EP3289109A1

EP3289109A1 - Martensitic stainless steel, method for the production of a semi-finished product from said steel, and cutting tool produced from the semi-finished product

Info

Publication number: EP3289109A1
Application number: EP16724302.1A
Authority: EP
Inventors: Francis Chassagne; Françoise Haegeli
Original assignee: Aperam SA
Current assignee: Aperam SA
Priority date: 2015-04-30
Filing date: 2016-04-29
Publication date: 2018-03-07
Anticipated expiration: 2036-04-29
Also published as: KR20170141250A; ES2796354T3; CA2984514A1; JP2018521215A; WO2016146857A1; US20180127858A1; MX2017013834A; RU2017137708A3; BR112017023361A2; WO2016174500A1; EP3289109B1; JP6767389B2; RU2017137708A; UA120119C2; CN107567507A; BR112017023361B1

Abstract

The invention relates to a martensitic stainless steel characterised in that its composition consists of, by weight percent: 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%; Cr + 16 N - 5 C ≥ 16.0%; preferably 17 Cr + 500 C + 500 N ≤ 570%; the remainder being iron and impurities resulting from production. The invention also relates to a method for the production of a semi-finished product from the aforementioned martensitic stainless steel, and a 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 or VOD 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 consists of, in weight percentages:

- 0.10% <C <0.45%; preferably 0.20% <C <0.38%; better 0.20% <C <0.35%; optimally 0.30% <C <0.35%;

- traces <Mn <1, 0%; preferably traces <Mn <0.6%;

- traces <If <1, 0%;

- traces <S <0.01%; preferably traces <S <0.005%;

- traces <P <0.04%;

- 15.0% <Cr <18.0%; preferably 15.0 <Cr <17.0%; better 15.2% <Cr <17.0%; even better 15.5% <Cr <16.0%;

- traces <Ni <0.50%;

- traces <Mo <0.50%; preferably traces <Mo <0.1%; better traces <Mo <

0.05%;

- traces <Cu <0.50%; preferably traces <Cu <0.3%;

- traces <V <0.50%; preferably traces <V <0.2%;

- 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%; preferably 0.15% <N <0.20%;

- C + N≥ 0.25%; preferably C + N ≥ 0.30%; better C + N≥ 0.45%;

- Cr + 16 N - 5 C≥ 16.0%;

preferably 17 Cr + 500 C + 500 N <570%;

the rest being iron and impurities resulting from the elaboration.

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 half-shaped product, if its Cr content is between 15 and 17%, can then be austenitized between 950 and 1150 ° C, and then cooled to a speed of at least

15 ° C / s to a temperature below 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 below or equal to 20 ° C, and then undergoes a cryogenic treatment at a temperature of -220 to -50 ° C, then an income 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 features 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. It should be clear that the ranges of contents of the various elements considered as preferential are independent of each other, and that any combination of the ranges defined in the description which follows is conceivable within the scope of the invention, provided that individual levels of C, N and Cr that they would allow simultaneously can respect the relations which must bind them according to the invention.

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. An excessive C content also leads, according to the austenitization temperature, either to a C content too high in the austenitic matrix which no longer makes it possible to obtain a sufficient fraction of martensite after quenching, or to the persistence of carbides M ₂₃ C ₆ undissolved which deplete the austenitic Cr matrix. They thus reduce the resistance to corrosion and impair polishability.

The C content must therefore 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. However, depending on the casting and solidification process used, it may be useful to limit the maximum C content a little further, in the event that this process might not guarantee homogeneity of the solidifying steel which would be sufficient to prevent precipitation of primary carbides M ₇ C ₃ . In this case, it is advisable to limit the C content to 0.38%. Of preferably 0.20% <C <0.38%, better 0.20% <C <0.35%, optimally 0.30% <C <0.35%.

The optimal range, in particular, allows to have a high hardness while limiting the formation of carbides in acceptable proportions, the possible loss of hardness resulting from the lowering of the maximum content of C compared to the most general range which can be compensated by the presence of sufficient nitrogen for this purpose, as will be seen later.

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 between traces resulting from the elaboration and 1.0%. Preferably its content is limited to 0.6% to help obtain an optimally low Ms temperature.

Si is a useful element in the process of making steel. It is very reducing, and it therefore reduces the Cr oxides in the steel reduction phase following the decarburization phase in the AOD or VOD converter. However, the Si content in the final steel must be between traces and 1.0% since this element has a heat-setting effect which limits the possibilities of hot deformation during hot rolling or during forging. Preferably its content is limited to 0.6% to help obtain an optimally low Ms temperature.

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. As such, the contents of S and P must respectively be between traces and, respectively, 0.01% and 0.04% by weight. Preferably, the S content does not exceed 0.005% to further ensure sufficient corrosion resistance.

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 temperature) below 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 15.0% and 18.0% by weight. However, it is advisable to limit the Cr content to 15.0-17.0%, better 15.2-17.0%, even better 15.5-16.0%, especially when a cryogenic treatment of steel is not carried out, so as not to have a temperature Ms of martensitic transformation start too high, and therefore not to leave too much residual austenite which would limit the hardness, so the tensile strength Rm, which is not desirable on a martensitic steel. If necessary, the reduction in the corrosion resistance induced by the reduction in the maximum Cr content may be offset by a high N content within the limits otherwise prescribed.

However, the solubility of N in the liquid metal decreases when the content of Cr decreases, so that it is no longer possible below 15% of Cr to keep enough dissolved N in the liquid metal at the solidification temperature of the steel, which leads to the formation of N ₂ bubbles during solidification, and no longer allows N to compensate for the decline of Cr with respect to the corrosion resistance. This low Cr limit for the solubility of N also increases when the ferrostatic pressure at solidification decreases. It may be preferable to increase the minimum Cr content from 15.0% to 15.2% or 15.5% depending on the type of casting process and the casting conditions used to guard against any risk of formation. N ₂ bubbles.

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, between traces and 0.50% by weight, preferably at most 0.10% for Mo. It is therefore not necessary to add after the merger raw materials. It is even more favorable that the Mo content does not exceed 0.05%, to help obtain an optimally low Ms temperature. For the same reason, it is preferable that the Cu content does not exceed 0.3%, and that the V content does not exceed 0.2%.

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 no longer be easily dissolved during austenitization, which limits the levels of C and N in the austenite, and therefore the corresponding hardness of martensite after quenching. The content of each of these elements must therefore be between traces and 0.03%.

The Al content must likewise be between traces and 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. martensite after quenching. The O content results from the process of making the steel and its composition. It must be between traces and not more than 0.0080% (80 ppm), so as to avoid forming too many and / or too large oxide inclusions, which could constitute privileged sites of initiation of corrosion. by stitching, and also take off during 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, depending on the requirements of users of the final product.

The contents of Pb, Bi and Sn can be limited to traces resulting from the elaboration, and must not each exceed 0.02% so as 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. If it is not desired a C content too high not to form too much precipitates, an addition of N compensates for the loss of hardness. 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 which form blisters (porosities) during the solidification of the steel, detrimental to the internal health of the metal .

For these various reasons the N content must be between 0.10 and 0.20% by weight, preferably between 0.15 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%, preferably C + N≥0.30%

In an even more 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 with different weight contents in Cr, N and C, the inventors have found a formula combining these various elements which makes it possible to ensure very good resistance to corrosion:

Cr + 16 N - 5 C≥ 16.0%

A preferred condition, though not mandatory, is that:

17Cr + 500C + 500N <570%

This condition makes it possible to ensure that one will have a temperature not too high, as its respect would represent a lowering of Ms of the order of 60 ° C compared to what would authorize the simultaneous satisfaction of the upper limits of the contents in C, N and Cr chosen.

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 14 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 up to 20 ° C or less, treatment is carried out cryogenic, that is to say the 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, Cr + 16 N - 5 C and 17Cr + 500C + 500N were also reported for each sample.

C Mn Si P Si Ni Cr Cu Mo V

11 0, 104 0.36 0.26 0.007 0.003 0.29 15.1 0.21 0.03 0.08

12 0, 1 12 0.47 0.43 0.012 0.003 0.34 16.7 0, 16 0.03 0.09

13 0.244 0.29 0.30 0.009 0.002 0.37 15.1 0, 10 0.03 0.07

14 0.443 0.36 0.31 0.024 0.003 0.26 16.8 0.24 0.02 0.13

0.445 0.32 0.29 0.009 0.001 0.34 15.3 0.22 0.03 0.1 1

16 0.410 0.39 0.42 0.007 0.001 0.41 16.8 0, 18 0.02 0.09

17 0.432 0.39 0.42 0.007 0.001 0.41 17.9 0, 18 0.02 0.09

18 0.345 0.31 0.38 0.010 0.001 0.25 15.3 0, 18 0.02 0.07

19 0.332 0.38 0.27 0.006 0.002 0.34 15.8 0.23 0.03 0.10

Invention 110 0.340 0.26 0.32 0.009 0.001 0.28 16.3 0.23 0.02 0.09

11 1 0.342 0.28 0.30 0.012 0.001 0.39 17.8 0, 14 0.02 0.08

112 0.376 0.34 0.35 0.015 0.003 0.30 16.1 0, 16 0.02 0.1 1

113 0.335 0.29 0.32 0.007 0.002 0.28 15.9 0.20 0.03 0.07

114 0.442 0.38 0.29 0.010 0.002 0.36 16.0 0, 14 0.03 0.06

115 0.245 0.34 0.33 0.016 0.001 0.40 16.1 0.01 0.02 0.10

116 0.366 0.28 0.28 0.013 0.002 0.29 16.0 0, 1 1 0.03 0.07

117 0.356 0.30 0.31 0.019 0.003 0.21 17.3 0, 18 0.02 0.12

118 0, 163 0.27 0.40 0.01 1 0.001 0.33 16.0 0.20 0.03 0.06

119 0.239 0.33 0.29 0.010 0.002 0.32 15.9 0, 15 0.03 0.07

R1 0.223 0.38 0.35 0.012 0.003 0, 18 13.4 0, 12 0.02 0.08

R2 0.312 0.33 0.42 0.008 0.001 0.35 13.8 0.08 0.03 0.09

R3 0.478 0.42 0.28 0.017 0.002 0.21 13.7 0, 13 0.02 0.1 1

References

R4 0.392 0.35 0.24 0.021 0.001 0.37 13.9 0.24 0.03 0.21

R5 0.298 0.26 0.35 0.006 0.002 0.36 14.3 0, 18 0.02 0.13

R6 0.465 0.27 0.43 0.007 0.002 0.28 16.3 0.28 0.02 0.08 R7 0.405 0.46 0.46 0.015 0.002 0.43 16.1 0, 14 0.02 0.07

R8 0.520 0.30 0.24 0.018 0.001 0.41 16.4 0, 19 0.03 0.14

R9 0.448 0.39 0.29 0.024 0.001 0.26 18.5 0, 14 0.02 0.09

R10 0, 1 12 0.27 0.34 0.010 0.001 0.34 15.1 0.07 0.02 0.10

R1 1 0.447 0.34 0.34 0.018 0.002 0.24 15.4 0, 14 0.02 0.17

R12 0.246 0.18 0.41 0.019 0.001 0.36 15.2 0, 14 0.02 0.10

R13 0, 123 0.41 0.31 0.016 0.002 0.38 16.7 0.23 0.02 0.23

R14 0.21 1 0.27 0.34 0.009 0.003 0.24 16.2 0, 15 0.02 0.10

17Cr +

Cr + 16N 500C +

Nb Ti Al Zr Sn 0 N C + N

- 5C 500N (preferred)

11 0.004 0.004 0.002 0.001 0.008 0.002 0.197 0.301 17.73 407.2

12 0.004 0.002 0.001 0.002 0.006 0.002 0.192 0.304 19.21 435.9

13 0.002 0.002 0.001 0.001 0.009 0.003 0.194 0.438 16.98 475.7

14 0.002 0.003 0.003 0.002 0.015 0.002 0.102 0.545 16.22 558.1

15 0.005 0.003 0.001 0.001 0.016 0.003 0.194 0.639 16, 18 579.6

16 0.003 0.002 0.002 0.001 0.007 0.003 0.184 0.594 17.69 582.6

17 0.003 0.002 0.002 0.001 0.007 0.003 0.175 0.607 18.54 607.8

18 0.003 0.005 0.002 0.001 0.006 0.001 0.179 0.524 16.44 522.1

19 0.002 0.002 0.003 0.001 0.010 0.003 0.176 0.508 16.96 522.6

Invention 10 0.004 0.004 0.002 0.002 0.012 0.002 0.180 0.520 17.48 537.1

1 1 0.003 0.003 0.002 0.002 0.009 0.002 0.178 0.520 18.94 562.6

12 0.004 0.002 0.001 0.001 0.013 0.001 0.182 0.558 17, 13 552.7

13 0.002 0.001 0.002 0.001 0.006 0.003 0.125 0.460 16.23 500.3

14 0.002 0.003 0.003 0.002 0.008 0.003 0.177 0.619 16.62 581, 5

15 0.003 0.002 0.001 0.001 0.010 0.002 0.105 0.350 16.56 447.2

16 0.002 0.003 0.002 0.002 0.007 0.003 0.134 0.500 16.31 522.0

17 0.004 0.005 0.002 0.001 0.01 1 0.003 0.106 0.462 17.22 525.1

18 0.003 0.004 0.002 0.001 0.010 0.003 0.1 12 0.275 16.98 409.5

19 0.003 0.002 0.001 0.002 0.012 0.002 0.164 0.403 17.33 471.8

R1 0.005 0.003 0.003 0.002 0.006 0.003 0.002 0.225 12.32 340.3

R2 0.002 0.002 0.003 0.001 0.01 1 0.002 0.003 0.315 12.29 392.1

R3 0.005 0.004 0.002 0.001 0.010 0.001 0.003 0.481 1 1, 36 473.4

R4 0.003 0.004 0.001 0.002 0.006 0.002 0.109 0.501 13.68 483.4

R5 0.002 0.001 0.002 0.002 0.009 0.004 0.197 0.495 15.96 490.6

References

R6 0.004 0.002 0.001 0.001 0.013 0.003 0.032 0.497 14.51 525.6

R7 0.003 0.002 0.001 0.001 0.014 0.003 0.253 0.658 18, 12 602.7

R8 0.005 0.002 0.002 0.002 0.012 0.003 0.198 0.718 16.97 637.8

R9 0.002 0.001 0.001 0.001 0.008 0.002 0.195 0.643 19.38 636.0

R10 0.002 0.003 0.003 0.001 0.006 0.002 0.1 14 0.226 16.36 369.7 R1 1 0.003 0.001 0.003 0.001 0.008 0.002 0.106 0.553 14.86 538.3

R12 0.002 0.001 0.002 0.002 0.012 0.002 0.105 0.351 15.65 433.9

R13 0.003 0.001 0.002 0.002 0.01 1 0.003 0.1 12 0.235 17.88 401, 4

R14 0.002 0.002 0.003 0.001 0.01 1 0.002 0.217 0.428 18.62 489.4

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.

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 low temperature, carried out by 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 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 insufficient if the average hardness is less than 500 HV. It is considered satisfactory if the average hardness is between 500 HV and 550 HV. It is considered very satisfactory if the average hardness is between 551 and 600 HV. It is considered excellent 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. Polishability is considered unsatisfactory if the density of defects conventionally called "comet tails" is greater than or equal to 100 / cm ² . The polishability is considered satisfactory if this density is between 10 / cm ² and 99 / cm ² . The polishability is considered very satisfactory if this density is between 1 and 9 / cm ² . The polishability is considered excellent if this density is less than 1 / cm ² .

The internal health is evaluated by observing the raw steel of solidification in section by optical metallography at magnification x25. The internal health is unsatisfactory and indicated by the value "0" in Table 2 if globular cavities (blowholes) reflecting the formation of nitrogen bubbles on solidification are observed. Otherwise the internal health is considered satisfactory and indicated by the value "1" in Table 2.

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, in all the samples examined, these are the only two phases in presence. In general, it would not be excluded that other phases are observed in the samples according to the invention. This is the martensite rate which is primarily to be considered in the context of the invention.

A martensite content greater than or equal to 75% after quenching at 20 ° C. and returned at 200 ° C., or greater than or equal to 75% after quenching at 20 ° C., cryogenic treatment at -80 ° C. and tempering at 200 ° C, is satisfactory. If a martensite rate of 75% or more can not be obtained by any of these treatments, the sample is considered unsatisfactory.

R1 1 445 583 68 1 100 100

R12 390 708 0 1 92 97

R13 605 489 0 1 100 100

R14 655 632 0 0 95 100

Table 2: Results of the Tests on the Samples of Table 1 The steels according to the invention 11 to 16, as well as the steels 18 to 19, combine good properties of resistance to corrosion, hardness and polishability, and exhibit good internal health, as well as a martensite rate greater than or equal to 75% after a quench at 20 ° C.

The steel according to the invention 17 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.

At a comparable N level, it is seen that the hardness increases between samples 11, 12 where C is between 0.10 and 0.20%, and samples 13 where C is between 0.20 and 20.0. and 0.30% and especially 18, 19, 110 where C is between 0.30 and 0.35%.

114, where C is even higher and N is of the same level as the previous ones, has a hardness less than them, because the martensite fraction after quenching begins to decrease by the decrease of the temperature Mf in relation with a high value 17Cr + 500C + 500N (see Table 1). Also at N levels and other comparable essential elements, it can be seen that the increase of Cr makes it possible to improve the resistance to corrosion, see samples 18 and 19. Conversely, the increase in the Cr content tends to decrease. the hardness, see samples 18, 110 and 11 1 whose compositions differ significantly only on Cr. Going beyond 18% Cr could increase corrosion resistance, but would lead to lower C and N levels to maintain a satisfactory Ms, and a correct hardness would no longer be ensured.

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 the lack of Cr by adding N restores a satisfactory corrosion resistance, but no longer allows to ensure good internal health because the Cr content is no longer sufficient to allow dissolution complete N in the liquid metal.

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. It is the same for the reference steel R14. 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 reference steels R12 and R13 would have compositions according to the invention on the individual contents of each element, but their sum Cr + 16 N - 5 C, which is less than 16.0%, is insufficient to guarantee a resistance to corrosion as high as that of steels which are in all respects according to the invention, including those which hardly exceed the value of 16.0% for this amount Cr + 16 N - 5 C.

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 consists of, in percentages by weight:

- traces <Mn <1, 0%; preferably traces <Mn <0.6%;

- traces <If <1, 0%;

- traces <S <0.01%; preferably traces <S <0.005%;

- traces <P <0.04%;

- 15.0% <Cr <18.0%; preferably 15.0 <Cr <17.0%, better 15.2% <Cr <17.0%; even better 15.5% <Cr <16.0%;

- traces <Ni <0.50%;

- traces <Mo <0.50%; preferably traces <Mo <0.1%; better traces <Mo <

0.05%;

- traces <Cu <0.50%; preferably traces <Cu <0.3%;

- traces <V <0.50%; preferably traces <V <0.2%;

- 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%; preferably 0.15% <N <0.20%;

- C + N≥ 0.25%; preferably C + N ≥ 0.30%; better C + N≥ 0.45%;

- Cr + 16 N - 5 C≥ 16.0%;

preferably 17 Cr + 500 C + 500 N <570%;

the rest being iron and impurities resulting from the elaboration.

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

3. - 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 claim 1; 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.

4. - Method according to claim 3, characterized in that said half-product is a sheet, and in that said shaping operation is a cold rolling.

5. Method according to claim 3, characterized in that said half-product is a rod or a wire rod, and in that said shaping operation is a forging.

6. - Method according to one of claims 3 to 5, characterized in that the steel has a composition according to claim 2, in that said shaped half-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.

7. - Method according to one of claims 3 to 5, 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 of less than or equal to 20 ° C, 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.

8. - cutting tool, characterized in that it was made from a semi-finished product prepared according to the method of one of claims 3 to 7.

9. A cutting tool according to claim 8, characterized in that it is a cutlery article such as a knife blade, a household robot blade, a scalpel, or a scissors blade,