents of more than 0.5%. The nominal content of silicon is around 0.2%. Manganese also exists as a residue from the manufacture of steel, and it binds the amounts of sulfur that can exist at low contents in the steel, by forming manganese sulphide. Manganese also promotes hardening capacity, which is favorable. However, as an austenite former manganese is undesirable in steel according to the invention, which means that the manganese content very desirably must be less than 0.5%, preferably less than 0.4%, and suitably less than 0.3%. The nominal content of manganese is around 0.3%. Chromium is an important nitride former, and together with nitrogen it forms chromium nitrides (Cr2N). These will give a steel with improved corrosion properties and a martensite that will normally have an unusually high hardness considering its stainless properties. Chromium nitrides also contribute to the desired wear resistance of the material. Chromium can also contribute to an increased hardness and a reduced corrosion rate of martensite by hardening in solution. Therefore, the chromium must exist at a content of at least 12%, preferably at least 12.5% and suitably at least 13%, in order to give the steel the desired corrosion resistance. However, chromium is a strong ferrite former, and to avoid ferrite after hardening at 1050-1150 ° C, the chromium content should not be more than 18%, preferably not more than 17% and adequately no more of 16%. The nominal content of chromium is around 14.5%. Since it is an austenite stabilizing compound, nickel is not desired in the steel according to the invention. However, the presence of nickel as an unavoidable contaminant can be tolerated, which can thus be as high as approximately 0.5%. Preferably, the nickel content is less than 0.4%. The nominal nickel content is around 0.3%. Cobalt is an optional compound, and as such it can optionally be included in a 2% content at most, in order to increase the hardness further by accelerating the conversion of the austenite residue into martensite, and to contribute to some degree by hardening in solution . Normally, however, no addition of cobalt is necessary to achieve the desired properties of the steel. Therefore, cobalt in steel can be allowed to exist as a contaminant in a content of up to 0.5%, emanating from the raw material included in steelmaking. Molybdenum must exist in steel to give it an adequate resistance to corrosion,. in particular, an adequate resistance to pitting corrosion, and an adequate hardening capacity. Molybdenum is also a valuable nitride builder. In its property of being a nitride former, molybdenum can nevertheless be replaced mainly by the double amount of tungsten. Consequently, the total content of Mo + W / 2 in steel should not be less than 1%, preferably at least 2%, suitably at least 2.5%. However, molybdenum and tungsten are strong ferrite formers which means that steel should not contain more than 5% when much, preferably 4% at most, adequately 3.5% at most (Mo + W / 2). The nominal content of - (Mo + W / 2) is 3.0%. However, tungsten does not give the same improvement in corrosion resistance and hardenability as molybdenum. Even more, due to atomic weight ratios, twice the amount of tungsten is required to replace molybdenum. Another disadvantage of tungsten is that the handling of the residual fragments becomes more difficult, that is, the use of residual products (fragments) that originate in the manufacture of the steel and its processing to give a final product. Therefore, in a preferred embodiment of the invention, the steel should not comprise any deliberately added tungsten, but can be tolerated as an unavoidable contaminant in the form of residual elements from the raw material included in the manufacture of the steel. Vanadium must be included in the steel, so that in this way together with the nitrogen and any existing carbon forms M (N, C) -nitrides, -carbons and / or -carbonitrides in the martensitic matrix of the steel in a hardened and tempered condition. Niobium is an element that has a strong tendency to form M (N, C) -nitrides, -carbons and / or -carbonitrides, and exists both as primary precipitated particles and in the form of smaller secondary precipitated particles. The M (N, C) -nitrides, precipitated primary -carbonitrides and / or -carbonitrides containing niobium have a considerably smaller size, <0.5 μm, than the M (N, C) -nitrides, -carbons and / or -carbonitrides without niobium, which have a size of about 1 μm. The niobium compounds can contribute to keep the grain size of the material low, and ga better hardness of the material to an approximately equal firmness. Along with vanadium, niobium contributes to improved wear resistance, so that steel should preferably comprise both of these two alloy materials. Titanium can also form M (N, C) -nitrides, -carbons and / or -carbonitrides, and contributes to the hardness of the material by precipitation of primary and secondary particles. In a preferred embodiment, the steel however does not comprise any amount of titanium added deliberately. The total content of (V + Nb / 2 + Ti) in steel should be a content of 1.5% at most, preferably 0.35-1.0%, suitably around 0.6%, of which a maximum of 1.0%, preferably 0.3 -0.7%, suitably around 0.5% is Nb, and a maximum of 0.5%, preferably 0.05-0.3%, suitably around 0.1% is V. The nominal content of (V + Nb / 2) is around 0.6 %. Apart from the aforementioned alloy materials, the steel does not need, and should not, comprise any additional alloying element in significant quantities. Some materials are explicitly undesirable, since they affect the properties of steel in an undesirable way. This - it is true for example for phosphorus, the. which should be kept at the lowest possible level, preferably 0.05% at most, most preferably 0.03% when much so as not to adversely affect the hardness of the steel. Sulfur is also an unwanted element that, among other things, deteriorates corrosion resistance. Its negateffect, mainly on the hardness, can be significantly neutralized with the help of 'manganese, which forms essentially harmless manganese sulphides. However, preferably, the steel normally comprises no more than 0.1% S at most. A nominal composition of the steel of the invention that is preferred is g below, in Table 2. The steel is intended primarily to be used for kn and tools, especially kn for manual kn and machines in the food industry, in accordance with previous. Other conceivable applications are for plastic molding tools and injection worms for plastics, tools for cutting paper-based laminated packaging products for food and beverages. Another conceivable field of application is a material for ball bearings.

Table 2 Nominal chemical composition in% by weight, the rest Fe and impurities that are not those given in the table The manufacture of the steel material preferably comprises the powder metallurgical manufacture of a steel powder, by gas atomization with nitrogen according to the well-known ASP process (the ASEA-STORA process), including ESH refining, which means Heating Electro Residual and that gives an extremely homogeneous steel powder with a very low content of residual fragments inclusions. However, the invention also comprises the manufacture of a steel according to the invention by other closely related manufacturing methods, such as spray formation. The metallurgically powdered steel powder is screened to a particle size of 500 μm at most, and a certain amount thereof is nitrided to a suitable nitrogen content, such as 1-5%, at a temperature of between 550 and 550 μm. 600 ° C and in an atmosphere consisting of a mixture of ammonia gas and nitrogen gas. The steel powder with a high content of nitrogen is then mixed with the rest of the non-nitrided steel powder having a lower nitrogen content, according to a special and precise procedure, and then it is filled into a capsule which is evacuated of air. The capsule is filled with inert nitrogen gas and sealed by air tight welding, where after the capsule is compacted by hot isostatic pressure (HIP), to form a homogeneous steel ingot. In a procedure. Alternatively, the entire amount of sieved steel powder is nitrided to an adequate nitrogen content, in which case the mixing procedure may be omitted. Subsequently, the material is worked hot to create bars or strips, after which it is tempered in soft so that the steel according to the invention obtains a hardness of 220-250 HB (Brinell hardness number), of preference 230-240HB. The steel is supplied as steel strips worked in hot and cold. After machining to a desired shape, especially the form of knives for manual knives and machines for use in the food industry and the pharmaceutical industry, or for plastic molding tools and injection worms for plastics, tools for cutting laminated products based on of paper for food and beverages, and for ball bearings, the product is heat treated by austenitizing it at a temperature of between 1000 and 1200 ° C, preferably at a temperature of between 1050 and 1150 ° C, most preferably at a temperature of temperature between 1100 and 1150 ° C A suitable retention time at the austenitization temperature is 10-30 minutes. From the aforementioned austenitization temperature, the steel is cooled by deep cooling to -80 - -200 ° C, in order to eliminate residual austenite. To achieve a desired secondary hardening, the product is tempered at least twice at a temperature between 400 and 600 ° C, preferably at a temperature between 460 and 520 ° C. After each of these tempering treatments, the product is cooled, suitably up to around room temperature. The retention time at the tempering temperature can be 1-10 hours, suitably about 1 hour. Other features and aspects of the invention are clear from the appended claims, and from the following number of tests that have been carried out. Brief description of the figures In the following number of tests that have been carried out, reference is made to the attached figures, in which: Figure 1 shows a graph of the effect of the nitrogen content in the material on its hardness, in six sample alloys. Figure 2 shows the micro structure of a steel according to the invention, at an extension of x 2000. Figure 3a shows a graph of the result in EPR tests with anodic polarization. Figure 3b shows a graph of the result in EPR tests with cathodic polarization. Figure 4 shows a graph of heat ductility in a material according to the invention. Detailed Description of the Invention Examples A number of steel alloys were manufactured as laboratory charges, and from these subsequently HIP: ed, 030x100 mm steel capsules were made, according to the manufacturing process described above. Each capsule has been divided into smaller parts and has been analyzed with respect to the included elements. Table 3 shows the compositions for these laboratory loads. The different materials were further examined with respect to hardness, corrosion resistance and heat ductility, in order to find the best possible composition. The wear resistance of the steel will be examined as a knife test, after the manufacture of the strip material intended for knives. This strip material is suitably manufactured from steel in a full-scale load, which unlike the laboratory load steels results in a material with an insignificant content of residual fragments. A low content of residual fragments gives the best possible conditions for fair results of the «resistance of steel from knife tests and mechanical tests. With a point of departure from laboratory load tests, which refers to the chemical composition of steel, the thermodynamic calculations of the composition of steel in chemical phases, among others of the phases of hard nitride, M (N, C) and Cr2N, the metallographic examination of the portion, that is, the size and number of the hard phase particles of the nitride phases, - and without forgetting the high hardness of the steel, it can be determined however that the material will most likely satisfy the wear resistance requirements.

Table 3 In the laboratory loads produced, the carbon content has been constantly maintained at a level of about 0.08% by weight, in a couple of cases at 0.11% by weight. The nitrogen content has been varied between 0.4 and 0.94% -in weight. The amounts of alloy materials molybdenum, vanadium, niobium and silicon have been varied in these charges. In one case, cobalt has been added. The most important result from these comparatively small variations in the composition has been restricted to a variation in the mechanical properties, especially those related to the hardness of the steel.

Microstructure Hardened and hardened steel has a microstructure consisting essentially of two different hard phases in a matrix of nitrogen martensite. Referring to Figure 2, the microstructure of a steel according to the invention, having a nominal composition corresponding to steel No. 10-1 in Table 3, will be described. The steel according to the invention has been subjected to a heat treatment comprising austenitization at 1100 ° C, deep cooling at -196 ° C and 3x1 hours tempering at 460 ° C. The microstructure is very thin - and the difference "in contrast to the phases is small, which means that it is - more difficult to illustrate clearly, than the common ASP steel.

Matrix phase Depending on the hardening temperature, 94-97% of the steel is called nitrogen martensite, which is a martensite in which the carbon in the main part has been replaced by nitrogen. The chemical content is, apart from iron, essentially chromium, molybdenum and nitrogen, and simulates the average composition of the alloy, with the exception of nitrogen, niobium and vanadium, however, the contents of which are lower. All these materials have more or less influence on the hardness of the matrix phase. This nitrogen martensite is unusually hard to have stainless properties. The hardness of Vickers has been medium to HV 600-700, which is achieved by precipitation- / secondary hardening of very small secondary particles. Probably, these small particles have a size similar to those of high speed steel, and then their size is 5-20 nm. In addition, hardening in solution from the materials nitrogen, carbon, chromium and molybdenum, can contribute to the hardness of the nitrogen martensite. The martensite of. Itrogen also contains 3-6% by weight of precipitated primary hard phase particles.

These hard phase primary particles are much larger, 100-500 nm, than the secondary particles. The nitrogen martensite also contains 5-20% residual austenite. The portion of this phase must be low, since the residual austenite is mild. An attempt is made to reduce the portion of residual austenite by repeated deep temperature quenching and / or cooling at low temperature, for example in liquid nitrogen. However, tests have shown that for the material according to the invention, a suitable hardness, > 62 HRC, can already be achieved after the two tempering treatments, and that additional tempering treatments only have a marginal effect on hardness.

The hard phases In Figure 2, very small and light particles of M (N, C) are observed, which is the hardest phase that has a Vickers hardness measured from HV 200-0-3000. The particles have a size usually smaller than 0.5 μm. This hard phase contains essentially chromium, niobium, some vanadium and molybdenum, and also quite a lot of nitrogen. The carbon content is almost negligible. The proportion of the alloy materials in this hard phase can be described according to the following: (Cr 0.66, Nb 0.27, V 0.07, Mo -0) (N 0.98 C0.02). Niobium is included in the particles of M (N, C), both as larger primary particles and as small secondary particles (during precipitation hardening), as well as vanadium. The niobium compound, which is more difficult to soluble at the hardening temperature than the corresponding compound with vanadium, also has the advantage that it prevents the growth of the grains in the austenitic phase. Cr2N is also harder than the matrix phase, (HV 1200-1600), but not as hard as M (N, C). In Figure 2, Cr2N appears as dark gray particles that have a size that is usually 0.2-1.0 μm. It contains essentially chromium and in small amounts, iron and vanadium, according to the following proportions: (Cr 0.79, Mo 0.07, Fe 0.09, V 0.05) 2 (N 0.98 C0.02) Since the carbon content is generally insignificant , this phase is simply indicated as Cr2N.

In Figure 2, the particles of M (N, C) have a light gray color, and exist in the material up to an amount of 1.5-2.0%. The Cr2N particles have a dark gray color, and exist in an amount of 4-1.5%, depending on the austenitization temperature, within the range of 1100-1150 ° C. Consequently, the Cr2N content (4%) is larger than the content of M (N, C) in the figure, due to the lower austenitizing temperature. According to the above, it is in particular the amount of Cr2N that is affected by the austenitization temperature. The hardening affects the hardness of the matrix phase, but also its resistance to corrosion, in such a way - that a high tempering temperature gives a higher hardness but a deteriorated corrosion resistance. Based on the results of the tests carried out, the tempering temperature has been limited to 450-500 ° C, in order to obtain the desired properties. The steel according to the invention has been subjected to a heat treatment comprising austenitization at 1100 ° C, deep cooling at -19.6 ° C and 3x1 hours tempering at 460 ° C.

Hardness In the hardened and tempered condition, the hardness of the steel according to the invention should be 58-65 HRC, preferably 60-64 HRC and it is preferred more than the hardness be in the range of 62-63 HRC. . The hardness that is achieved depends on the choice of the hardening temperature, whether the material is subjected to deep cooling or not, and the choice of tempering temperature. Deep cooling essentially iminates the presence of residual austenite, which gives a desired hardness. If deep cooling is excluded, the hardness will be 1-1.5 HRC units lower than if deep cooling is applied. Furthermore, the hardness of the material depends on the content of the alloy materials included, as described above. Mainly nitrogen has been shown to have a great impact on the hardness of the material, through the formation of nitrogen martensite and hard phase particles. A number of produced laboratory loads, having compositions according to table 3, were tested with respect to Rockwell hardness (HRC), and the result is shown in the graph of figure 1. It is evident that a content of Higher nitrogen contributes to a higher hardness of the material.

Corrosion resistance Corrosion resistance depends on the amount of nitrogen, chromium and molybdenum alloy materials, which are dissolved in the steel matrix, but is negatively affected by an increased carbon content. One way of expressing corrosion resistance, in particular the level of protection against pitting corrosion which is the most severe type of corrosion, is through the so-called PREN number, which is obtained by the following calculation: Cr + 3.33 Mo + l € N (% by weight). Table 4 shows a comparison between some commercial steels (A, B, E) and a steel according to the invention, in which the hardness and PREN numbers of the materials are shown. Table 4 A number of the laboratory loads produced were examined according to two different test methods, to determine in this way their corrosion properties. One of the test methods focuses on determining the resistance of the material against pitting corrosion, and is defined in EN ISO 8442.2. These tests have been carried out at the Swedish Corrosion Institute. The second test method is focused on determining the resistance of the material against intercrystalline corrosion, also called intergranular corrosion, and Electrochemical Potentiokinetic Reaction (EPR) is indicated. Those tests have been carried out at Aubert &Duval. An important aspect in this context is that the inventive steel is intended to be tempered at a temperature of between 400 and 560 ° C. This gives a great advantage to the mechanical properties of the steel, that is to say, a high hardness and dimensional stability, within a wide range of temperatures up to the tempering temperature. At the same time, high tempering leads to a higher impact on the corrosion resistance of steel. Therefore, most competitive materials are low tempered in order to withstand corrosion tests.

EN ISO 8442.2 According to one aspect of the invention, it is desirable that the material has a corrosion resistance that meets the requirements of the test method EN ISO 8442-2. This test method is intended to test materials that come in contact with food, especially cutting tools and kitchen tools that are at risk of pitting corrosion after contact with chlorine-containing water. Seven of the laboratory loads produced were produced in 2-4 different variants, which had variable nitrogen contents. The loads were subjected to the following heat treatment before the tests. Austenitization at 11O0 ° C, deep cooling in liquid nitrogen at -196 ° C, 3x1 hours of tempering at 460 ° C. In this corrosion test, the series of alloys indicated 10-1, 10-2 and 10-3 have been tempered at a higher temperature than that of the other materials, 3x1 h at 500 ° C. To be approved, it is required that the material does not have more than 3 points with a diameter of between 0.4 and 0.75 mm, and no more than 1 point with a diameter of more than 0.75 mm, by 20 cm. All materials passed the test, in the form of double samples, but some of the samples having the lowest nitrogen contents displayed a slight discoloration due to corrosion in areas around large inclusions of residual fragments. Comparative tests were made on a commercial martensitic stainless steel, here indicated F. The composition of the material is shown in table 5. Two tests of this material were tested. Both samples were austenitized at 1050 ° C, but one of them was tempered at high temperature (FHT) and the other was tempered at a low temperature (FLT). None of the samples passed the test. The following table -6 shows the results for a choice of the materials tested.

Table 5 Composition of steel F Table 6 Result of the corrosion test according to EN ISO 8422.2 Electrochemical potentiokinetic reactivation (EPR) The resistance of laboratory loads against intercrystalline corrosion has been examined by an electrochemical test method called Electrochemical Potentiometric Reactivation (EPR). With the help of the EPR method, the corrosion resistance of the material can be determined both in the matrix and in the contours of the grain. The intercrystalline corrosion is very severe for the hardness of the material, and appears due to the precipitation of chromium carbide in the contours of the grain during hardening of the hardened material. This causes depletion of the chromium in the adjacent areas of the material around the contours of the grain, and therefore the material is sensitized to corrosion attacks. The result of this examination is shown in Figures 3a and 3b, and shows, among other things, the following in comparison with other reference materials tempered at high temperature (Ht) and at low temperature (Lt) respectively: • Absence of a corrosion mechanism intergranular initiated. • A very low dissolution of the matrix in 1% sulfuric acid in the presence of oxygen from the air. In the figures, the current density measured in the examination is shown in relation to the hardness of the material.

A low current density corresponds to a high resistance to corrosion, and the material according to the invention has the best result of the materials tested. Moreover, the examination showed very surprisingly that passivation is reinforced in repeated potential cycles, which is seen in the figures as the second current peak that has a lower value than the first peak current. In Figure 3a (anodic polarization), similar results are achieved for the reference material which is indicated A, but in Figure 3b (cathodic polarization) this material also exhibits deteriorated corrosion properties in the second current peak. This is especially interesting since the reference material has 0.4% by weight of nitrogen, and in this way one might expect it to react in a manner similar to that of the materials 2-1 and 10-1 according to the invention. In addition, the material A has a hardness worse than that of the two materials according to the invention. Accordingly, the examination shows that the stainless steel for knives according to the invention have the best combination of hardness and corrosion resistance, as compared to the other high and low temperature hardened reference steels examined. Table 7 Current density (μA / cm2) at the 450 mV peak The polarization has been measured for two cycles, in order to investigate whether the passivation was reinforced or deteriorated. If the second value is the lowest, the passivation was reinforced. .

Heat Ductility The heat ductility of the material 10-1, within the temperature range of 900-1210 ° C, is shown in figure 4. Test dimension 015x85mm, elongation speed ß. € s-1, temperature Increasingly high-for T > 1120 ° C and increasingly lower temperature for T = 1120 ° C. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers.

Claims (21)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A steel material that has an adequate resistance to corrosion, characterized in that it consists of an alloy containing in% by weight: maximum 0.12 of C 0.5-1.5 of N 12-18 of Cr maximum 0.5 of Mn maximum 0.5 of Ni 1- 5 of (Mo + W / 2) maximum 1.5 of (V + Nb / 2 + Ti) 0.1-0.5 of Si from traces and up to a maximum of 2.0 of Co from traces and up to a maximum of 0.1 of S the rest being iron and essentially only impurities in normal contents.

2. The steel material according to claim 1, characterized in that after hardening and hardening, it has a hardness of 58-65 HRC and a microstructure which -contains 3-6% -in volume of the two hard phases M (N) , C) and Cr2N in a matrix that consists essentially of tempered nitrogen martensite, nitrogen martensite comprising 5-20% residual austenite.

3. The steel material according to claim 1 or 2, characterized in that it contains a maximum of 0.11 of C, preferably 0.02-0.10 of C.

4. The steel material according to any of claims 1-3, characterized in that it contains 0.7-1.2, preferably 0.8-1.0 N. The steel material according to any of claims 1-4, characterized in that it contains 12.5-17, preferably 13-16 Cr. 6. The steel material according to any of claims 1-5, characterized in that it contains a maximum of 0.4, preferably a maximum of 0.3 of Mn. The steel material according to any of claims 1-6, characterized in that it contains a maximum of 0.4, preferably a maximum of 0.3 of Ni. The steel material according to any of claims 1-7, characterized in that it contains 2-4, preferably 2.5-3.5 of (Mo + W / 2). 9. The steel material according to any of claims 1-8, characterized in that it contains 0.05-0.3, preferably 0.1 of V. 10. The steel material according to any of claims 1-9, characterized in that it contains 0.3-0.7, preferably 0.5 of Nb. 11. The steel material according to any of claims 2-10, characterized in that it has been hardened - by austenitization at 1000-1200 ° C, preferably at 1050-1150 ° C and most preferably at 1100-1150 ° C. , cooled to -80 - -200 ° C deep, and then tempered at a temperature of 400-560 ° C, preferably at 430-500 ° C and most preferably at 460-50 ° C. 12. The steel material according to claim 11, characterized in that it has a hardness of 60-64 HRC, preferably around 62-63 HRC. The steel material according to any of the preceding claims, characterized in that the M in the hard phase M (N, C) contains essentially chromium, niobium, vanadium and molybdenum according to the following composition: 0.66 Cr, 0.27 Nb , 0.07 V + Mo, where the content of V is predominant, and where (N, C) contains essentially nitrogen but also a certain amount of carbon according to the following composition: 0.98 N, 0.02 C. 14. The material of steel according to any of the preceding claims, characterized in that Cr in the hard phase Cr2N contains essentially chromium, molybdenum, iron and vanadium, according to the following composition: 0.79 Cr, 0.07 Mo, 0.09 Fe and 0.05 V, and wherein (N, C) contains essentially nitrogen but also a certain amount of carbon according to the following composition: 0.98 N, 0.02 C. 1

5. The steel material according to claim 1 or with any of the claims. is 3-10, characterized in that it is tempered in soft and because in the tempered condition in soft it has a hardness of 220-250 HB (Brinell hardness), preferably 230-240 HB. 1

6. The steel material according to any of the preceding claims, characterized in that it is a metallurgically manufactured metal material. 1

7. Use of the steel material according to claim 15, in the manufacture of knives and tools. 1

8. Use of the steel material according to claim 15, in the manufacture of knives for manual knives and machines for the food industry. 1

9. Use of the steel material according to claim 15, in the manufacture of tools for molding plastics and injection worms for plastics. 20. Use of the steel material according to claim 15, in the manufacture of tools for cutting paper-based laminates for food and beverages. 21. Use of the steel material according to claim 15, in the manufacture of wave bearings. ..