CA2984514A1 - Martensitic stainless steel, method for the production of a semi-finished product from said steel, and cutting tool produced from the semi-finished product - Google Patents

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, process for producing a semi-finished product 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, in particular of articles of cutlery, such as scalpels, scissor blades, or blades of knives or blades of household robots.
Steels for cutlery must be resistant to corrosion, high polishability and hardness.
The martensitic stainless steels currently used to make the blades of cutting tools, such as steels of the types EN 1.4021, EN 1.4028 and in 1.4034, have Cr contents 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 IN
1.4028 and 0.43-0.50% for EN 1.4034. The hardness level of the steel depends mainly from this C content.
When even better corrosion resistance is sought, the shade 1.4419 at 0.36-0.42% C, 13.0-14.5% Cr and 0.60-1.00% Mo may be used.
During their manufacture, these steels are typically elaborated in a AOD or VOD converter, then continuously cast in the form of slabs, blooms or of billet, then hot-rolled to lead to a coil, a bar laminated or wire machine. They are then annealed in order to obtain a structure ferritic containing carbides, which is mild enough to allow for a cold rolling for flat products, or to facilitate sawing before forging the half product hot rolled for long products.
The product then undergoes recrystallization annealing. In this sweetened state of recrystallized ferrite containing carbides, the product is cut for him confer final shape, for example that of a knife blade, before undergoing a treatment thermal process including 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, all the more more high carbon content is important, but it also presents a big fragility. A treatment of income, 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 quality of cut and sound aesthetic appearance.

2 None of the four shades mentioned allows a good resistance to Corrosion, a good surface condition and a high hardness, for a cost reasonable.
EN 1.4419 grade with good corrosion resistance and hardness high, but it has a prohibitive cost due to the addition of Mo in large quantities.
The EN 1.4034 grade has a high hardness, but also an appearance of poor surface after polishing, because of the presence in large numbers of carbides undissolved during austenitization, because of the high C content of this shade.
Corrosion resistance is insufficient because the Cr content is not quite high in the matrix, especially since some of the Cr is trapped in the carbides not dissolved.
Moreover it happens regularly that the cutting edge of the blade is the seat corrosion cavernous, originating from the decohesion of large primary carbides which appear in end of solidification in continuous casting.
Shades less loaded in C EN 1.4021 and 1.4028 have more hardnesses weak, without having sufficient resistance to corrosion due to of contents in Cr too weak.
The present invention aims to solve the problems mentioned above.
In particular, it aims to propose a martensitic stainless steel for cutting tool as economical as possible, which, however, has both a good resistance to corrosion, 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% 5 C 5 0.45%; preferably 0.20% C, 0.38%; better 0.20% 5 C 5 0.35%; optimally 0.30% 5 C 5 0.35%;
traces 5 Mn 5 1.0%; preferably traces 5 Mn 0.6%;
- traces 5 Si 5 1.0%;
traces 5 S 5 0.01%; preferably 5% traces 0.005%;
traces 5 P 5 0.04%;
- 15.0% 5 Cr 5 18.0%; preferably 15.0 5 Cr 5 17.0%; better 15.2% 5 Cr 5 17.0%; even better 15.5% 5 Cr 5 16.0%;
traces 5 Ni 5 0.50%;
traces 5 MO 5 0.50%; preferably traces 5 MO 5 0.1%; best tracks 5 MB 5 0.05%;
traces of Cu 5 0.50%; preferably traces of 0.3% Cu;
traces 5 V 5 0.50%; preferably traces 5 V 0.2%;
traces 5 Nb 5 0.03%;
traces 5 Ti 5 0.03%;

3 traces 5 Zr 5 0.03%;
traces 5 Al 5 0.01 0%;
traces 5 0.0080%;
traces 5 Pb 0.02%;
traces 5 Bi 5 0.02%;
traces 5 Sn 5 0.02%;
- 0.10% 5 N 5 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 semi-finished product in steel martensitic stainless, characterized in that:
a semi-finished product is produced and cast in a steel having the composition previous ;
said half-product is heated to a temperature greater than or equal to 100000;
- 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 range between 700 and 900 C;
and performing a shaping operation on said sheet, said bar or said machine wire.
Said half-product may be a sheet, and said forming operation can be a cold rolling.
Said semi-finished product may be a bar or a machine wire, and said operation of shaping can be a forging.
Said semifinished product, if its Cr content is between 15 and 17%
can then be austenitized between 950 and 1150 C, then cooled to a speed at least 15 C / s to a temperature below or equal to 20 C, then undergoes a income to 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 lower or equal to 20 C, then undergoes a cryogenic treatment at a temperature of -220 to -50 C, then an income at a temperature between 100 and 300 C.
The invention also relates to a cutting tool, characterized in that has been made from a half-product prepared according to the preceding method.

4 The cutting tool can be a cutlery item such as a blade of knife, a household robot blade, a scalpel, or a scissors blade, As will be understood, the invention consists in using, to achieve the tool cut, a martensitic stainless steel of particular composition, free items expensive at high levels but containing amounts of nitrogen relatively in a well-defined range. Also, a balancing particular Cr, C and N contents are necessary.
Other features and advantages of the invention will become apparent in the course of the description below given as an example and made with reference to the figure 1 Annex, 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, 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 must be clear that the ranges of contents of various elements considered as preferential are independent of each other of the other, and any combination of ranges defined in the description that follows is envisaged in the context of the invention, provided that individual C, N and Cr they would allow simultaneously can respect the relationships that must bind according to the invention.
C increases the hardness in the martensitic state after austenitization, quenching and returned. However, it also promotes the precipitation of primary carbides NA7C3 during solidification, which can be removed during polishing or the sharpening of the blade, which degrades the surface appearance of the product. The sites where they are before polishing can also become the seat of cavernous corrosion. A
C content excessive conduct also, depending on the austenitization temperature, either at a C content too high in the austenitic matrix that no longer allows to obtain a fraction sufficient martensite after quenching, either at the persistence of carbides M23C6 no dissolved which deplete the austenitic matrix in Cr. They reduce resistance to corrosion and detract from polishability.
The C content must therefore be at least 0.10% to obtain a hardness sufficient and not more than 0.45% to obtain good corrosion resistance and one satisfactory surface appearance after polishing. According to the casting process and of solidification, it may be useful to limit a little the more the content maximum in C, in the event that this process might not guarantee a homogeneity solidification steel which would be sufficient to avoid a precipitation of primary carbides NA7C3. In this case, it is advisable to limit the C content at 0.38%. Of preferably 0.20% 5 C 5 0.38%, better 0.20% 5 C 5 0.35%, optimally 0.30% 5 C

5 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 5 Hardness from the lowering of the maximum C content in relation to at the range more general that can be offset by a sufficient nitrogen presence at this effect, as will be seen later.
In addition, the C content must satisfy formulas linking it with the content in N and with the contents of N and Cr, as will be explained later.
Mn is a so-called gammagenic element because it stabilizes the austenitic structure.
A
excessive Mn content leads to an insufficient martensite treatment of austenitization and quenching, which leads to a decrease in hardness. For this reason the Mn content must be between traces resulting from the elaboration and 1.0%. Of preferably its content is limited to 0.6% to help achieve a Ms temperature optimally low.
Si is a useful element in the process of making steel. He is very reducer, and it therefore makes it possible to reduce the Cr oxides in the phase of reduction of steel that follows the decarburization phase in the AOD or VOD converter.
However the Si content in the final steel must be between traces and 1.0% because this element with a heat-curing effect which limits the possibilities of hot deformation during hot rolling or during forging. Preferably we limit its 0.6% content to help obtain an optimally low Ms temperature.
S and P are impurities that decrease hot ductility. P segregate easily at grain boundaries and facilitates their decohesion. Furthermore reduces the resistance to pitting corrosion by forming with Mn compounds which serve as sites initiators for this type of corrosion. As such, the contents of S and P
have to be between 0.01% and 0.04% respectively in weight.
Preferably, the S content does not exceed 0.005% to further ensure a 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 (temperature end of martensitic transformation) below ambient temperature.
it would lead, after austenitization and quenching to room temperature, to a Martensitic transformation too incomplete and insufficient hardness.
For these different reasons, the Cr content must be between 15.0% and 18.0%
in weight. he however, it is advisable to limit the Cr content to 15.0-17.0%, better 15.2-17.0%

6 even better 15.5-16.0%, especially when cryogenic treatment of steel is not performed, so as not to have a Ms start transformation temperature martensitic too high, and therefore not to leave too much austenite residual limit the hardness, so the tensile strength Rm, which is not desirable on a martensitic steel. If necessary, the decrease in corrosion resistance induced by the reduction in the maximum level of Cr may be offset by a NOT
within the limits otherwise prescribed.
However, the solubility of N in the liquid metal decreases when the content of Cr decreases, so it is no longer possible below 15% of Cr to keep in the liquid metal enough N dissolved at the solidification temperature of steel, this which leads to the formation of N2 bubbles during solidification, and does not allows more N to compensate for the decline in Cr with respect to corrosion resistance. This low limit in Cr for the solubility of N also increases when the ferrostatic pressure at the solidification decreases. It may be preferable to increase the minimum content in Cr of 15.0% to 15.2% or 15.5% depending on the type of casting process and the conditions casting practiced to guard against any risk of N2 bubble formation.
The Cr content must also satisfy a formula which binds it to N and VS
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%
weight, preferably at most 0.10% for Mo. It is therefore not necessary to add after the merger of the raw materials. It is even more favorable than the Mo content does not exceed 0.05%, to help obtain a temperature Ms optimally low. For the same reason, it is preferable that the Cu content does not exceed not 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 that the carbides and nitrides of Cr. These elements are however undesirable, because their respective carbides and nitrides, once formed in the process of manufacture, can not to be easily dissolved during austenitization, which limits the contents in C and N
in austenite, and therefore the corresponding hardness of martensite after tempering. The 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% for avoid forming Al nitrides, whose dissolution temperature would be too high and which would decrease the N content of the austenite, so the hardness of the martensite after tempering.

WO 2016/14685

7 The 0 content results from the steel making process and its composition.
It must be between traces and 0.0080% (80 ppm) maximum, of way to avoid forming too many and / or too large oxide inclusions, who could constitute privileged sites of initiation of corrosion by sting, and also take off when polishing, so that the surface appearance of the product does not would not be satisfactory. The 0 content also influences the mechanical properties of steel, and may, in a conventional way, set a limit not to exceed lower than 80 ppm, depending on the end user's requirements.
The contents of Pb, Bi and Sn may be limited to traces resulting from the elaboration, and each must not exceed 0,02% in order not to render too difficult hot transformations.
Control of N content at a well-defined level is essential of the invention. Just like C, it allows, when in solid solution, to increase the hardness of martensite without having the disadvantage of forming precipitates during of the solidification. If you do not want a C content too high to not train too much precipitated, an addition of N makes it possible to compensate for the loss of hardness. Nitrides form at lower temperatures than carbides which facilitates their implementation.
solution when of austenitization. The presence of N in solid solution also improves the held at the corrosion.
However an excessive content of N no longer allows its complete dissolution during solidification, and leads to the formation of N2 bubbles that form blowholes (porosity) during the solidification of steel, detrimental to health internal metal.
For these 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 linking it to the levels of Cr and vs.
Indeed, the hardness of martensite depends on its C and N contents.
inventors have shown that the hardening effects of these two elements are similar, and therefore the hardness of martensite is dependent on its content global in C + N. It has been established by the inventors that the hardness after quenching and tempering will be sufficient if the following formula is followed:
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%.

8 Three elements have an effect on the corrosion resistance. Cr and N are beneficial, whereas C has a negative effect as it is not usually possible to Dissolve all Cr carbides during austenitization, for reasons of productivity and cost which limit in industrial practice the duration and temperature treatment. Undissolved Cr carbides reduce the Cr content of the matrix austenitic, and thereby reduce the corrosion resistance.
From the study of the corrosion resistance of martensitic steels to different weight contents in Cr, N and C, the inventors have found a formula combining these different elements that ensures a very good resistance to corrosion:
Cr + 16 N ¨ 5 C 16.0%
A preferred condition, though not mandatory, is that:
17Cr + 5000 + 500N 5 570%
This condition ensures that we will have a temperature Ms not too much high, as his respect would represent a lowering of Ms of the order of 60 C by in relation to the simultaneous satisfaction of the limits higher grades 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 speed of cooling greater than 100 C / s, followed by an income of 200 C in order to vary the proportion of dissolved carbides, and therefore the carbon content in the austenite then in the martensite after quenching. The martensite rate as well as the Vickers hardness have summer measured in order to trace the evolution of the hardness according to the rate of martensite, and The results are shown in FIG. 1, for a steel having the composition example 14 of Table 1.
We see in Figure 1 that hardness begins to grow with the decline of rate martensite, because martensite hardens by carbon enrichment. The hardness reaches a maximum, then decrease when the martensite rate becomes too low. In below 75% of martensite, the hardening of martensite does not compensate anymore softening related to the presence of residual austenite of lower hardness. For this reason, in a preferred embodiment of the invention, adapted to the manufacture of cutting tool from steel cast, the martensite ratio of the steel after austenitization, quenching to a speed of at minus 15 C / s to a temperature less than or equal to 20 C, then back 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, we proceed to a treatment

9 cryogenic, that is to say the realization of a quenching in a medium to very low temperature 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. he can also have ferrite.
As non-limiting 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. Underlined values are those that do not are not according to the invention. We have also reported the values of C + N, Cr +

and 17Cr + 500C + 500N for each sample.
C Mn If PS 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 0.112 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 0.11 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 0.09 111 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 0.11 113 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 0.19 0.02 0.10 116 0.366 0.28 0.28 0.013 0.002 0.29 16.0 0.11 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 0.011 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 0.21 137 0.13 0.02 0.11 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 Q 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 0.112 0.27 0.34 0.010 0.001 0.34 15.1 0.07 0.02 0.10 R11 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 0.211 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

(prefer) 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 0.005 0.003 0.001 0.001 0.016 0.003 0.194 0.639 16.18 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 110 0.004 0.004 0.002 0.002 0.012 0.002 0.180 0.520 17.48 537.1 111 0.003 0.003 0.002 0.002 0.009 0.002 0.178 0.520 18.94 562.6 112 0.004 0.002 0.001 0.001 0.013 0.001 0.182 0.558 17.13 552.7 113 0.002 0.001 0.002 0.001 0.006 0.003 0.125 0.460 16.23 500.3 114 0.002 0.003 0.003 0.002 0.008 0.003 0.177 0.619 16.62 581 5 115 0.003 0.002 0.001 0.001 0.010 0.002 0.105 0.350 16.56 447.2 116 0.002 0.003 0.002 0.002 0.007 0.003 0.134 0.500 16.31 522.0 117 0.004 0.005 0.002 0.001 0.011 0.003 0.106 0.462 17.22 525.1 118 0.003 0.004 0.002 0.001 0.010 0.003 0.112 0.275 16.98 409.5 119 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 0.011 0.002 0 003 0.315 12 29 392.1 R3 0.005 0.004 0.002 0.001 0.010 0.001 p.003 0.481 11 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 0.114 0 226 16.36 369.7 R11 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 0.011 0.003 0.112 0 235 17.88 401.4 R14 0.002 0.002 0.003 0.001 0.011 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 3 mm, annealed at a temperature of temperature of 800 C, 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 then underwent austenitization treatment of minutes at 1050 C followed by quenching with water until the temperature of 20 C.
After cutting the sheets into two parts, one of the parts was then diving during 10 minutes in a bath thermostated at -80 C, so as to be able to evaluate the effects a cryogenic treatment that would add to the simple quenching with water.
An income of 1h at 200 C was then made on each part of the sheet.
Table 2 presents the results of tests and observations made on these steels. The underlined values correspond to judged performances insufficient.
Internal health is evaluated on a raw state of solidification after casting, knowing that subsequent processing operations will not degrade it not.
The martensite content is measured after quenching with water at 20 ° C. and after a cryogenic treatment by quenching at -80 C, this quenching, or the second of these tempers, having been followed by income at 200 C. When the martensite rate is superior or equal to 75% after quenching with water at 20 C, the other results given in table 2 concern the soaked state at 20 C followed by the income at 200 C. When the martensite less than 75% after quenching with water at 20 C, the other results given in the Table 2 concern the state after cryogenic treatment (quenching up to a very low temperature, for example in dry ice) at -80 ° C, followed by income at 200 C.
The corrosion resistance is evaluated by an electrochemical corrosion test by puncture in a medium composed of 0.02M NaCl, at 23 ° C. and at a pH of 6.6. The test electrochemical test carried out on 24 samples makes it possible to determine the potential E01 for which the elementary probability of pitting is equal to 0.1 cm-2. The clothe Corrosion is considered unsatisfactory if the potential E01 is less than 350 mV, measured compared to the KCI saturated calomel electrode (350 mV / ECS). She is considered as satisfactory if the potential E01 is between 350 mV / ECS and 450 mV / ECS.

It is considered very satisfactory if the potential E01 is higher at 450 mV / SCE.
Vickers hardness is measured in the thickness on a mirror polished cut, under a load of 1kg with a pyramidal diamond point of square base, following the EN ISO 6507. The average of the hardnesses obtained is calculated by realizing

10 fingerprints. Hardness is considered insufficient if the average hardness is less than 500 HV. It is considered satisfactory if the hardness average is between 500 HV and 550 HV. It is considered very satisfactory if the average hardness is between 551 and 600 HV. She is considered excellent if the average hardness is greater than 600 HV.
Polishability is evaluated by flat polishing until mid thickness of the sample, using successively SiC 180 papers, 320, 500, 800 and 1200 under a force of 30 N, then a polishing on cloth soaked in paste diamond of particle size 3 μm and then 1 μm under a force of 20 N. The surface is then observed under an optical microscope at the magnification of x100. Polishability is considered as unsatisfactory if the density of defects classically called tails of comet is greater than or equal to 100 / cm2. Polishability is considered as satisfactory if this density is between 10 / cm2 and 99 / cm2. Polishability is considered as very satisfactory if this density is between 1 and 9 / cm 2. The polishability is considered excellent if this density is less than 1 / cm2.
Internal health is assessed by observing the raw steel of solidification in chopped off by optical metallography at magnification x25. Internal health is not satisfactory and indicated by the value 0 in Table 2 if globular cavities (Blisters) translating the formation of nitrogen bubbles to solidification are observed.
In the case contrary 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 characteristic rays of martensite compared to the intensity of rays characteristic of austenite knowing that in all samples examined, this are the only two phases involved. Generally, it would not be excluded that other phases are observed marginally in samples according to the invention. It is the martensite rate which is first and foremost to be considered in the context of the invention.
A martensite rate 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 rate of 75% or more can not be obtained by any of these treatments, the sample is considered like no satisfactory.
polishability martensite Eco Hardness Health Martensite (/ 0) (tails of (%) (mV / DHW) HV internal quenching 20 C
comet / cm2) quenching -80 VS

11,610,554 0 1,100 100

12 695 536 0 1 97 100

13,570,650 0 1 95,100

14 510 698 1 88 95

15 510 689 36 1 78 86

16,610,648 1,76 85

17,660,687 51 1 69 81

18,515,700 0.8 1 97,100

19 565 690 06 1 96 100 , Invention 110 580 689 0.5 1 94 97 111 690 680 0.5 1 90 94 113 510 670 0.4 1 95 100 114,540,628 1 76 86 115,535,580 0 1 98,100 118,565,500 1,100 100 , R3 190,683 124 1 97,100 References 0.2 Table 2: Results of Tests on Table 1 Samples The steels according to the invention 11 to 16, as well as the steels 18 to 19, combine of good corrosion resistance, hardness and polishability properties, and present a good internal health and a martensite rate greater than or equal to 75%
soon after a quench at 20 C.
The steel according to the invention 17 combines good holding properties with corrosion, hardness and polishability, and has good internal health and rate martensite greater than or equal to 75%, but provided that treatment is cryogenic at -80 C. Indeed, after a simple quenching with water at 20 C, the rate martensite is not yet sufficient, which is to relate to the presence of Cr at one level higher than that of the other samples according to the invention.
At comparable N level, we see that the hardness increases between on the one hand the samples 11, 12 where C is between 0.10 and 0.20%, and on the other hand the samples 13 where C is between 0.20 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, a Hardness less than them, because the martensite fraction after quenching begins at to decrease by the decrease of the temperature Mf in relation with a value high of 17Cr + 5000 + 500N (see Table 1). Also at N levels and other comparable essential elements, we see that the increase in Cr allows to improve corrosion resistance, see samples 18 and 19.
Conversely, the increase in Cr content tends to decrease the hardness, see the samples 18, 110 and 111 whose compositions differ only significantly in Cr. Go to-more than 18%
of Cr could increase corrosion resistance, but would lead to decrease contents in C and N to keep a satisfactory Ms, and a correct hardness will not more assured.
The reference steels R1 to R3 have Cr and N contents, as well as are C + N and / or Cr + 16 N ¨ 5 C insufficient, which does not allow a held at the satisfactory corrosion.

The R4 and R5 reference steels have insufficient Cr contents. Without compensation by adding N, the R4 steel also has a Cr + combination 16 N ¨ 5 Insufficient C leading to unsatisfactory corrosion resistance. For R5 steel, the compensation for the lack of Cr by adding N restores an outfit to the corrosion 5 satisfactory, but no longer ensures good internal health because Cr content is no longer sufficient to allow a complete dissolution of N in the liquid metal.
Reference steel R6 has a high C content and an N content insufficient. The excessively high C content does not allow for a polishing sufficient because of the formation of carbides too important.
The reference steel R7 has an excessively high N content, which degrades health internal. It is the same for the reference steel R14. Reference steel R8 at a excessive C content, which leads to poor polishability and rate martensite too low even after cryogenic tempering at -80 C.
reference R9 contains too much Cr, which leads to an insufficient martensite rate even after a Cryogenic quenching at -80 ° C.
The reference steels R10 and R11 have low C contents as well as insufficient C + N sums, leading to too low hardnesses. The steels reference R12 and R13 would have compositions according to the invention on the contents of each element, but their sum Cr + 16 N - 5 C, which is lower than 16.0% is insufficient to guarantee corrosion resistance too high than that steels that are in all respects in accordance with the invention, including those who does not only slightly exceed the value of 16.0% for this amount Cr + 16 N ¨ 5 C.
The steels according to the invention are used profitably for the manufacture of tools cutting, such as scalpels, scissors, knife blades or of the circular blades of household robots.

Claims (9)

1.- Martensitic stainless steel, characterized in that its composition consists of, in percentages by weight:
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 5% traces 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% 5 Cr <= 16.0%;
- traces <= Ni <= 0.50%;
- traces <= Mo <= 0.50%; preferably <= Mo traces <= 0.1%; better traces <= MB <=
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 <= O <= 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 has at least 75% martensite. 3.- Method for manufacturing a semi-finished product made of stainless steel martensitic characterized in that a semi-finished product is produced and cast in a steel having the composition according to claim 1;

said half-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 range between 700 and 900 ° C;
and performing a shaping operation on said sheet, said bar or said machine wire. 4. A process according to claim 3, characterized in that said semi-finished product is sheet metal, and in that said shaping operation is cold. 5. Method according to claim 3, characterized in that said half-product is a bar or a machine wire, and in that said shaping operation is a forging. 6. Method according to one of claims 3 to 5, characterized in that steel has a composition according to claim 2, wherein said semi-finished product is form is then austenitized between 950 and 1150 ° C, then cooled to a speed at least 15 ° C / s to a temperature of 20 ° C or less, then undergoes a income to a temperature between 100 and 300 ° C. 7.- Method according to one of claims 3 to 5, characterized in that steel has a composition according to claim 1 or 2, wherein said semi-finished product shaped is then austenitized between 950 and 1150 ° C, then cooled to a speed of at least 15 ° C / s to a temperature of 20 ° C or less, then undergoes a cryogenic treatment at a temperature of -220 to -50 ° C, then an income at a temperature between 100 and 300 ° C. 8. - cutting tool, characterized in that it was made from a half 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,