WO2016174500A1

WO2016174500A1 - Martensitic stainless steel, method for producing a semi-finished product made from said steel and cutting tool produced from said semi-finished product

Info

Publication number: WO2016174500A1
Application number: PCT/IB2015/053144
Authority: WO
Inventors: Francis Chassagne; Françoise Haegeli
Original assignee: Aperam
Priority date: 2015-04-30
Filing date: 2015-04-30
Publication date: 2016-11-03
Also published as: RU2017137708A3; RU2017137708A; CN107567507A; MX2017013834A; JP6767389B2; CA2984514A1; JP2018521215A; WO2016146857A1; KR20170141250A; UA120119C2; ES2796354T3; BR112017023361B1; EP3289109A1; EP3289109B1; US20180127858A1; BR112017023361A2

Abstract

martensitic stainless steel, characterised in that the composition thereof is, in percentage by weight: 0.10% ≤ C ≤ 0.45%; traces ≤ Mn ≤ 1.0%; traces ≤ Si ≤ 1.0%; traces ≤ S ≤ 0.01 %; traces ≤ P ≤ 0.04%; 15.0% ≤ Cr ≤ 18.0%; traces ≤ Ni ≤ 0.50%; traces ≤ Mo ≤ 0.50%; traces ≤ Cu ≤ 0.50%; traces ≤ V ≤ 0.50%; traces ≤ Nb ≤ 0.03%; traces ≤ Ti ≤ 0.03%; traces ≤ Zr ≤ 0.03%; traces ≤ Al ≤ 0.010%; traces ≤ O ≤ 0.0080%; traces ≤ Pb ≤ 0.02%; traces ≤ Bi ≤ 0.02%; traces ≤ Sn ≤ 0.02%; 0.10%≤ N ≤ 0.20%; C + N ≥ 0.25%; preferably C + N ≥ 0.45%; Cr + 16 N - 5 C ≥ 14.0%; preferably Cr + 16 N - 5 C ≥ 16%; the remainder being iron and impurities resulting from the production. Method for producing a semi-finished product from said martensitic stainless steel, and cutting tool produced from said semi-finished product.

Description

Martensitic stainless steel, method of manufacturing a semi-finished product of this steel and cutting tool made from this semi-finished product

The invention relates to a martensitic stainless steel. This steel is mainly intended for the manufacture of cutting tools, especially cutlery items, such as scalpels, scissors blades, or knife blades or blades of household robots.

Steels for cutlery must have high corrosion resistance, polishing ability and hardness.

The martensitic stainless steels currently used to make the blades of the cutting tools, such as the steels of the types EN 1 .4021, EN 1 .4028 and EN 1 .4034, have Cr contents of less than or equal to 14 or 14, 5% by weight and variable C contents, ie 0.16% -0.25% for EN 1 4021, 0.26-0.35% for EN 1 4028 and 0.43-0.50. % for EN 1 4034. The hardness level of the steel depends mainly on this C content.

When an even better corrosion resistance is sought, the grade EN 1 .4419 at 0.36-0.42% C, 13.0-14.5% Cr and 0.60-1.00% of Mo can be used.

In their manufacture, these steels are typically made in an AOD converter, then continuously cast as slabs, blooms, or billets, and then hot-rolled to a reel, roll bar, or wire rod. They are then annealed to obtain a ferritic structure containing carbides, which is sufficiently soft to allow cold rolling for the flat products, or to facilitate sawing before forging the hot-rolled semi-finished product for long products.

The product then undergoes recrystallization annealing. In this softened state of recrystallized ferrite containing carbides, the product is cut to give it its final shape, for example that of a knife blade, before undergoing a heat treatment comprising a high temperature austenitization, typically between 950 ° C. and 1150 ° C, followed by quenching to room temperature which leads to a predominantly martensitic structure.

In this martensitic state the product has a high hardness, the higher the carbon content is important, but it also has great fragility. A tempering treatment, typically between 100 ° C and 300 ° C, is then performed to reduce brittleness without too much lowering hardness. The blade then undergoes various operations including sharpening and polishing to give it its cutting quality and aesthetic appearance. None of the four shades mentioned at the same time allows a good resistance to corrosion, a good surface condition and a high hardness, for a reasonable cost.

The grade EN 1 .4419 has good corrosion resistance and high hardness, but it is prohibitively expensive due to the addition of Mo in large quantities.

The grade EN 1 .4034 has a high hardness, but also a poor surface appearance after polishing, because of the presence in large numbers of undissolved carbides during austenitization, because of the high C content of this grade. . The corrosion resistance is insufficient because the Cr content is not high enough in the matrix, especially since part of the Cr is trapped in the undissolved carbides. Moreover it happens regularly that the cutting edge of the blade is the seat of a crevice corrosion, from the decohesion of large primary carbides which appear at the end of solidification in continuous casting.

The less loaded grades of C EN 1 4021 and 1 4028 have lower hardnesses, without having sufficient resistance to corrosion due to too low Cr contents.

The present invention aims to solve the problems mentioned above. In particular, it aims to provide a martensitic stainless steel for cutting tool as economical as possible, which however has both good corrosion resistance, good polishing ability and high hardness.

For this purpose the invention relates to a martensitic stainless steel, characterized in that its composition is, in percentages by weight:

- 0.10% ≤ C≤ 0.45%; preferably 0.10% ≤ C≤ 0.40%; better 0.10%

0.25%;

- traces≤ Mn≤ 1, 0%;

- traces≤ Si≤ 1, 0%;

- traces≤ S≤ 0.01%;

- traces≤ P≤0.04%;

- 15.0% ≤Cr≤ 18.0%;

- traces≤ Ni≤ 0.50%;

- traces≤ Mo≤ 0.50%;

- traces≤ Cu≤ 0.50%;

- traces≤ V≤ 0.50%;

- traces≤Nb≤0.03%;

- traces ≤ Ti ≤ 0.03%;

- traces≤Zr≤0.03%;

- traces≤ Al≤ 0.010%; - traces ≤ 0 ≤ 0.0080%;

- traces≤ Pb≤ 0.02%;

- traces≤ Bi≤ 0.02%;

- traces≤ Sn≤ 0.02%;

- 0.10% ≤N≤0.20%;

- C + N≥ 0.25%; preferably C + N≥ 0.45%;

- Cr + 16 N - 5 C ≥ 14.0%; preferably Cr + 16 N - 5 C≥16%;

the rest being iron and impurities resulting from the elaboration.

Its Cr content can be between 15 and 17%.

Its microstructure preferably comprises at least 75% of martensite

The subject of the invention is also a process for producing a martensitic stainless steel semi-finished product, characterized in that:

a semi-finished product is produced and cast into a steel having the preceding composition;

said semi-product is heated to a temperature greater than or equal to 1000 ° C .;

- It is hot rolled to obtain a sheet, bar or wire machine;

said sheet, said bar or said machine wire is annealed at a temperature of between 700 and 900 ° C .;

and performing a shaping operation on said sheet, said bar or said wire rod.

Said half-product may be a sheet, and said forming operation may be cold rolling.

Said half-product may be a bar or a wire rod, and said shaping operation may be forging.

Said semi-shaped product, if its Cr content is between 15 and 17%, can then be austenitized between 950 and 1150 ° C, and then cooled at a rate of at least 15 ° C / s to a temperature lower than or equal to 20 ° C, then undergoes an income at a temperature between 100 and 300 ° C.

Said semi-finished product can then be austenitized between 950 and 1150 ° C, then cooled at a rate of at least 15 ° C / s to a temperature of 20 ° C or lower, and then undergoes a treatment. cryogenic at a temperature of -220 to -50 ° C, then a tempering at a temperature of between 100 and 300 ° C.

The invention also relates to a cutting tool, characterized in that it was made from a semi-finished product prepared according to the preceding method.

The cutting tool may be a cutlery article such as a knife blade, a food processor blade, a scalpel, or a scissors blade, As will be understood, the invention consists in using, in order to produce the cutting tool, a martensitic stainless steel of particular composition, free from expensive elements at high levels, but containing relatively large amounts of nitrogen located in a well defined range. Also, a particular balancing of the contents of Cr, C and N is necessary.

Other characteristics and advantages of the invention will become apparent from the following description given by way of example and with reference to the appended FIG. 1, which shows the evolution of the Vickers hardness of steel under a load of 1 kg, depending on the martensite rate after austenitization, quenching and tempering, of a steel according to the invention.

With regard to the chemical composition of the steel according to the invention, the following justifications are advanced.

C increases martensitic hardness after austenitization, quenching and tempering. However, it also promotes the precipitation of primary carbides M ₇ C ₃ during solidification, which can be removed during polishing or sharpening of the blade, which degrades the surface appearance of the product. The sites where they were before polishing can also become the site of cavernous corrosion. Excessive C content also leads, according to the austenitization temperature, either to a C content that is too high in the austenitic matrix that no longer makes it possible to obtain a sufficient fraction of martensite after quenching, or to the persistence of M ₂ carbides. 3C ₆ undissolved which deplete the austenitic matrix in Cr. They thus reduce the resistance to corrosion and impair polishability.

The C content must be at least 0.10% to obtain sufficient hardness and at most 0.45% to obtain good corrosion resistance and a satisfactory surface appearance after polishing. Depending on the casting process used, however, it may be useful to limit a little more the maximum content of C in case this process might not guarantee homogeneity of the solidifying steel which would be sufficient to avoid a precipitation of primary carbides M ₇ C ₃ . In this case, it is advisable to limit the C content to 0.40%, better still to 0.25%.

In addition, the content of C must satisfy formulas linking it with the N content and with the N and Cr contents, as will be explained below.

Mn is a so-called gammagenic element because it stabilizes the austenitic structure. An excessive content of Mn leads to an insufficient martensite rate after austenitization and quenching treatment, which leads to a decrease in hardness. For this reason the Mn content must be less than or equal to 1.0%.

Si is a useful element in the process of making steel. It is very reducing, and it allows to reduce the Cr oxides in the reduction phase of the steel that follows the decarburization phase in the AOD converter. However, the Si content in the final steel must be less than or equal to 1.0%, since this element has a heat-curing effect which limits the possibilities of hot deformation during hot rolling or during forging.

S and P are impurities that decrease hot ductility. P easily segregates at the grain boundaries and facilitates their decohesion. In addition, S reduces the resistance to pitting corrosion by forming compounds with Mn which serve as initiating sites for this type of corrosion. In this respect, the contents of S and P must respectively be less than or equal to 0.01% and 0.04% by weight.

Cr is an essential element for corrosion resistance. However, its content must be limited because a high content may lower the temperature Mf martensitic transformation end below the ambient temperature. This would lead, after austenitization and quenching to room temperature, to a martensitic transformation that is too incomplete and to an insufficient hardness. For these reasons, the Cr content must be between 15% and 18% by weight. However, it is advisable to limit the Cr content to 17% when cryogenic treatment of steel is not performed.

The Cr content must also satisfy a formula linking it to the N and C contents as will be explained later.

The elements Ni, Cu, Mo and V are expensive and also reduce the temperature Mf.

The content of each of these elements must therefore be limited to 0.50% by weight. It is therefore not necessary to add after the merger of raw materials.

Nb, Ti and Zr are so-called "stabilizing" elements, which means that they form, in the presence of N and C and at high temperature, carbides and nitrides more stable than the carbides and nitrides of Cr. These elements are however undesirable because their respective carbides and nitrides once formed during the manufacturing process, can not be easily dissolved during austenitization, which limits the levels of C and N in the austenite, and therefore the hardness corresponding martensite after quenching. The content of each of these elements must therefore be less than or equal to 0.03%.

The Al content must likewise be less than or equal to 0.010% to avoid forming nitrides of AI, whose dissolution temperature would be too high and which would reduce the N content of the austenite, hence the hardness of martensite after quenching.

The O content results from the process of making the steel and its composition. It must be limited to 0.0080% (80 ppm) maximum, so as to avoid forming too many and / or too large oxide inclusions, which could constitute privileged sites of initiation of pitting corrosion, and also take off shoes during the polishing so that the surface appearance of the product would not be satisfactory. The O content also influences the mechanical properties of the steel, and it may optionally, in a conventional manner, set a limit not to exceed lower than 80 ppm according to the requirements of users of the final product.

The contents of Pb, Bi and Sn must not exceed 0.02% each in order not to make the hot transformations too difficult.

The control of the N content at a well defined level is an essential element of the invention. Like C, it allows, when in solid solution, to increase the hardness of martensite without having the disadvantage of forming precipitates during solidification. Nitrides are formed at lower temperatures than carbides, which makes them easier to dissolve during austenitization. The presence of N in solid solution also improves the resistance to corrosion.

However an excessive content of N no longer allows its complete dissolution during solidification, and leads to the formation of N ₂ bubbles detrimental to the internal health of the metal.

For these reasons, the N content must be between 0.10 and 0.20% by weight.

The N content must also satisfy various formulas that bind it to Cr contents and

vs.

Indeed, the hardness of the martensite depends on its contents in C and N. The inventors have shown that the hardening effects of these two elements are similar, and therefore that the hardness of the martensite is dependent on its overall content. C + N. It has been established by the inventors that the hardness after quenching and tempering will be sufficient if the following formula is respected:

C + N≥ 0.25%

In a preferred embodiment of the invention, an even higher hardness is obtained after quenching and tempering if the following formula is respected:

C + N≥ 0.45%.

Three elements have an effect on the corrosion resistance. Cr and N are beneficial, while C has a negative effect because it is generally not possible to dissolve all the carbides of Cr during austenitization, for reasons of productivity and cost that limit in industrial practice the duration and the temperature of the treatment. Undissolved Cr carbides reduce the Cr content of the austenitic matrix, and thereby reduce the corrosion resistance.

From the study of the corrosion resistance of martensitic steels at different weight contents in Cr, N and C, the inventors have found a formula combining these different elements that ensures good resistance to corrosion:

Cr + 16 N - 5 C≥ 14.0%

In a preferred embodiment of the invention, even higher corrosion resistance is achieved if the following formula is followed:

Cr + 16 N - 5 C≥ 16.0%

It should be understood that the preferential values of C + N and Cr + 16 N - 5 C may not be respected simultaneously. At the same time, C + N 0,4 0.45% and Cr + 16 N -5 C may be between 14 and 16.0% if it is desired to favor an optimum hardness and to accept a corrosion resistance corresponding only to the minimum required by the invention. . Cr + 16 N - 5 C ≥ 16.0% and C + N may also be simultaneously between 0.25 and 0.45% if, on the contrary, optimum corrosion resistance is desired but hardness corresponding only to minimum required by the invention.

Steels according to the invention have been subjected to austenitization tests at different temperatures before quenching with water at 20 ° C. with a cooling rate greater than 100 ° C./s, followed by an income of 200 ° C. ° C, in order to vary the proportion of dissolved carbides, and consequently the carbon content in the austenite then in martensite after quenching. The martensite rate and the Vickers hardness were measured in order to plot the evolution of the hardness as a function of the martensite content, and the results are shown in FIG. 1, for a steel having the composition of Example 16 of table 1.

We see in Figure 1 that the hardness begins to grow with the drop in the martensite rate, because martensite hardens by carbon enrichment. The hardness reaches a maximum, then decreases when the martensite rate becomes too low. Below 75% of martensite, the hardening of martensite no longer offsets the softening associated with the presence of residual austenite of lower hardness. For this reason, in a preferred embodiment of the invention, suitable for the manufacture of cutting tool from cast steel, the martensite rate of the steel after austenitization, quenching at a speed of at least 15 ° C / s up to a temperature below or equal to 20 ° C, and then returned to a temperature of 100 to 300 ° C, typically 200 ° C, is greater than or equal to 75%.

Achieving a high martensite content of up to 100% can be better ensured if, after quenching to 20 ° C or less, cryogenic treatment, i.e. quenching in a very low temperature medium ranging from -220 to -50 ° C, typically in liquid nitrogen at -196 ° C or in dry ice at -80 ° C, before proceeding to 100-300 ° C. When the martensite content does not reach 100%, the remaining microstructure typically consists essentially of residual austenite. There may also be ferrite.

By way of nonlimiting examples, the following results will show the advantageous characteristics conferred by the invention.

The compositions of the various samples of steel tested are shown in Table 1, expressed in% by weight. The underlined values are those which do not conform to the invention. The values of C + N and Cr + 16 N - 5 C were also reported for each sample.

Nb Ti Al Zr Sn 0 N C + N Cr + 16N - 5C

11 0.004 0.004 0.002 0.001 0.008 0.002 0.197 0.301 17.73

12 0.004 0.002 0.001 0.002 0.006 0.002 0.192 0.304 19.21

13 0.002 0.001 0.002 0.002 0.012 0.002 0.105 0.351 15.65

14 0.002 0.002 0.001 0.001 0.009 0.003 0.194 0.438 16.98

0.003 0.001 0.003 0.001 0.008 0.002 0.106 0.553 14.86

Invention 16 0.002 0.003 0.003 0.002 0.015 0.002 0.102 0.545 16.22

17 0.005 0.003 0.001 0.001 0.016 0.003 0.194 0.639 16, 18

18 0.003 0.002 0.002 0.001 0.007 0.003 0.184 0.594 17.69 19 0.003 0.002 0.002 0.001 0.007 0.003 0.175 0.607 18.54

R1 0.005 0.003 0.003 0.002 0.006 0.003 0.002 0.225 12.32

R2 0.002 0.002 0.003 0.001 0.01 1 0.002 0.003 0.315 12.29

R3 0.005 0.004 0.002 0.001 0.010 0.001 0.003 0.481 1 1, 36

R4 0.003 0.004 0.001 0.002 0.006 0.002 0.109 0.501 13.68

R5 0.002 0.001 0.002 0.002 0.009 0.004 0.197 0.495 15.96

References R6 0.004 0.002 0.001 0.001 0.013 0.003 0.032 0.497 14.51

R7 0.003 0.002 0.001 0.001 0.014 0.003 0.253 0.658 18.12

R8 0.005 0.002 0.002 0.002 0.012 0.003 0.198 0.718 16.97

R9 0.002 0.001 0.001 0.001 0.008 0.002 0.195 0.643 19.38

R10 0.002 0.003 0.003 0.001 0.006 0.002 0.1 14 0.226 16.36

R1 1 0.003 0.001 0.002 0.002 0.01 1 0.003 0.1 12 0.235 17.88

Table 1: Compositions of the samples tested

After casting, these steels were heated to a temperature above 1100 ° C, hot rolled to a thickness of 3mm, annealed at a temperature of 800 ° C, and then pickled and cold rolled to a thickness of 1, 5mm.

The steel sheets were then annealed at a temperature of 800 ° C. The annealed steel sheets were then subjected to a 15-minute austenitization treatment at 1050 ° C. followed by quenching with water to a temperature of 20 ° C.

After cutting the sheets into two parts, one of the parts was then immersed for 10 minutes in a bath thermostated at -80 ° C., so as to be able to evaluate the effects of a cryogenic treatment which would be added to the simple quenching. the water.

An income of 1 h at 200 ° C was then made on each part of the sheet.

Table 2 shows the results of tests and observations made on these steels. The underlined values correspond to performances deemed insufficient. The "+" correspond to results deemed satisfactory, the "-" correspond to results deemed insufficient.

Internal health is evaluated on a post-pouring solidification state, knowing that subsequent processing operations will not degrade it.

The martensite rate is measured after quenching with water at 20 ° C. and after a cryogenic quenching treatment at -80 ° C., this quenching, or the second of these quenchings, having been followed by a tempering at 200 ° C. . When the martensite content is greater than or equal to 75% after quenching with water at 20 ° C., the other results given in Table 2 relate to the quenched state at 20 ° C. followed by the tempering at 200 ° C. When the martensite content is less than 75% after quenching with water at 20 ° C., the other results given in Table 2 concern the state after a cryogenic treatment (quenching to a very high temperature). low temperature, for example in dry ice) at -80 ° C, followed by income at 200 ° C.

The corrosion resistance is evaluated by an electrochemical pitting corrosion test in a medium composed of 0.02M NaCl, at 23 ° C. and at a pH of 6.6. The electrochemical test carried out on 24 samples to determine the potential E ₀ .i for which the probability element of pitting is equal to 0.1 cm ^"2. The corrosion resistance is considered unsatisfactory if the potential E ₀ is less .i 350 mV, measured against the saturated calomel electrode at KCI (350 mV / SCE). it is considered satisfactory if the potential E ₀ .i is between 350 mV / SCE 450 mV / SCE. it is considered to be very satisfactory if the potential E ₀ .i is greater than 450 mV / ECS.

The Vickers hardness is measured in the thickness on a mirror-polished cut, under a load of 1 kg with a square base diamond pyramidal tip, according to EN ISO 6507. The average hardness obtained is calculated by making 10 impressions. Hardness is considered unsatisfactory if the average hardness is less than 500 HV. It is considered satisfactory if the average hardness is between 500 HV and 600 HV. It is considered very satisfactory if the average hardness is greater than 600 HV.

The polishability is evaluated by carrying out a flat polishing up to the mid-thickness of the sample, successively using the SiC papers 180, 320, 500, 800 and 1200 under a force of 30 N, then a polishing on a soaked cloth of diamond paste of particle size 3 μηι then 1 μηι under a force of 20 N. The surface is then observed under an optical microscope at the magnification of x100. The polishability is considered unsatisfactory if the density of defects conventionally called "comet tails" is greater than 100 / cm ² . The polishability is considered satisfactory if this density is between 10 / cm ² and 100 / cm ² . The polishability is considered very satisfactory if this density is less than 10 / cm ² .

The internal health is evaluated by observing the raw steel of solidification in section by optical metallography at magnification x25. Internal health is not satisfactory if globular holes showing the formation of nitrogen bubbles on solidification are observed. Otherwise, internal health is considered satisfactory.

The martensite rate is determined by X-ray diffraction by measuring the intensity of the lines characteristic of martensite compared to the intensity of the lines characteristic of austenite knowing that they are the only two phases in the presence. A martensite content greater than or equal to 75% after quenching at 20 ° C. and returned to 200 ° C., or greater than or equal to 75% after quenching at 20 ° C., a treatment cryogenic at -80 ° C and an income at 200 ° C, is satisfactory. If a martensite level of 75% or more can not be achieved by any of these treatments, the sample is considered unsatisfactory.

Table 2: Results of Tests on Table 1 Samples

The steels according to the invention 11 to 18 combine good properties of resistance to corrosion, hardness and polishability, and have good internal health, as well as a martensite content greater than or equal to 75% after quenching. ° C. The steel according to the invention 19 combines good properties of resistance to corrosion, hardness and polishability, and has good internal health and a martensite rate greater than or equal to 75%, but provided to perform cryogenic treatment at -80 ° C. Indeed, after a simple quenching with water at 20 ° C, the martensite rate is not yet sufficient, which is to relate to the presence of Cr at a higher level than other samples according to the invention.

The reference steels R1 to R3 have Cr and N contents, as well as insufficient C + N and / or Cr + 16 N - 5 C sums, which does not allow a satisfactory corrosion resistance.

The R4 and R5 reference steels have insufficient Cr contents. Without compensation by addition of N, the steel R4 also has a combination Cr + 16 N - 5 C insufficient leading to an unsatisfactory corrosion resistance. For R5 steel, compensating for lack of Cr by adding N restores satisfactory corrosion resistance, but no longer ensures good internal health.

The R6 reference steel has a high C content and an insufficient N content. The excessively high C content does not allow sufficient polishing ability due to excessive carbide formation.

The reference steel R7 has too high a N content, which degrades internal health.

The R8 reference steel has an excessive C content, which leads to poor polishability and a low martensite rate even after cryogenic quenching at -80 ° C.

The R9 reference steel contains too much Cr, which leads to an insufficient martensite rate even after cryogenic quenching at -80 ° C.

The reference steels R10 and R1 1 have too low C contents as well as insufficient C + N sums, leading to too low hardnesses.

The steels according to the invention are used with advantage for the manufacture of cutting tools, such as for example scalpels, scissors, knife blades or circular blades of household robots.

Claims

1. - Martensitic stainless steel, characterized in that its composition is, in percentages by weight:

- 0.10% ≤ C≤ 0.45%; preferably 0.10% ≤ C≤ 0.40%; better 0.10% ≤ C≤

0.25%;

- traces≤ Mn≤ 1, 0%;

- traces≤ Si≤ 1, 0%;

- traces≤ S≤ 0.01%;

- traces≤ P≤0.04%;

- 15.0% ≤Cr≤ 18.0%;

- traces≤ Ni≤ 0.50%;

- traces≤ Mo≤ 0.50%;

- traces≤ Cu≤ 0.50%;

- traces≤ V≤ 0.50%;

- traces≤Nb≤0.03%;

- traces ≤ Ti ≤ 0.03%;

- traces≤Zr≤0.03%;

- traces≤ Al≤ 0.010%;

- traces ≤ 0 ≤ 0.0080%;

- traces≤ Pb≤ 0.02%;

- traces≤ Bi≤ 0.02%;

- traces≤ Sn≤ 0.02%;

- 0.10% ≤N≤0.20%;

- C + N≥ 0.25%; preferably C + N≥ 0.45%;

- Cr + 16 N - 5 C ≥ 14.0%; preferably Cr + 16 N - 5 C≥16%;

the rest being iron and impurities resulting from the elaboration.

2. - Steel according to claim 1, characterized in that its weight content of Cr is between 15 and 17%.

3. - Steel according to claim 1, characterized in that its microstructure comprises at least 75% of martensite.

4. - Process for producing a martensitic stainless steel semi-finished product, characterized in that:

a semi-finished product is produced and cast in a steel having the composition according to one of claims 1 or 2;

said semi-product is heated to a temperature greater than or equal to 1000 ° C .; - It is hot rolled to obtain a sheet, bar or wire machine;

and performing a shaping operation on said sheet, said bar or said wire rod.

5. - Method according to claim 4, characterized in that said half-product is a sheet, and in that said shaping operation is a cold rolling.

6. Method according to claim 4, characterized in that said half-product is a bar or a wire rod, and in that said shaping operation is a forging.

7. - Method according to one of claims 4 to 6, characterized in that the steel has a composition according to claim 2, in that said semi-shaped product is then austenitized between 950 and 1150 ° C, then cooled at a rate of at least 15 ° C / s to a temperature of less than or equal to 20 ° C and then tempered at a temperature of between 100 and 300 ° C.

8. - Method according to one of claims 4 to 6, characterized in that the steel has a composition according to claim 1 or 2, in that said semi-shaped product is then austenitized between 950 and 1150 ° C, then cooled at a rate of at least 15 ° C / s to a temperature below or equal to 20 ° C, and then undergoes a cryogenic treatment at a temperature of -220 to -50 ° C, and then an income at a temperature of between 100 and 300 ° C.

9. - cutting tool, characterized in that it was made from a semi-finished product prepared according to the method of one of claims 4 to 8.

10. - Cutting tool according to claim 9, characterized in that it is a cutlery article such as a knife blade, a household robot blade, a scalpel, or a scissors blade,