EP2678459B1 - Use of a chromium steel having a martensitic microstructure and carbidic inclusions - Google Patents

Description

The invention relates to the use of a chromium steel with martensitic structure and carbide inclusions, and a method for heat treating such a steel.

Such steels are known in large numbers and are suitable depending on their composition for a very different range of uses.

The prior art includes one from the German patent application 10 2009 038 382 known stainless martensitic chromium steel having 0.40 to 0.80% carbon, 0.20 to 1.50% silicon, 0.15 to 1.00% nickel, 0.30 to 1.00% manganese, 0.015 to 0.035% sulfur , 16 to 18% chromium, 1.25 to 1.50% molybdenum, at most 0.8% tungsten, 0.04 to 0.08% nitrogen, 0.15 to 0.20% vanadium, each up to 0.05% Titanium and niobium, 0.001 to 0.03% aluminum, 0.02 to 0.5% copper, at most 0.5% cobalt and not more than 0.004% boron, balance iron, including impurities caused by melting. This steel is suitable as a corrosion-resistant, in particular puncture-resistant material for objects subject to frictional wear. Nothing is known about the suitability of the alloy for further use beyond this specific use, although the practice knows a number of wear-resistant steel alloys.

That's how it describes U.S. Patent 3,990,892 a high temperature wear resistant steel having 0.8 to 1.2% carbon, 1.0 to 2.5% silicon, 0.5 to 3.5% nickel, 0.2 to 1.0% manganese, 15 to 25 % Chromium, 0.3 to 3.5% molybdenum, 0.5 to 3.5% tungsten, to 0.3% nitrogen, to 0.5% vanadium, to 0.3% titanium, to 1.0% niobium , to 0.5% aluminum, to 1.0% copper, 0.3 to 5.0% cobalt, balance iron.

DE 10 2004 051 629 discloses a stainless martensitic chromium steel which is useful as a material for making knives and mercer elements.

Furthermore, the German patent specification describes 100 27 049 B4 also a martensitic chrome steel, but with 0.4 to 0.75% carbon, to 0.7% silicon, to 0.2% nickel, 0.4 to 1.6% manganese, 0.02 to 0.15 % Sulfur, 12 to 19% chromium, 0.5 to 1.5% molybdenum, up to 1.5% tungsten, up to 0.1% nitrogen and 0.05 to 0.3% vanadium, titanium and niobium individually or side by side as well to 0.008% boron. This steel has good processability, corrosion resistance and low plastic deformability as well as wear and abrasion resistance; he is therefore suitable without a galvanic coating as a material for industrial needles and in particular allows a high sewing speed.

Prior to this prior art, the invention is based on the problem of proposing a stainless martensitic chromium steel, the surface of which is sufficiently stable in a certain stress spectrum and which is due to its properties for another or special use.

• 0,50 bis 0,78% Kohlenstoff
• 0,20 bis 1,30% Silizium
• 0,15 bis 0,80% Nickel
• 0,30 bis 1,00% Mangan
• 0,01 bis 0,035% Schwefel
• 16 bis 18% Chrom
• 1,25 bis 1,50% Molybdän
• 0,001 bis 0,8% Wolfram
• 0,04 bis 0,08% Stickstoff
• 0,15 bis 0,20% Vanadium
• 0,001 bis 0,04% Titan
• 0,001 bis 0,04% Niob
• 0,001 bis 0,03% Aluminium
• 0,02 bis 0,5% Kupfer
• 0,001 bis 0,04% Bor
• Rest Eisen

einschließlich erschmelzungsbedingter Verunreinigungen als kontaminations- oder seewasserbeständiger oder auch als verschleißbeständiger Werkstoff mit im praktischen Einsatz geringem Materialabtrag für die Lebens- und die Genussmittelindustrie oder auch als Werkstoff für chirurgische Instrumente wie Skalpelle und Klingen. Darüber hinaus ist der vorgeschlagene Chromstahl aber auch als Werkstoff zum Herstellen von Filettiermessern für die Fischverarbeitung und für die Lebens- und die Genussmittelindustrie sowie zum Herstellen von Industrienadeln geeignet.The solution to this problem is the use of a martensitic chromium steel with

• 0.50 to 0.78% carbon
• 0.20 to 1.30% silicon
• 0.15-0.80% nickel
• 0.30 to 1.00% manganese
• 0.01 to 0.035% sulfur
• 16 to 18% chromium
• 1.25 to 1.50% molybdenum
• 0.001 to 0.8% tungsten
• 0.04 to 0.08% nitrogen
• 0.15 to 0.20% vanadium
• 0.001 to 0.04% titanium
• 0.001 to 0.04% niobium
• 0.001 to 0.03% aluminum
• 0.02 to 0.5% copper
• 0.001 to 0.04% boron
• Rest iron

Including melting-related contaminants as contamination or seawater resistant or wear-resistant material with in the practical Use of low material removal for the food and luxury food industry or as a material for surgical instruments such as scalpels and blades. In addition, the proposed chromium steel is also suitable as a material for the manufacture of Filettiermessern for fish processing and for the food and luxury food industry and for the production of industrial needles.

Of particular importance for the wear resistance is that the steel is both dry and wet wear resistant. The steel is therefore particularly suitable where the use of a contaminating lubricant is not possible, such as in the food and beverage industry and in contact with water. The steel is therefore particularly suitable for use due to its high wet corrosion resistance. But even in the presence of corrosive media comparatively little material is removed and accordingly released only a small amount of toxic components of the steel such as nickel.

Because of its high resistance or its extremely low release rate of allergenic substances such as the toxic nickel acting under wear stress, the proposed chromium steel is particularly suitable for use in the field of surgery and as a material for braces. Here, the material is of particular importance, because there are usually several influencing factors, which cooperatively reduce the functionality of instrument tips and cutting due to wear. This is the case, for example, due to the interaction between the material to be processed and its influence by other materials, for example by abrasion and by loading or processing tools under the influence of the conditions of use, for example in high-speed machines, by pressure and temperature.

The consequence of this is a removal of material, which affects the life of functional elements in machines such as knife blades, needles and static or dynamically installed thread guide elements as wear. In addition, such erosion through wear can result in harmful contamination of biological tissues or in contact with food to produce allergens. For example, nickel and cobalt act as toxic allergens, which is associated with a significant health risk in terms of the health consequences of interactions between the metal and biological tissue.

Thus, in the food and beverage industry, toxicological problems often arise with metal ions as a result of corrosive wear in fish processing, for example, with the aid of filleting machines and deburring knives in the presence of seawater and in the manufacture of fabrics such as dressings or for clothing. Another problem area for metal ion-induced damage to health are medical instruments. Here, in a wet atmosphere or in contact with biological material pitting corrosion plays a significant role. It is a local metal dissolution associated with a porous surface of the metal on the one hand and, for example, allergies triggering contamination in particular by harmful heavy metals.

Rust and acid-resistant chromium-nickel steels are generally characterized by a low rate of release of the nickel, since their surface is protected by an initially forming passivating topcoat. The steels have a low carbon content and are therefore easily deformable but can not be hardened. As a material for example for knives, blades and needles they are therefore not suitable. These require martensitic steels with higher carbon content compared to austenitic chromium-nickel steels and carbide formers such as titanium, vanadium and tungsten. With the help of a heat treatment, a high hardness can be achieved with these steels. The disadvantage, however, is that these steels are not resistant to salt solutions and therefore are subject to pitting corrosion. As far as dry machining is concerned, there are no corrosion problems with cutting and punching knives, for example. However, this is different in the presence of a saline atmosphere or in salty water with biological tissue and complex chemical reactions that can lead to severe pitting corrosion.

The contact with a saline atmosphere is disadvantageous in two respects, because the pitting corrosion reduces the functionality of, for example, knives, blades, blades, needles or drills and thus leads to porous surfaces by material dissolution and hole formation. Furthermore, the release of metal ions promotes or intensifies the development of allergies. In addition, especially in the food sector, the high or increasing concentration of heavy metals in the liquid range.

Furthermore, it comes in the structure of steel to the emergence of chromium carbides, which is naturally associated with a local depletion of chromium dissolved in the structure. The material is then in the area of the depletion areas close to the surface to a great extent endangered by pitting corrosion, as shown in the schematic representation in FIG Fig. 1 makes it clear. Coarse-grained chromium carbides 2, which are surrounded by a chromium depletion zone 3 and, as a consequence of pitting corrosion, lead to dissolution of the metal or pitting in region 4, are present in the corrosion-resistant matrix 1. In addition, high surface and shear stresses, in particular in the area of coarse-grained carbides, impair the corrosion resistance.

The steel also has a low release rate which is at least as good as that of the known implant material Ti-50at% Ni with the same number of titanium and nickel atoms (US Pat. Xiao-Xiang, Journal of Materials Science Letters, 17 (1998), 375/376 ). The steel is therefore suitable as a contamination-resistant, low-wear and seawater-resistant material for the food and luxury food industry and as a material for surgical instruments such as scalpels and blades, but also for the manufacture of filleting knives for fish processing.

Experiments and more detailed investigations have also shown that the pitting corrosion above a carbide grain size of about 15 microns is particularly critical. This is especially true for areas with high interfacial and shear stresses. These arise preferably between the coarse carbides and the surrounding chromium depletion zones with those empty lattice sites on which there are no chromium atoms as a result of chromium depletion. This leads to dislocations and lattice defects with local stresses as the cause of the pitting corrosion in aqueous solutions; Over time, it releases the near-surface carbides until they break out, resulting in severe wear and ultimately unusability in the case of cutting.

The diagram of Fig. 2 illustrates the danger of near-surface pitting corrosion on the basis of the frequency distribution as a function of the carbide grain size. In this case, the fully drawn curve indicates the steel of experiment 4 of Table II, the hatched area with the dotted line the critical corrosion range under the aspect of grain size and the dot-dash line the particle size distribution of the comparative steel 6 in experiment 10 with a mean particle size of 16 .mu.m and a maximum grain size of 32 μm (Table II). The course of the fully extended bell curve in the diagram of Fig. 2 illustrates the importance of maximum carbide grain size in minimizing pitting corrosion and associated chromium depletion in a surface zone. From the course of the left curve of Fig. 2 shows that in the steel of Experiment 4, the majority of the carbides corresponding to the culmination point of the left bell curve has a particle size of less than 10 microns. At the same time, the curve shows that the proportion of carbides with a particle size of 15 μm is extremely small; it is less than 2%.

The frequency maximum of the comparative steel 6 in Experiment 10 (Table II), however, is 15 microns and only for carbides with a particle size of 38 to 40 microns results in a low frequency, as it has the steel of Experiment 4 at 15 microns. The comparative steel 6 is therefore much more because of its coarse carbides pitting sensitive. The hatched area in the diagram of Fig. 2 makes it clear that coarse carbides of the size of 20 to 30 microns and more already at relatively low frequency and with 15 microns are already critical. In contrast, the left frequency curve for the steel of Experiment 4 is far from the hatched critical region, which proves its corrosion resistance, and makes it clear that only very little nickel is released.

Decisive for this is the composition of the steel on the one hand and the ability to largely suppress the formation of coarse chromium carbides by means of a preferably multi-stage heat treatment with each subsequent phase transformation and cold working. The invention uses the knowledge that carbides and carbonitrides in the austenitic structure have both different solubilities and different diffusion coefficients, which exploits the multi-stage annealing in combination with the phase transformation during annealing and cold working to the proportion of chromium carbides having a size below 15 .mu.m at least 98%.

The invention uses the knowledge of the importance of the particle size distribution for the pitting corrosion to influence them in such a way that it degrades coarser carbides with a particle size over 15 to 35 microns and more and thereby increases the proportion of small-grained carbides with only narrow zones of chromium depletion.

The influencing of the particle size distribution is done by means of a multi-stage, preferably at least three-stage heat treatment with gradually decreasing annealing temperatures and a hot working between two annealing treatments and a final curing and preferably a final annealing. The annealing time and the annealing temperature in the first annealing stage 18 to 24 hours at 1100 to 1250 ° C and in the second stage 0.5 to 2 hours at a temperature of 1,000 to 1,100 ° C amount. In the case of any third annealing stage, the annealing time should be 0.5 to 1 hour and the annealing temperature 720 to 780 ° C. This can be a hardening of 10 to 20 min. at 1,000 to 1,080 ° C, optionally with a preheating at 300 to 600 ° C for 15 to 35 minutes, followed by an optional one to four hour tempering at 100 to 500 ° C.

The duration of the annealing within the aforementioned periods of course depends on the annealing material and / or the Glühgutquerschnitt. Because the annealing time must ensure in individual cases that the annealed material is thoroughly warmed to ensure a homogeneous annealing structure.

The structural changes associated with the stepwise annealing, in conjunction with the respective deformation between each two annealing stages, mean that even in the first annealing stage, the precipitated fine carbides are almost completely dissolved and the volume of the coarse carbides is reduced by about 20 to 40%. there plays the role of nitrogen of the steel so far, as form at low nitrogen contents of, for example, about 0.08 to 0.1% sparingly soluble carbonitrides, go in glowing only in a small volume of volume in solution and obtained in the other annealing stages as coarse carbonitrides stay.

This illustrates the diagram of Fig. 3 with the dependence of the carbide grain size on the carbon content of the steel on the basis of two curves for a sample with a maximum nitrogen content of 0.08% according to the invention and a comparison curve for a steel not exceeding the invention containing more than 0.08% nitrogen at carbon levels of 0.50 to 0.78%.

In the illustration after Fig. 3 the full circles and triangles refer to the maximum carbide size, ie KG max. according to Table II. The negative influence of higher nitrogen contents of more than 0.08% becomes clear. The open circles and triangles, on the other hand, refer to the grain size with the highest frequency, which is given in Table II as KG means. The two lines relate to the increase in grain size (KG mean in Table II). They show that with the comparison steels 5 to 8, the maximum grain size deviates more and more from the already high line of mean grain size with increasing carbon content. This is a sign of a very strong grain growth.

Specifically, the annealing causes the first annealing stage to form an austenitic microstructure that, upon cooling and subsequent deformation, converts to an austenitic-martensitic mixed structure having a relatively high residual austenite content. Even this mixed structure is characterized by more advantageous material properties, because it comes in the mixed structure for the precipitation of carbides, such as chromium carbides and the emergence of chromium depletion zones, but have the advantage of a finer distribution.

The subsequent annealing stages, each with a lower annealing temperature, each with an intermediate deformation effect a volume reduction of the already smaller chromium depletion zones from the previous annealing stage. This is done in such a way that during the intermediate deformation, the chromium depletion zones are destroyed and thinned out as a result of the flow and sliding processes occurring in the steel. In the respective subsequent annealing stage at a lower temperature, this is associated with a degradation of the chromium depletion zones by way of diffusion without any appreciable growth of the carbides.

As far as the composition of the steel is concerned, the carbon promotes the formation of austenite and reacts with chromium, titanium, vanadium, niobium and tungsten.

This produces carbides with the disadvantage of reducing the corrosion resistance and the advantage of a substantial increase in hardness. In addition, with increasing carbon content, the strength increases accompanied with an increase in hardenability. However, with increasing carbon content, it is difficult to control carbide segregations and agglomeration, so the maximum carbon content is 0.78%.

Silicon stabilizes the ferrite and binds traces of oxygen in the steel itself; the steel therefore contains at least 0.2% silicon, but to avoid intermetallic phases with other elements it is not more than 1.0%.

Nickel is ferromagnetic in addition to iron and thus extends the field of gamma-iron; it also stabilizes the austenite at the expense of the ferrite. In addition, the nickel stabilizes the martensitic transformation, which is why the steel contains 0.15% nickel, but not more than 1.0% nickel, because higher nickel contents stabilize the austenite too much.

The steel contains 0.3 to 1.0% manganese, which, like nickel, stabilizes the austenite microstructure, thus contributing to the uniform emergence of the martensite phase.

Sulfur is not a pollutant, but imperative to form sulfides, which, however, promote corrosion under the action of aqueous salt solutions. The steel therefore contains at least 0.01% and at most 0.035% sulfur.

Chrome gives the necessary corrosion resistance with a minimum content of 16% steels. However, carbon also forms chromium carbides which increase the hardness and wear resistance of the steel. However, a disadvantage is the formation of chromium carbides, which is associated with a consumption of chromium on the one hand and, consequently, the risk of impairing the corrosion resistance, combined with a chromium depletion in the carbides and thus an impairment of the corrosion resistance in aqueous salt solutions. Associated with this is a dissolution of the carbides to their breaking out in a surface zone as the cause of the extremely detrimental pitting corrosion. In addition, since the chromium is a ferrite-forming agent, it stabilizes a ferritic microstructure, and therefore the chromium content is only 16 to 18%, in order to ensure a hardenable martensitic structure required for knives, bone drills and industrial needles.

Molybdenum lowers the critical cooling rate and, together with the carbon for cutting steels, forms important carbides. In addition, molybdenum improves corrosion resistance in the presence of chromium. Because of its high atomic weight, the solid state diffusion of molybdenum is relatively low, which the solubility of mixed carbides deteriorates. The molybdenum content is therefore at a minimum content of 1.25% at most 1.50%.

Tungsten also has a ferrite-stabilizing effect and promotes hardening by means of mixed carbide formation. However, at levels above 0.8%, it makes deformation more difficult.

aufeinander abgestimmt.Nitrogen forms hard nitrides with chromium, titanium, vanadium, aluminum, and niobium and is incorporated into the carbide crystal lattice in the presence of carbon or carbides to form carbonitrides that expand the crystal lattice, causing internal stress and increasing hardness. However, since the nitrogen deteriorates the solubility of carbonitride precipitates, solution annealing to reduce the amount of coarse carbides tends to lose its effect. The nitrogen content is therefore at a lower limit of 0.04%, below which at increased costs the nitrogen does not bring any significant improvements, at most 0.08%. Preferably, the contents of the carbide or carbide formers which are decisive for the grain size become nitrogen and carbon according to the equation $$\left(\%\mathrm{N}\right)=\mathrm{0}.\mathrm{1}{\left(<figure-callout\; id="2"\; label="\%\; C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">\%\; </figure-callout>\mathrm{C}\mathrm{}\right)}^{\mathrm{2}}+\mathrm{0}.\mathrm{02}$$

coordinated.

Titanium and niobium are strong deoxidizers that form stable oxides, carbides and nitrides with high hardness and low solubility in iron. In addition, the mentioned titanium and niobium compounds or precipitates in the structure can hardly be dissolved during annealing; they therefore act in a supersaturated microstructure as germs for excreta. The maximum content of titanium and / or niobium is therefore 0.04% with an optional minimum content of 0.001%.

Aluminum is one of the most effective deoxidizers, which also forms extremely stable Al ₂ O ₃ precipitates with oxygen and, depending on the nitrogen content, also aluminum nitride. The aluminum content is therefore 0.001 to 0.03%.

Copper is used to harden the steel and thus improves its usability within a copper content of 0.02 to 0.05%. In particular, copper promotes the precipitation of ultrafine carbides with a grain size below 2 microns, because the relatively large copper atom at relatively low temperatures in the solid state of the steel in the iron grid makes room for the carbon and thus create one of the conditions for the emergence of Feinstkarbiden, while the Copper associated with other, equally larger atoms, with an overall increase in hardness is connected. At higher copper contents, however, with the Risk of copper clusters forming, which adversely affect toughness.

Boron increases strength in low concentrations and forms finely divided boride precipitates with other constituents of the steel, such as titanium, vanadium and molybdenum, but also embrittles at higher levels. The boron content is therefore at most 0.04% with a minimum content of preferably 0.0001%.

The steel is particularly suitable for knife blades, sewing and weaving needles, surgical drills, blades and instruments, as well as for filleting and deburring knives, coater blades, meat saws, leaf valves, photochemically etched filter plates, electric shaver cutting elements and extruders.

tries

The composition of trial steels is shown in Table I.

The steels of Table I were melted in a medium frequency oven under inert gas, poured into an elongated mold and ground after cooling to room temperature to round rods. These were annealed for the widest possible dissolution of the carbides in the temperature range T1, occasionally also for comparison purposes at lower temperatures under inert gas and rapidly cooled. Subsequently, elongated rods with a diameter of about 20 mm were forged and turned off by means of carbide cutting plates. These rods were each used for annealing and deformation tests as well as for structural, corrosion and abrasion tests. <b> Table I </ b> stole C (%) N (%) Cr (%) Ni (%) Mn (%) Not a word (%) Cu (%) V (%) S (%) 1 0.49 0.05 16.57 0.37 0.45 1.30 0.202 0.15 0,010 2 0.56 0.07 17.30 0.82 0.55 1.40 0.230 0.19 0,013 3 0.75 0.05 17.82 0.21 0.32 1.28 0,450 0.16 0,018 4 0.76 0.05 16.15 0.90 0.85 1.35 0,320 0.18 0,015 5 0.75 0.15 16.50 0.90 0.50 2.25 0,015 0.28 0.005 6 0.80 0.18 18.50 2.05 0.62 1.90 0.023 0.08 0,008 7 0.42 0.12 15.40 1.52 0.55 1.32 0,730 0.10 0,012 8th 0.51 0.20 17,20 1.80 0.71 0.60 0,035 0.10 0,010

In order to evaluate the material properties, first of all a wire rod with a diameter of about 5 mm was produced from the rods of the trial alloys and this wire rod in several stages, each with an intermediate annealing at the temperatures T2 and T3 shown in Table II, to form a wire with a diameter of 0.5 mm, then hardened three times and finally hardened and tempered. Carbide grain sizes were determined microscopically and are set forth in Table II. For grade evaluation, wire sample lengths of 80 mm were flexed 90 ° back and bent back three times in a flex-rebound test, then fracture and bump due to localized solidification due to bending evaluated from Table II. <b> Table II </ b> attempt stole Glowing ° C T hardening T tempering KG mean μm KG Max. Μm Abrasion mg K Wire pull Φ 0.5 mm bending test T1 T2 T3 wet dry 1 1 1050 980 760 1040 480 3 6 5 5 0.75 Good good (3) 2 1 1050 960 720 1040 420 1.5 5.5 6 5.5 0.72 Good good (3) 3 2 1050 960 740 1080 300 2.5 5.5 4.5 4.5 0.73 Good good (3) 4 3 1050 960 780 1080 400 3.5 7 4 3.9 0.76 Good good (3) 5 3 1080 960 760 1060 200 2.5 5.5 4 4 0.71 Good good (3) 6 4 1080 1000 780 1080 260 3 6 4 4 0.74 Good good (3) 7 4 1100 1000 780 1080 280 2 5.5 4.5 5 0.72 Good good (3) 8th 5 870 940 700 1020 320 16 25 10 4 1.30 Good Fraction (1) 9 6 800 1030 750 1050 340 18 38 14 3.5 1.60 fracture - 10 6 1080 960 720 1080 300 16 32 13 4 1.30 fracture - 11 6 1080 960 760 1030 380 14 25 11 4 1.29 fracture - 12 6 1080 960 720 1080 300 15 35 13 4 1.40 fracture - 13 7 1050 960 760 1050 290 9 15 11 4 1.14 Good Humpback (3) 14 7 850 960 720 1050 400 10 16 9 5.5 1.14 Good Humpback (2) 15 8th 800 980 760 1050 350 16 20 12 5 1.26 Good Breakage (3) T1 1050 ... 1150 () Number of bending tests
T2 960 ... 1040 good (3) good after 3 bends
T3 720 ... 780 Break (1) Break after 1 bend
Humpback (2) Hunchback after 2 bends

To assess the pitting resistance or the nickel release rate, the samples were compared with a NiTi shape memory alloy, as is also the case with medical implants. For this purpose, in a first step, samples of the same size of 4 cm ² were prepared and placed in each case in 50 ml of an electrolyte from an aqueous solution of 4% NaCl, 0.5% lactic acid and 1% urea.

mit den Ergebnissen der Tabelle II berechnet. Ein Vergleich der Daten zeigt, dass die Freisetzungsraten der Versuche 1 bis 7 ganz erheblich unter den Werten für die Vergleichsstähle der Versuche 8 bis 15 liegen, was den wesentlichen Unterschied in der Lösungsstabilität deutlich macht.In this electrolyte, the samples were left as well as a control sample of the material Ti-50At% Ni for a period of 190 hours. Thereafter, the dissolution of the material due to corrosion, or the nickel content passed through corrosion in the electrolyte, was determined by ASS (atomic absorption) and the rate of release K corresponding to the relationship $$\mathrm{K}=\frac{\mathrm{Ni}-\mathrm{Concentration\; in\; the\; test\; sample\; solution}}{\mathrm{Ni}-\mathrm{Concentration\; in\; the\; reference\; sample\; solution}}$$

calculated with the results of Table II. A comparison of the data shows that the release rates of Experiments 1 to 7 are considerably below the values for the comparative steels of Experiments 8 to 15, which shows the significant difference in the solution stability.

The evaluation of wear resistance and wear resistance was material abrasion during a grinding test. For this purpose cylindrical samples with a diameter of 5 mm were installed in a grinding device and pressed under a contact pressure of 5 N against a grinding wheel rotating slowly at 10 revolutions per minute. The samples were tested both in the dry contact, as well as under rinsing with salt water as electrolyte to also detect the influence of corrosion on the wear resistance. The electrolyte used was a saline solution, as was used for the nickel release rate.

The results of tests 1 to 7 consistently show, according to the data in Table II, that their steels have a K value <1, which indicates that the pitting corrosion or the nickel release rate is lower than in the NiTi comparison alloy. In contrast, the comparison steels 5 to 8 with a K value> 1 show a much greater tendency to salmon cracking corrosion and nickel release than the steels 1 to 4. This is due to the fact that in the small carbides of steels 1 to 4 also only narrow areas of chromium depletion have arisen, which are characterized by Dissipate diffusion easily. The unfavorable effect of large carbides ( Fig. 1 ) with the broad chromium depletion zones and internal stresses surrounding them, is particularly evident from the K values of Experiments 8 and 9 and the "abrasion result wet". All steels (1 to 8, tests 1 to 15) show good and favorable abrasion behavior under dry conditions. With simultaneous action of a salt solution, or a corrosive electrolyte, however, only the fine-grained variants (1 to 4) have turned out to be good, indicating a low corrosion and is supported by a K value <1. In the case of steels with coarse carbides, on the other hand, corrosion reactions and outbreaks as well as material releases ( Fig. 1 ), which indicates a high K value (eg experiments 9, 10, 11).

The experiments thus show that a low nitrogen content is advantageous in order to largely avoid pitting corrosion and nickel release rate. This becomes particularly clear when comparing the steels 3 and 5 (test No. 4, 5 with 8), which have a different nitrogen content for the same carbon content. The advantage of the low nitrogen content of 0.04 to 0.08% lies in the fact that no carbonitrides form, which ensures a good solubility of the primary carbides during annealing. As a result, in the annealing treatment according to the invention ( Fig. 4 ) achieves a low carbide grain size, good corrosion resistance and low nickel release rate.

It is essential to lower the nitrogen content in order to improve the carbide solubility and to avoid the formation of sparingly soluble carbonitrides, ie the incorporation of nitrogen into the carbides. Furthermore, it is essential that in the production of functional elements and structurable parts with a combination of at least two, preferably three (or more) temperature-dropping annealing treatments each with a forming after a final hardening and tempering, a further improvement of the material properties, like the data of Table II in conjunction with the diagram of Fig. 4 occupy.

Claims (8)

1. Use of a martensitic chromium steel with

   0.50 to 0.78% carbon,

   0.20 to 1.30% silicon,

   0.15 to 0.80% nickel,

   0.30 to 1.00% manganese,

   0.01 to 0.035% sulphur,

   16 to 18% chromium,

   1.25 to 1.50% molybdenum,

   0.001 to 0.8% tungsten,

   0.04 to 0.08% nitrogen,

   0.15 to 0.20% vanadium,

   0.001 to 0.04% titanium,

   0.001 to 0.04% niobium,

   0.001 to 0.03% aluminium,

   0.02 to 0.05% copper,

   0.001 to 0.04% boron,

   the remainder iron,

   including impurities introduced by melting, as a contamination- or seawater-resistant material or as a wear-resistant material for the food and beverages industry, for surgical instruments such as scalpels and blades, as a material for the production of filetlng knives for fish processing and for the food and beverages industry and for the production of industrial needles.
2. Use of a steel according to Claim 1, the contents of nitrogen and carbon nevertheless satisfy the condition $$\left(\%\mathrm{Ni}\right)=\mathrm{0.1}{\left(\%\mathrm{C}\right)}^{\mathrm{2}}+\mathrm{0.02.}$$

   
3. Method for heat treatment of a martensitic chromium steel with

   0.50 to 0.78% carbon,

   0.20 to 1.30% silicon,

   0.15 to 0.80% nickel,

   0.30 to 1.00% manganese,

   0.01 to 0.035% sulphur,

   16 to 18% chromium,

   1.25 to 1.50% molybdenum,

   0.001 to 0.8% tungsten,

   0.04 to 0.08% nitrogen,

   0.15 to 0.20% vanadium,

   0.001 to 0.04% titanium,

   0.001 to 0.04% niobium,

   0.001 to 0.03% aluminium,

   0.02 to 0.05% copper,

   0.001 to 0.04% boron,

   the remainder iron,

   including Impurities introduced by melting, characterised by a multi-stage annealing with cooling and cold working between each of the annealing stages and final hardening.
4. Method according to Claim 3, characterised in that the annealing time and the annealing temperature (T) in the first stage are 18 to 24 hours at 1,100 to 1,250°C and in the second stage 0.5 to 2 hours at a temperature of 1,000 to 1,100°C.
5. Method according to Claim 4, characterised in that the annealing time in a third stage is 0.5 to 1 hour and the annealing temperature 720 to 780°C.
6. Method according to any one of Claims 3 to 5, characterised by hardening at 1,000 to 1,080°C with a duration of 10 to 20 minutes.
7. Method according to Claim 6, characterised by preheating of 15 to 35 minutes at 300°C to 600°C.
8. Method according to any one of Claims 4 to 7, characterised by an at least one final tempering for between one and four hours at 100 to 500°C.

Images