EP3289109B1 - 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

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

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EP3289109B1
EP3289109B1 EP16724302.1A EP16724302A EP3289109B1 EP 3289109 B1 EP3289109 B1 EP 3289109B1 EP 16724302 A EP16724302 A EP 16724302A EP 3289109 B1 EP3289109 B1 EP 3289109B1
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traces
finished product
semi
steel
temperature
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EP3289109A1 (en
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Francis Chassagne
Françoise Haegeli
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Aperam SA
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
    • C21C5/005—Manufacture of stainless steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/068—Decarburising
    • C21C7/0685—Decarburising of stainless steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1222—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/18—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for knives, scythes, scissors, or like hand cutting tools

Definitions

  • the invention relates to martensitic stainless steel.
  • This steel is mainly intended for the manufacture of cutting tools, in particular cutlery articles, such as scalpels, scissor blades, or knife blades or blades of household robots.
  • Steels for cutlery must have high corrosion resistance, polishability and hardness.
  • the martensitic stainless steels currently used to make the blades of cutting tools such as steels of types EN 1.4021, EN 1.4028 and EN 1.4034, have Cr contents less than or equal to 14 or 14.5% by weight and variable C contents, i.e. 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 mainly depends on this C content.
  • these steels are typically produced in an AOD or VOD converter, then continuously cast in the form of slabs, blooms or billets, then hot rolled to lead to a coil, a rolled bar or a wire rod. They are then annealed in order to obtain a ferritic structure containing carbides, which is soft enough to allow cold rolling for flat products, or to facilitate sawing before forging of the hot-rolled semi-finished product for the long products.
  • the product then undergoes recrystallization annealing.
  • the product is cut to give it its final shape, for example that of a knife blade, before undergoing a heat treatment comprising austenitization at high temperature, typically between 950 ° C. and 1150 ° C, followed by quenching to room temperature which leads to a predominantly martensitic structure.
  • the product has a high hardness, the higher the higher the carbon content, but it also has a great brittleness.
  • a tempering treatment typically between 100 ° C and 300 ° C, is then carried out to reduce the brittleness without reducing the hardness too much.
  • the blade then undergoes various operations including a sharpening and polishing to give it its quality of cut and its aesthetic appearance.
  • the EN 1.4419 grade has good corrosion resistance and high hardness, but it has a prohibitive cost due to the addition of Mo in large quantities.
  • EN 1.4034 grade has a high hardness, but also a poor surface appearance after polishing, due to the presence of a large number of undissolved carbides during austenitization, due to the high C content of this grade.
  • the corrosion resistance is insufficient because the Cr content is not high enough in the matrix, especially as part of the Cr is trapped in the undissolved carbides.
  • the edge of the blade is the site of cavernous corrosion, coming from the decohesion of large primary carbides which appear at the end of solidification in continuous casting.
  • the grades less charged in C EN 1.4021 and 1.4028 have lower hardnesses, without having sufficient corrosion resistance due to too low Cr contents.
  • EP-A-0 638 658 describes a martensitic stainless steel for surgical instruments with a high Mo content (1-2.5%) and necessarily containing V, and the production diagram of which includes remelting under an electroconductive slag.
  • WO-A-2012/137070 describes an austenitic stainless steel with antimicrobial activity for making knives and surgical instruments, several elements of which may be present in very wide ranges. It always contains V, significant or even massive amounts of Mo and W.
  • the present invention aims to solve the problems mentioned above. It aims in particular to propose a martensitic stainless steel for cutting tool as economical as possible, which however has both good corrosion resistance, good polishability and high hardness.
  • Said semi-finished product may be a sheet, and said forming operation may be a cold rolling.
  • Said semi-finished product can be a rod or a wire rod, and said forming operation can be a forging.
  • Said shaped semi-finished product if its Cr content is between 15 and 17%, can then be austenitized between 950 and 1150 ° C, then cooled at a speed of at least 15 ° C / s to a temperature less than or equal to 20 ° C, then undergoes tempering at a temperature between 100 and 300 ° C.
  • Said shaped semi-finished product can then be austenitized between 950 and 1150 ° C, then cooled at a speed of at least 15 ° C / s to a temperature less than 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 it was produced from a semi-finished product prepared according to the preceding process.
  • the cutting tool can be a cutlery item such as a knife blade, a food processor blade, a scalpel, or a scissor blade,
  • the invention consists in using, to produce the cutting tool, a martensitic stainless steel of particular composition, free from expensive elements at high contents, but containing relatively large quantities of nitrogen located in a well defined range. Also, a particular balancing of the contents of Cr, C and N is necessary.
  • figure 1 shows the evolution of the Vickers hardness of steel under a load of 1 kg, as a function of the rate of martensite after austenitization, quenching and tempering, of a steel according to the invention.
  • 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.
  • the optimal range makes it possible to have a high hardness while limiting the formation of carbides in acceptable proportions, the possible loss of hardness coming from the lowering of the maximum content of C compared to the most general range can be compensated by the presence of sufficient nitrogen for this purpose, as will be seen below.
  • the C content 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 gamma element, because it stabilizes the austenitic structure.
  • An excessive Mn content leads to an insufficient rate of martensite after austenitization and quenching treatment, which leads to a reduction in hardness.
  • the Mn content must be between traces resulting from the production and 1.0%.
  • its content is limited to 0.6% to help obtain an optimally low temperature Ms.
  • Si is a useful element during the steel making process. It is very reducing, and it therefore makes it possible to reduce the Cr oxides in the steel reduction phase which follows the decarburization phase in the AOD or VOD converter.
  • the Si content in the final steel must be between traces and 1.0%, because this element with a hot hardening effect which limits the possibilities of hot deformation during hot rolling or during forging.
  • its content is limited to 0.6% to help obtain an optimally low temperature Ms.
  • S and P are impurities which reduce ductility when hot. P segregates easily at grain boundaries and facilitates their decohesion.
  • S reduces the resistance to pitting corrosion by forming compounds with Mn which serve as initiating sites for this type of corrosion.
  • the contents of S and P must be respectively between traces and, respectively, 0.01% and 0.04% by weight.
  • the S content does not exceed 0.005% in order to better ensure sufficient corrosion resistance.
  • Cr is an essential element for corrosion resistance.
  • its content must be limited since a high content risks lowering the temperature Mf (the temperature at the end of martensitic transformation) below ambient temperature. This would lead, after austenitization and quenching to room temperature, to an excessively incomplete martensitic transformation and to insufficient hardness.
  • the Cr content must be between 15.0% and 18.0% by weight.
  • the Cr content it is however advisable to limit the Cr content to 15.0-17.0%, better 15.2-17.0%, even better 15.5-16.0%, especially when cryogenic treatment of the steel is not made, so as not to have an excessively high martensitic transformation start temperature Ms, and therefore not to leave too much residual austenite which would limit the hardness, therefore the tensile strength Rm, which doesn is not desirable on martensitic steel.
  • the decrease in corrosion resistance induced by the decrease in the maximum Cr content may be compensated by a high N content within the limits prescribed elsewhere.
  • the solubility of N in the liquid metal decreases when the Cr content decreases, so that it is no longer possible below 15% of Cr to keep sufficient dissolved N in the liquid metal at the solidification temperature of steel, which leads to the formation of N2 bubbles during solidification, and no longer allows N to compensate for the decrease in Cr with respect to corrosion resistance.
  • This low limit in Cr 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 in order to guard against any risk of formation of N2 bubbles.
  • the Cr content must also satisfy a formula linking it to the N and C contents as will be explained below.
  • 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 any after the fusion of the raw materials. It is even more favorable that the Mo content does not exceed 0.05%, to help obtain an optimally low temperature Ms. 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 elements called “stabilizers", which means that they form, in the presence of N and C and at high temperature, carbides and nitrides more stable than carbides and nitrides of Cr.
  • stabilizers which means that they form, in the presence of N and C and at high temperature, carbides and nitrides more stable than 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 contents of C and N in austenite, and therefore the corresponding hardness of the 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 AI nitrides, the dissolution temperature of which would be too high and which would decrease the N content of the austenite, therefore the hardness. martensite after quenching.
  • the O content results from the process for the production of steel and its composition. It must be between traces and 0.0080% (80 ppm) at most, so as to avoid forming too many and / or too large oxide inclusions, which could constitute privileged sites of initiation of corrosion. by pitting, 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 will be possible, in a conventional manner, to set a limit not to be lower than 80 ppm, according to the requirements of the users of the final product.
  • Pb, Bi and Sn can be limited to traces resulting from the production, and must not each exceed 0.02% in order not to make the hot transformation too difficult.
  • 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 one does not want a C content which is too high not to form too many precipitates, an addition of N makes it possible to compensate for the loss of hardness. Nitrides are formed at lower temperatures than carbides which facilitates their dissolution during of austenitization. The presence of N in solid solution also improves the resistance to corrosion.
  • 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 Cr and C contents.
  • an even higher hardness is obtained after quenching and tempering if the following formula is observed: VS + NOT ⁇ 0.45 ⁇ % .
  • a preferred condition, although not mandatory, is that: 17 ⁇ Cr + 500 ⁇ VS + 500 ⁇ NOT ⁇ 570 ⁇ %
  • Steels according to the invention were 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 tempering at 200 ° C, in order to vary the proportion of dissolved carbides, and consequently the carbon content in the austenite and then in the martensite after quenching.
  • the rate of martensite as well as Vickers hardness were measured in order to plot the evolution of hardness as a function of the rate of martensite, and the results are represented on the figure 1 , for a steel having the composition of Example I4 in Table 1.
  • the rate of martensite in the steel after austenitization, quenching at a speed of at least 15 ° C / s up to a temperature less than or equal to 20 ° C, then returned to a temperature of 100 to 300 ° C, typically 200 ° C, is greater than or equal to 75%.
  • Obtaining a high martensite content of up to 100% can be better ensured if, after quenching to 20 ° C or less, a cryogenic treatment is carried out, that is to say the realization of '' quenching in a medium at 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 tempering 100-300 ° C.
  • the remaining microstructure typically consists essentially of residual austenite. There may also be ferrite.
  • compositions of the various steel samples tested are shown in Table 1, expressed in% by weight.
  • the values underlined are those which are not in accordance with the invention.
  • the values of C + N, Cr + 16 N - 5 C and 17Cr + 500C + 500N were also reported for each sample.
  • these steels were reheated to a temperature above 1100 ° C, hot rolled to a thickness of 3mm, annealed at a 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 were then subjected to an austenitization treatment for 15 minutes at 1050 ° C. followed by quenching with water up to the temperature of 20 ° C.
  • one of the parts was then immersed for 10 minutes in a bath thermostatically controlled at -80 ° C, so as to be able to evaluate the effects of a cryogenic treatment which would be added to the simple quenching at the water.
  • Table 2 presents the results of tests and observations carried out on these steels. The underlined values correspond to performances considered insufficient.
  • the internal health is evaluated on a raw solidification state after casting, knowing that the subsequent transformation operations will not degrade it.
  • the rate of martensite 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 quenchings, having been followed by tempering at 200 ° C. .
  • the rate of martensite 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 tempering 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 pH 6.6.
  • the electrochemical test carried out on 24 samples makes it possible to determine the potential E 0.1 for which the elementary probability of pitting is equal to 0.1 cm -2 .
  • the corrosion resistance is considered unsatisfactory if the potential E 0.1 is less than 350 mV, measured with respect to the calomel electrode saturated with KCI (350 mV / DHW). It is considered satisfactory if the potential E 0.1 is between 350 mV / DHW and 450 mV / DHW. It is considered very satisfactory if the potential E 0.1 is greater than 450 mV / DHW.
  • Vickers hardness is measured in thickness on a mirror polished cut, under a load of 1 kg with a pyramidal point in square base diamond, according to standard EN ISO 6507. The average hardness obtained is calculated by making 10 impressions. The 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 performing a flat polishing up to the mid-thickness of the sample, successively using SiC papers 180, 320, 500, 800 and 1200 under a force of 30 N, then polishing on a cloth soaked of diamond paste with a particle size of 3 ⁇ m then 1 ⁇ m under a force of 20 N. The surface is then observed under an optical microscope at a magnification of x100.
  • the 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 to be 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 .
  • Internal health is evaluated by observing the solidification solid steel in section by optical metallography at x25 magnification. Internal health is not satisfactory and indicated by the value "0" in Table 2 if globular cavities (blisters) reflecting the formation of nitrogen bubbles upon solidification are observed. Otherwise internal health is considered satisfactory and indicated by the value "1" in Table 2.
  • the rate of martensite 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 marginally in samples according to the invention. It is the rate of martensite which is above all to be considered in the context of the invention.
  • Table 2 results of the tests carried out on the samples in Table 1 E 0.1 (mV / DHW) HV hardness Polishability (comet tails / cm 2 ) Internal health Martensite (%) quenching 20 ° C Martensite (%) quenching -80 ° C Invention I1 610 554 0 1 100 100 I2 695 536 0 1 97 100 I3 570 650 0 1 95 100 I4 510 698 47 1 88 95 I5 510 689 36 1 78 86 I6 610 648 43 1 76 85 I7 660 687 51 1 69 81 I8 515 700 0.8 1 97 100 I9 565 690 0.6 1 96 100 I10 580 689 0.5 1 94 97 I11 690 680 0.5 1 90 94 I12 565 689 8 1 92 98 I13 510 670 0.4 1 95 100 I14 540 628 49 1 76 86 I15 535 580 0 1 98 100 I16 520 6
  • the steels according to the invention I1 to I6, as well as the steels I8 to I9, combine good properties of resistance to corrosion, hardness and polishability, and have good internal health, as well as a higher rate of martensite or equal to 75% immediately after quenching at 20 ° C.
  • the steel according to the invention I7 combines good properties of resistance to corrosion, hardness and polishability, and has good internal health as well as a rate of martensite greater than or equal to 75%, but on condition that it performs cryogenic treatment at -80 ° C. Indeed after a simple water quenching at 20 ° C, the rate of Martensite is not yet sufficient, which is to be linked to the presence of Cr at a level higher than that of the other samples according to the invention.
  • the reference steels R1 to R3 have insufficient Cr and N contents, as well as insufficient C + N and / or Cr + 16 N - 5 C sums, which does not allow satisfactory corrosion resistance.
  • the reference steels R4 and R5 have insufficient Cr contents. Without compensation by adding N, steel R4 also has an insufficient Cr + 16 N - 5 C combination, leading to unsatisfactory corrosion resistance. For R5 steel, compensating for the lack of Cr by adding N restores satisfactory corrosion resistance, but no longer ensures good internal health since the Cr content is no longer sufficient to allow dissolution. complete with N in the liquid metal.
  • the reference steel R6 has too high a C content and an insufficient N content.
  • the excessively high C content does not allow sufficient polishability due to the formation of too large carbides.
  • the reference steel R7 has too high an N content, which degrades internal health. The same is true for the reference steel R14.
  • the reference steel R8 has an excessive content of C, which leads to poor polishability and to a rate of martensite which is too low even after a cryogenic quenching at -80 ° C.
  • the reference steel R9 contains too much Cr, which leads to an insufficient rate of martensite even after a cryogenic quenching at -80 ° C.
  • the reference steels R10 and R11 have too low C contents as well as insufficient C + N sums, leading to too low hardnesses.
  • Steels of reference R12 and R13 would have compositions in accordance with 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 corrosion resistance also higher than that of steels which are in all respects in accordance with the invention, including those which only slightly exceed the value 16.0% for this sum Cr + 16 N - 5 C.
  • the steels according to the invention are used with profit for the manufacture of cutting tools, such as for example scalpels, scissors, knife blades or circular blades of household robots.

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Description

L'invention concerne un acier inoxydable martensitique. Cet acier est principalement destiné à la fabrication d'outils de coupe, notamment d'articles de coutellerie, tels que des scalpels, des lames de ciseaux, ou des lames de couteaux ou des lames de robots ménagers.The invention relates to martensitic stainless steel. This steel is mainly intended for the manufacture of cutting tools, in particular cutlery articles, such as scalpels, scissor blades, or knife blades or blades of household robots.

Les aciers destinés à la coutellerie doivent présenter une résistance à la corrosion, une aptitude au polissage et une dureté élevées.Steels for cutlery must have high corrosion resistance, polishability and hardness.

Les aciers inoxydables martensitiques actuellement utilisés pour réaliser les lames des outils de coupe, tels que les aciers de types EN 1.4021, EN 1.4028 et EN 1.4034, ont des teneurs en Cr inférieures ou égales à 14 ou à 14,5% en poids et des teneurs en C variables, soit 0,16%-0,25% pour le EN 1.4021, 0,26-0,35% pour le EN 1.4028 et 0,43-0,50% pour le EN 1.4034. Le niveau de dureté de l'acier dépend principalement de cette teneur en C.The martensitic stainless steels currently used to make the blades of cutting tools, such as steels of types EN 1.4021, EN 1.4028 and EN 1.4034, have Cr contents less than or equal to 14 or 14.5% by weight and variable C contents, i.e. 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 mainly depends on this C content.

Lorsqu'une résistance à la corrosion encore meilleure est recherchée, la nuance EN 1.4419 à 0,36-0,42% de C, 13,0-14,5% de Cr et 0,60-1,00% de Mo peut être utilisée.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% Mo be used.

Lors de leurs fabrications, ces aciers sont typiquement élaborés dans un convertisseur AOD ou VOD, puis coulés en continu sous forme de brames, de blooms ou de billettes, puis laminés à chaud pour conduire à une bobine, une barre laminée ou un fil machine. Ils subissent ensuite un recuit afin d'obtenir une structure ferritique contenant des carbures, qui est suffisamment douce pour permettre de réaliser un laminage à froid pour les produits plats, ou pour en faciliter le sciage avant forgeage du demi-produit laminé à chaud pour les produits longs.During their manufacture, these steels are typically produced in an AOD or VOD converter, then continuously cast in the form of slabs, blooms or billets, then hot rolled to lead to a coil, a rolled bar or a wire rod. They are then annealed in order to obtain a ferritic structure containing carbides, which is soft enough to allow cold rolling for flat products, or to facilitate sawing before forging of the hot-rolled semi-finished product for the long products.

Le produit subit ensuite un recuit de recristallisation. Dans cet état adouci de ferrite recristallisée contenant des carbures, le produit est découpé pour lui conférer sa forme finale, par exemple celle d'une lame de couteau, avant de subir un traitement thermique comprenant une austénitisation à haute température, typiquement entre 950°C et 1150°C, suivie d'une trempe jusqu'à la température ambiante qui conduit à une structure majoritairement martensitique.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 austenitization at high temperature, typically between 950 ° C. and 1150 ° C, followed by quenching to room temperature which leads to a predominantly martensitic structure.

Dans cet état martensitique le produit présente une dureté élevée, d'autant plus élevée que la teneur en carbone est importante, mais il présente aussi une grande fragilité. Un traitement de revenu, typiquement entre 100°C et 300°C, est alors effectué pour réduire la fragilité sans trop abaisser la dureté. La lame subit ensuite diverses opérations dont un affûtage et un polissage pour lui conférer sa qualité de coupe et son aspect esthétique.In this martensitic state the product has a high hardness, the higher the higher the carbon content, but it also has a great brittleness. A tempering treatment, typically between 100 ° C and 300 ° C, is then carried out to reduce the brittleness without reducing the hardness too much. The blade then undergoes various operations including a sharpening and polishing to give it its quality of cut and its aesthetic appearance.

Aucune des quatre nuances citées ne permet à la fois une bonne résistance à la corrosion, un bel état de surface et une dureté élevée, pour un coût raisonnable.None of the four grades mentioned provides good corrosion resistance, a good surface finish and high hardness, at a reasonable cost.

La nuance EN 1.4419 à une bonne résistance à la corrosion et une dureté élevée, mais elle a un coût prohibitif dû à l'ajout de Mo en grande quantité.The EN 1.4419 grade has good corrosion resistance and high hardness, but it has a prohibitive cost due to the addition of Mo in large quantities.

La nuance EN 1.4034 présente une dureté élevée, mais aussi un aspect de surface médiocre après polissage, à cause de la présence en grand nombre de carbures non dissous lors de l'austénitisation, en raison de la teneur élevée en C de cette nuance. La résistance à la corrosion est insuffisante car la teneur en Cr n'est pas assez élevée dans la matrice, d'autant qu'une partie du Cr est piégée dans les carbures non dissous. Par ailleurs il arrive régulièrement que le tranchant de la lame soit le siège d'une corrosion caverneuse, provenant de la décohésion de gros carbures primaires qui apparaissent en fin de solidification en coulée continue.EN 1.4034 grade has a high hardness, but also a poor surface appearance after polishing, due to the presence of a large number of undissolved carbides during austenitization, due to the high C content of this grade. The corrosion resistance is insufficient because the Cr content is not high enough in the matrix, especially as part of the Cr is trapped in the undissolved carbides. Furthermore, it often happens that the edge of the blade is the site of cavernous corrosion, coming from the decohesion of large primary carbides which appear at the end of solidification in continuous casting.

Les nuances moins chargées en C EN 1.4021 et 1.4028 ont des duretés plus faibles, sans pour autant avoir une tenue à la corrosion suffisante en raison de teneurs en Cr trop faibles.The grades less charged in C EN 1.4021 and 1.4028 have lower hardnesses, without having sufficient corrosion resistance due to too low Cr contents.

Le document WO-A-2014/014246 décrit un acier inoxydable martensitique et son procédé de fabrication, destiné à la coutellerie. Il est fabriqué par coulée de bandes minces. Sa composition en % pondéraux est C = 0,4-0,5%, N = 0,1-0,2%, Cr = 13-15%, Si = 0.1%-1,0%, Mn = 0.1%-1,0%, Ni ≤ 1% ; C+N ≥ 0.5%, N/C ≥ 02%, le reste étant du Fe et des impuretés. Les carbures résiduels ont une taille de 10 µm ou moins et la dureté est d'au moins 55 HRC.The document WO-A-2014/014246 describes martensitic stainless steel and its manufacturing process for cutlery. It is produced by casting thin strips. Its composition in% by weight is C = 0.4-0.5%, N = 0.1-0.2%, Cr = 13-15%, Si = 0.1% -1.0%, Mn = 0.1% - 1.0%, Ni ≤ 1%; C + N ≥ 0.5%, N / C ≥ 02%, the rest being Fe and impurities. Residual carbides have a size of 10 µm or less and the hardness is at least 55 HRC.

Le document EP-A-0 638 658 décrit un acier inoxydable martensitique pour instruments chirurgicaux à haute teneur en Mo (1-2,5%) et contenant obligatoirement du V, et dont le schéma d'élaboration comporte une refusion sous laitier électroconducteur.The document EP-A-0 638 658 describes a martensitic stainless steel for surgical instruments with a high Mo content (1-2.5%) and necessarily containing V, and the production diagram of which includes remelting under an electroconductive slag.

Le document WO-A-2012/137070 décrit un acier inoxydable austénitique à activité antimicrobienne pour fabriquer des couteaux et des instruments chirurgicaux, dont plusieurs éléments peuvent être présents dans des plages très larges. Il contient toujours du V, du Mo en quantité significative voire massive et du W.The document WO-A-2012/137070 describes an austenitic stainless steel with antimicrobial activity for making knives and surgical instruments, several elements of which may be present in very wide ranges. It always contains V, significant or even massive amounts of Mo and W.

C'est aussi le cas du document CN-A-101 768 700 .This is also the case with the document CN-A-101 768 700 .

La présente invention a pour but de résoudre les problèmes évoqués ci-dessus. Elle vise en particulier à proposer un acier inoxydable martensitique pour outil de coupe aussi économique que possible, qui présente cependant à la fois une bonne résistance à la corrosion, une bonne aptitude au polissage et une dureté élevée.The present invention aims to solve the problems mentioned above. It aims in particular to propose a martensitic stainless steel for cutting tool as economical as possible, which however has both good corrosion resistance, good polishability and high hardness.

Dans ce but l'invention à pour objet un acier inoxydable martensitique, caractérisé en ce que sa composition consiste en, en pourcentages pondéraux :

  • 0,10% ≤ C ≤ 0,45% ; de préférence 0,20% ≤ C ≤ 0,38% ; mieux 0,20% ≤ C ≤ 0,35% ; optimalement 0,30% ≤ C ≤ 0,35% ;
  • traces ≤ Mn ≤ 1,0% ; de préférence traces ≤ Mn ≤ 0,6% ;
  • traces ≤ Si ≤ 1,0% ;
  • traces ≤ S ≤ 0,01% ; de préférence traces ≤ S ≤ 0,005% ;
  • traces ≤ P ≤ 0,04% ;
  • 15,0% ≤ Cr ≤ 18,0% ; de préférence 15,0 ≤ Cr ≤ 17,0% ; mieux 15,2% ≤ Cr ≤ 17,0% ; encore mieux 15,5% ≤ Cr ≤ 16,0% ;
  • traces ≤ Ni ≤ 0,50% ;
  • traces ≤ Mo ≤ 0,50% ; de préférence traces ≤ Mo ≤ 0,1% ; mieux traces ≤ Mo ≤ 0,05% ;
  • traces ≤ Cu ≤ 0,50% ; de préférence traces ≤ Cu ≤ 0,3% ;
  • traces ≤ V ≤ 0,50% ; de préférence 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% ; de préférence 0,15% ≤ N ≤ 0,20% ;
  • C + N ≥ 0,25% ; de préférence C + N ≥ 0,30% ; mieux C + N ≥ 0,45% ;
  • Cr+ 16 N - 5 C ≥ 16,0% ;
  • de préférence 17 Cr + 500 C + 500 N ≤ 570% ;
le reste étant du fer et des impuretés résultant de l'élaboration.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 ≤ Si ≤ 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 ≤ 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 remainder being iron and impurities resulting from processing.

Sa microstructure comporte, de préférence, au moins 75% de martensite. L'invention a également pour objet un procédé de fabrication d'un demi-produit en acier inoxydable martensitique, caractérisé en ce que :

  • on élabore et on coule un demi-produit en un acier ayant la composition précédente ;
  • on chauffe ledit demi-produit à une température supérieure ou égale à 1000°C ;
  • on le lamine à chaud pour obtenir une tôle, une barre ou un fil machine ;
  • on recuit ladite tôle, ladite barre ou ledit fil machine à une température comprise entre 700 et 900°C ;
  • et on exécute une opération de mise en forme sur ladite tôle, ladite barre ou ledit fil machine.
Its microstructure preferably includes at least 75% martensite. The subject of the invention is also a process for manufacturing a semi-finished product in martensitic stainless steel, characterized in that:
  • a semi-finished product is produced and poured into a steel having the preceding composition;
  • said semi-finished product is heated to a temperature greater than or equal to 1000 ° C;
  • hot rolled to obtain a sheet, bar or wire rod;
  • said sheet metal, said bar or said wire rod are annealed at a temperature between 700 and 900 ° C;
  • and a shaping operation is carried out on said sheet, said bar or said wire rod.

Ledit demi-produit peut être une tôle, et ladite opération de mise en forme peut être un laminage à froid.Said semi-finished product may be a sheet, and said forming operation may be a cold rolling.

Ledit demi-produit peut être une barre ou un fil machine, et ladite opération de mise en forme peut être un forgeage.Said semi-finished product can be a rod or a wire rod, and said forming operation can be a forging.

Ledit demi-produit mis en forme, si sa teneur en Cr est comprise entre 15 et 17%, peut être ensuite austénitisé entre 950 et 1150°C, puis refroidi à une vitesse d'au moins 15°C/s jusqu'à une température inférieure ou égale à 20°C, puis subit un revenu à une température comprise entre 100 et 300°C.Said shaped semi-finished product, if its Cr content is between 15 and 17%, can then be austenitized between 950 and 1150 ° C, then cooled at a speed of at least 15 ° C / s to a temperature less than or equal to 20 ° C, then undergoes tempering at a temperature between 100 and 300 ° C.

Ledit demi-produit mis en forme peut être ensuite austénitisé entre 950 et 1150°C, puis refroidi à une vitesse d'au moins 15°C/s jusqu'à une température inférieure ou égale à 20°C, puis subit un traitement cryogénique à une température de -220 à -50°C, puis un revenu à une température comprise entre 100 et 300°C.Said shaped semi-finished product can then be austenitized between 950 and 1150 ° C, then cooled at a speed of at least 15 ° C / s to a temperature less than 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.

L'invention a également pour objet un outil de coupe, caractérisé en ce qu'il a été réalisé à partir d'un demi-produit préparé selon le procédé précédent.The invention also relates to a cutting tool, characterized in that it was produced from a semi-finished product prepared according to the preceding process.

L'outil de coupe peut être un article de coutellerie tel qu'une lame de couteau, une lame de robot ménager, un scalpel, ou une lame de ciseaux,
Comme on l'aura compris, l'invention consiste à utiliser, pour réaliser l'outil de coupe, un acier inoxydable martensitique de composition particulière, exempte d'éléments coûteux à des teneurs élevées, mais contenant des quantités d'azote relativement importantes situées dans une gamme bien définie. Egalement, un équilibrage particulier des teneurs en Cr, C et N est nécessaire.The cutting tool can be a cutlery item such as a knife blade, a food processor blade, a scalpel, or a scissor blade,
As will be understood, the invention consists in using, to produce the cutting tool, a martensitic stainless steel of particular composition, free from expensive elements at high contents, but containing relatively large quantities of nitrogen located in a well defined range. Also, a particular balancing of the contents of Cr, C and N is necessary.

D'autres caractéristiques et avantages de l'invention apparaîtront au cours de la description ci-dessous donnée à titre d'exemple et faite en référence à la figure 1 annexée, qui montre l'évolution de la dureté Vickers de l'acier sous une charge de 1 kg, en fonction du taux de martensite après austénitisation, trempe et revenu, d'un acier selon l'invention.Other characteristics and advantages of the invention will appear during the description below given by way of example and made with reference to the figure 1 annexed, which shows the evolution of the Vickers hardness of steel under a load of 1 kg, as a function of the rate of martensite after austenitization, quenching and tempering, of a steel according to the invention.

En ce qui concerne la composition chimique de l'acier selon l'invention, les justifications suivantes sont avancées. Il doit être clair que les gammes de teneurs des divers éléments envisagées comme préférentielles sont indépendantes les unes des autres, et que toute combinaison des gammes définies dans la description qui suit est envisageable dans le cadre de l'invention, pourvu que des teneurs individuelles en C, N et Cr qu'elles autoriseraient simultanément puissent respecter les relations qui doivent les lier selon l'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 envisaged as preferred are independent of each other, and that any combination of the ranges defined in the description which follows is conceivable within the framework of the invention, provided that individual contents in C, N and Cr which they authorize simultaneously can respect the relationships which must bind them according to the invention.

C augmente la dureté à l'état martensitique après austénitisation, trempe et revenu. Cependant il favorise aussi la précipitation de carbures primaires M7C3 au cours de la solidification, qui peuvent être déchaussés lors du polissage ou de l'affûtage de la lame, ce qui dégrade l'aspect de surface du produit. Les sites où ils se trouvaient avant le polissage peuvent aussi devenir le siège d'une corrosion caverneuse. Une teneur en C excessive conduit aussi, suivant la température d'austénitisation, soit à une teneur en C trop élevée dans la matrice austénitique qui ne permet plus d'obtenir une fraction suffisante de martensite après trempe, soit à la persistance de carbures M23C6 non dissous qui appauvrissent la matrice austénitique en Cr. Ils réduisent ainsi la résistance à la corrosion et nuisent à la polissabilité.C increases the hardness in the martensitic state after austenitization, quenching and tempering. However, it also promotes the precipitation of primary carbides M 7 C 3 during solidification, which can be removed when polishing or sharpening 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, depending on the austenitization temperature, either to a too high C content in the austenitic matrix which no longer makes it possible to obtain a sufficient fraction of martensite after quenching, or to the persistence of M 23 carbides C 6 undissolved which deplete the austenitic matrix in Cr. They thus reduce corrosion resistance and affect polishability.

La teneur en C doit donc être d'au moins 0,10% pour obtenir une dureté suffisante et d'au plus 0,45% pour obtenir une bonne résistance à la corrosion et un aspect de surface satisfaisant après polissage. Selon le procédé de coulée et de solidification employé, il peut cependant s'avérer utile de limiter un peu plus la teneur maximale en C, pour le cas où ce procédé risquerait de ne pas garantir une homogénéité de l'acier en cours de solidification qui serait suffisante pour éviter une précipitation de carbures primaires M7C3. Dans ce cas, on conseille de limiter la teneur en C à 0,38%. De préférence 0,20% ≤ C ≤ 0,38%, mieux 0,20% ≤ C ≤ 0,35%, optimalement 0,30% ≤ C ≤ 0,35%.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. Depending on the casting and solidification process used, it may however prove useful to limit the maximum C content a little more, in the event that this process might not guarantee homogeneity of the steel during solidification which would be sufficient to avoid precipitation of primary carbides M 7 C 3 . In this case, it is advisable to limit the C content to 0.38%. Preferably 0.20% ≤ C ≤ 0.38%, better 0.20% ≤ C ≤ 0.35%, optimally 0.30% ≤ C ≤ 0.35%.

La gamme optimale, en particulier, permet d'avoir une dureté élevée tout en limitant la formation de carbures dans des proportions acceptables, la possible perte de dureté provenant de l'abaissement de la teneur maximale en C par rapport à la gamme la plus générale pouvant être compensée par une présence d'azote suffisante à cet effet, comme cela sera vu plus loin.The optimal range, in particular, makes it possible to have a high hardness while limiting the formation of carbides in acceptable proportions, the possible loss of hardness coming from the lowering of the maximum content of C compared to the most general range can be compensated by the presence of sufficient nitrogen for this purpose, as will be seen below.

De plus, la teneur en C doit satisfaire des formules la liant avec la teneur en N et avec les teneurs en N et Cr, comme il sera expliqué plus loin.In addition, the C content must satisfy formulas linking it with the N content and with the N and Cr contents, as will be explained below.

Mn est un élément dit gammagène, car il stabilise la structure austénitique. Une teneur excessive en Mn conduit à un taux de martensite insuffisant après traitement d'austénitisation et trempe, qui conduit à une baisse de la dureté. Pour cette raison la teneur en Mn doit être comprise entre des traces résultant de l'élaboration et 1,0%. De préférence on limite sa teneur à 0,6% pour aider à l'obtention d'une température Ms optimalement basse.Mn is a so-called gamma element, because it stabilizes the austenitic structure. An excessive Mn content leads to an insufficient rate of martensite after austenitization and quenching treatment, which leads to a reduction in hardness. For this reason, the Mn content must be between traces resulting from the production and 1.0%. Preferably, its content is limited to 0.6% to help obtain an optimally low temperature Ms.

Si est un élément utile lors du procédé d'élaboration de l'acier. Il est très réducteur, et il permet donc de réduire les oxydes de Cr dans la phase de réduction de l'acier qui suit la phase de décarburation dans le convertisseur AOD ou VOD. Cependant la teneur en Si dans l'acier final doit être comprise entre des traces et 1,0%, car cet élément à un effet durcissant à chaud qui limite les possibilités de déformation à chaud lors du laminage à chaud ou lors du forgeage. De préférence on limite sa teneur à 0,6% pour aider à l'obtention d'une température Ms optimalement basse.Si is a useful element during the steel making process. It is very reducing, and it therefore makes it possible to reduce the Cr oxides in the steel reduction phase which 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 hot hardening 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 temperature Ms.

S et P sont des impuretés qui diminuent la ductilité à chaud. P ségrége facilement aux joints des grains et facilite leur décohésion. De plus, S réduit la résistance à la corrosion par piqûre en formant avec Mn des composés qui servent de sites initiateurs pour ce type de corrosion. A ce titre les teneurs en S et en P doivent être respectivement comprises entre des traces et, respectivement, 0,01% et 0,04% en poids. De préférence, la teneur en S ne dépasse pas 0,005% pour encore mieux assurer une résistance à la corrosion suffisante.S and P are impurities which reduce ductility when hot. P segregates easily at 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 be respectively between traces and, respectively, 0.01% and 0.04% by weight. Preferably, the S content does not exceed 0.005% in order to better ensure sufficient corrosion resistance.

Cr est un élément essentiel pour la tenue à la corrosion. Cependant sa teneur doit être limitée car une teneur élevée risque d'abaisser la température Mf (la température de fin de transformation martensitique) en dessous de la température ambiante. Cela conduirait, après austénitisation et trempe jusqu'à la température ambiante, à une transformation martensitique trop incomplète et à une dureté insuffisante. Pour ces différentes raisons, la teneur en Cr doit être comprise entre 15,0% et 18,0% en poids. Il est cependant conseillé de limiter la teneur en Cr à 15,0-17,0%, mieux 15,2-17,0%, encore mieux 15,5-16,0%, surtout lorsqu'un traitement cryogénique de l'acier n'est pas effectué, afin de ne pas avoir une température Ms de début de transformation martensitique trop élevée, et donc de ne pas laisser trop d'austénite résiduelle qui limiterait la dureté, donc la résistance à la traction Rm, ce qui n'est pas désirable sur un acier martensitique. Si nécessaire, la baisse de la résistance à la corrosion induite par la diminution de la teneur maximale en Cr pourra être compensée par une teneur en N élevée dans les limites prescrites par ailleurs.Cr is an essential element for corrosion resistance. However, its content must be limited since a high content risks lowering the temperature Mf (the temperature at the end of martensitic transformation) below ambient temperature. This would lead, after austenitization and quenching to room temperature, to an excessively incomplete martensitic transformation and to insufficient hardness. For these various reasons, the Cr content must be between 15.0% and 18.0% by weight. It is however advisable to limit the Cr content to 15.0-17.0%, better 15.2-17.0%, even better 15.5-16.0%, especially when cryogenic treatment of the steel is not made, so as not to have an excessively high martensitic transformation start temperature Ms, and therefore not to leave too much residual austenite which would limit the hardness, therefore the tensile strength Rm, which doesn is not desirable on martensitic steel. If necessary, the decrease in corrosion resistance induced by the decrease in the maximum Cr content may be compensated by a high N content within the limits prescribed elsewhere.

Cependant la solubilité de N dans le métal liquide baisse quand la teneur en Cr diminue, de sorte qu'il n'est plus possible en dessous de 15% de Cr de garder dans le métal liquide suffisamment de N dissous à la température de solidification de l'acier, ce qui conduit à la formation de bulles de N2 lors de la solidification, et ne permet plus à N de compenser la baisse du Cr vis à vis de la résistance à la corrosion. Cette limite basse en Cr pour la solubilité de N augmente aussi lorsque la pression ferrostatique à la solidification diminue. Il peut être préférable d'augmenter la teneur minimale en Cr de 15,0% à 15,2% ou 15,5% suivant le type de procédé de coulée et les conditions de coulée pratiquées afin de se prémunir de tout risque de formation de bulles de N2.However, the solubility of N in the liquid metal decreases when the Cr content decreases, so that it is no longer possible below 15% of Cr to keep sufficient dissolved N in the liquid metal at the solidification temperature of steel, which leads to the formation of N2 bubbles during solidification, and no longer allows N to compensate for the decrease in Cr with respect to corrosion resistance. This low limit in Cr 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 in order to guard against any risk of formation of N2 bubbles.

La teneur en Cr doit aussi satisfaire une formule la liant aux teneurs en N et C comme ce sera expliqué par la suite.The Cr content must also satisfy a formula linking it to the N and C contents as will be explained below.

Les éléments Ni, Cu, Mo et V sont onéreux et réduisent aussi la température Mf. La teneur de chacun de ces éléments doit donc être limitée, entre des traces et 0,50% en poids, de préférence au plus à 0,10% pour Mo. Il n'est donc pas nécessaire d'en ajouter après la fusion des matières premières. Il est encore plus favorable que la teneur en Mo ne dépasse pas 0,05%, pour aider à l'obtention d'une température Ms optimalement basse. Pour la même raison, il est préférable que la teneur en Cu ne dépasse pas 0,3%, et que la teneur en V ne dépasse pas 0,2%.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 any after the fusion of the raw materials. It is even more favorable that the Mo content does not exceed 0.05%, to help obtain an optimally low temperature Ms. 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 et Zr sont des éléments dit « stabilisants », ce qui signifie qu'ils forment, en présence de N et C et à haute température, des carbures et des nitrures plus stables que les carbures et nitrures de Cr. Ces éléments sont cependant indésirables, car leurs carbures et nitrures respectifs, une fois formés lors du procédé de fabrication, ne peuvent plus être aisément dissous lors de l'austénitisation, ce qui limite les teneurs en C et N dans l'austénite, et donc la dureté correspondante de la martensite après trempe. La teneur de chacun de ces éléments doit donc être comprise entre des traces et 0,03%.Nb, Ti and Zr are elements called "stabilizers", which means that they form, in the presence of N and C and at high temperature, carbides and nitrides more stable than 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 contents of C and N in austenite, and therefore the corresponding hardness of the martensite after quenching. The content of each of these elements must therefore be between traces and 0.03%.

La teneur en Al doit, de même, être comprise entre des traces et 0,010% pour éviter de former des nitrures d'AI, dont la température de dissolution serait trop élevée et qui diminueraient la teneur en N de l'austénite, donc la dureté de la martensite après trempe.The Al content must likewise be between traces and 0.010% to avoid forming AI nitrides, the dissolution temperature of which would be too high and which would decrease the N content of the austenite, therefore the hardness. martensite after quenching.

La teneur en O résulte du procédé d'élaboration de l'acier et de sa composition. Elle doit être comprise entre des traces et 0,0080% (80 ppm) au maximum, de façon à éviter de former des inclusions d'oxydes trop nombreuses et/ou trop grosses, qui pourraient constituer des sites privilégiés d'initiation de la corrosion par piqûre, et aussi se déchausser lors du polissage, de sorte que l'aspect de surface du produit ne serait pas satisfaisant. La teneur en O influence aussi les propriétés mécaniques de l'acier, et on pourra éventuellement, de façon classique, fixer une limite à ne pas dépasser plus basse que 80 ppm, suivant les exigences des utilisateurs du produit final.The O content results from the process for the production of steel and its composition. It must be between traces and 0.0080% (80 ppm) at most, so as to avoid forming too many and / or too large oxide inclusions, which could constitute privileged sites of initiation of corrosion. by pitting, 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 will be possible, in a conventional manner, to set a limit not to be lower than 80 ppm, according to the requirements of the users of the final product.

Les teneurs en Pb, Bi et Sn peuvent être limitées à des traces résultant de l'élaboration, et ne doivent chacune pas dépasser 0,02% pour ne pas rendre trop difficiles les transformations à chaud.The contents of Pb, Bi and Sn can be limited to traces resulting from the production, and must not each exceed 0.02% in order not to make the hot transformation too difficult.

Le contrôle de la teneur en N a un niveau bien défini est un élément essentiel de l'invention. Tout comme C, il permet, lorsqu'il est en solution solide, d'augmenter la dureté de la martensite sans avoir l'inconvénient de former des précipités au cours de la solidification. Si on ne désire pas une teneur en C trop élevée pour ne pas former trop de précipités, un ajout de N permet de compenser la perte de dureté. Les nitrures se forment à des températures plus faibles que les carbures ce qui facilite leur mise en solution lors de l'austénitisation. La présence de N en solution solide améliore aussi la tenue à la corrosion.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 one does not want a C content which is too high not to form too many precipitates, an addition of N makes it possible to compensate for the loss of hardness. Nitrides are formed at lower temperatures than carbides which facilitates their dissolution during of austenitization. The presence of N in solid solution also improves the resistance to corrosion.

Cependant une teneur excessive en N ne permet plus sa dissolution complète lors de la solidification, et conduit à la formation de bulles de N2 qui forment des soufflures (porosités) lors de la solidification de l'acier, préjudiciables à la santé interne du métal.However, an excessive content of N no longer allows its complete dissolution during solidification, and leads to the formation of bubbles of N2 which form blisters (porosities) during the solidification of the steel, detrimental to the internal health of the metal.

Pour ces différentes raisons la teneur en N doit être comprise entre 0,10 et 0,20% en poids, de préférence entre 0,15 et 0,20% en poids.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.

La teneur en N doit aussi satisfaire diverses formules la liant aux teneurs en Cr et C.The N content must also satisfy various formulas linking it to the Cr and C contents.

En effet, la dureté de la martensite dépend de ses teneurs en C et en N. Les inventeurs ont mis en évidence que les effets durcissants de ces deux éléments sont similaires, et donc que la dureté de la martensite est dépendante de sa teneur globale en C + N. Il a été établi par les inventeurs que la dureté après trempe et revenu sera suffisante si la formule suivante est respectée : C + N ≥ 0,25 ⁢ % , de préférence C + N ≥ 0,30 ⁢ %

Figure imgb0001
de préférence C + N ≥ 0,30%Indeed, the hardness of martensite depends on its C and N contents. The inventors have shown that the hardening effects of these two elements are similar, and therefore that the hardness of 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 observed: VS + NOT ≥ 0.25 ⁢ % , of preference VS + NOT ≥ 0.30 ⁢ %
Figure imgb0001
 preferably C + N ≥ 0.30%

Dans un mode encore plus préféré de l'invention, une dureté encore plus élevée est obtenue après trempe et revenu si la formule suivante est respectée : C + N ≥ 0,45 ⁢ % .

Figure imgb0002
In an even more preferred embodiment of the invention, an even higher hardness is obtained after quenching and tempering if the following formula is observed: VS + NOT ≥ 0.45 ⁢ % .
Figure imgb0002

Trois éléments ont un effet sur la résistance à la corrosion. Cr et N sont bénéfiques, alors que C a un effet négatif car il n'est généralement pas possible de dissoudre tous les carbures de Cr lors de l'austénitisation, pour des raisons de productivité et de coût qui limitent dans la pratique industrielle la durée et la température du traitement. Les carbures de Cr non dissous réduisent la teneur en Cr de la matrice austénitique, et de ce fait réduisent la tenue à la corrosion.Three elements have an effect on corrosion resistance. Cr and N are beneficial, whereas C has a negative effect because it is generally not possible to dissolve all of the carbides of Cr during austenitization, for reasons of productivity and cost which limit in industrial practice the duration and the processing temperature. Undissolved Cr carbides reduce the Cr content of the austenitic matrix, thereby reducing the corrosion resistance.

A partir de l'étude de la résistance à la corrosion d'aciers martensitiques à différentes teneurs pondérales en Cr, N et C, les inventeurs ont trouvé une formule associant ces différents éléments qui permet d'assurer une très bonne résistance à la corrosion : Cr + 16 N − 5 C ≥ 16,0 ⁢ %

Figure imgb0003
From the study of the corrosion resistance of martensitic steels with different weight contents of Cr, N and C, the inventors have found a formula combining these different elements which makes it possible to ensure very good corrosion resistance: Cr + 16 NOT - 5 VS ≥ 16.0 ⁢ %
Figure imgb0003

Une condition préférée, sans être toutefois obligatoire, est que : 17 ⁢ Cr + 500 ⁢ C + 500 ⁢ N ≤ 570 ⁢ %

Figure imgb0004
A preferred condition, although not mandatory, is that: 17 ⁢ Cr + 500 ⁢ VS + 500 ⁢ NOT ≤ 570 ⁢ %
Figure imgb0004

Cette condition permet d'assurer que l'on aura une température Ms pas trop élevée, comme son respect représenterait un abaissement de Ms de l'ordre de 60°C par rapport à ce qu'autoriserait la satisfaction simultanée des limites supérieures des teneurs en C, N et Cr choisies.This condition makes it possible to ensure that there will be a temperature Ms not too high, as its compliance would represent a reduction of Ms of the order of 60 ° C compared to what would allow the simultaneous satisfaction of the upper limits of the contents in C, N and Cr chosen.

Des aciers selon l'invention ont fait l'objet d'essais d'austénitisation à différentes températures avant une trempe à l'eau à 20°C avec une vitesse de refroidissement supérieure à 100°C/s, suivie par un revenu à 200°C, afin de faire varier la proportion de carbures dissous, et par conséquent la teneur en carbone dans l'austénite puis dans la martensite après trempe. Le taux de martensite ainsi que la dureté Vickers ont été mesurés afin de tracer l'évolution de la dureté en fonction du taux de martensite, et les résultats sont représentés sur la figure 1, pour un acier ayant la composition de l'exemple I4 du tableau 1.Steels according to the invention were 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 tempering at 200 ° C, in order to vary the proportion of dissolved carbides, and consequently the carbon content in the austenite and then in the martensite after quenching. The rate of martensite as well as Vickers hardness were measured in order to plot the evolution of hardness as a function of the rate of martensite, and the results are represented on the figure 1 , for a steel having the composition of Example I4 in Table 1.

On voit sur la figure 1 que la dureté commence par croître avec la baisse du taux de martensite, car la martensite durcit par enrichissement en carbone. La dureté atteint un maximum, puis baisse lorsque le taux de martensite devient trop faible. En dessous de 75% de martensite, le durcissement de la martensite ne compense plus l'adoucissement lié à la présence d'austénite résiduelle de dureté plus faible. Pour cette raison, dans un mode préféré de l'invention, adapté à la fabrication d'outil de coupe à partir de l'acier coulé, le taux de martensite de l'acier après austénitisation, trempe à une vitesse d'au moins 15°C/s jusqu'à une température inférieure ou égale à 20°C, puis revenu à une température de 100 à 300°C, typiquement 200°C, est supérieur ou égal à 75%.We see on the figure 1 that hardness begins to increase with the drop in the rate of martensite, because martensite hardens by carbon enrichment. The hardness reaches a maximum, then decreases when the rate of martensite becomes too low. Below 75% martensite, the hardening of the martensite no longer compensates for the softening linked to the presence of residual austenite of lower hardness. For this reason, in a preferred embodiment of the invention, suitable for the manufacture of cutting tools from cast steel, the rate of martensite in the steel after austenitization, quenching at a speed of at least 15 ° C / s up to a temperature less than or equal to 20 ° C, then returned to a temperature of 100 to 300 ° C, typically 200 ° C, is greater than or equal to 75%.

L'obtention d'une teneur en martensite élevée pouvant atteindre 100% peut être mieux assurée, si, après la trempe jusqu'à 20°C ou moins, on procède à un traitement cryogénique, c'est-à-dire la réalisation d'une trempe dans un milieu à très basse température allant de -220 à -50°C, typiquement dans de l'azote liquide à -196°C ou dans de la neige carbonique à -80°C, avant de procéder au revenu à 100-300°C.Obtaining a high martensite content of up to 100% can be better ensured if, after quenching to 20 ° C or less, a cryogenic treatment is carried out, that is to say the realization of '' quenching in a medium at 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 tempering 100-300 ° C.

Lorsque la teneur en martensite n'atteint pas 100%, la microstructure restante est typiquement constituée essentiellement d'austénite résiduelle. Il peut aussi y avoir de la ferrite.When the martensite content does not reach 100%, the remaining microstructure typically consists essentially of residual austenite. There may also be ferrite.

A titre d'exemples non limitatifs, les résultats suivants vont montrer les caractéristiques avantageuses conférées par l'invention.By way of nonlimiting examples, the following results will show the advantageous characteristics conferred by the invention.

Les compositions des différents échantillons d'acier testés figurent dans le tableau 1, exprimées en % pondéraux. Les valeurs soulignées sont celles qui ne sont pas conformes à l'invention. On a également reporté les valeurs de C + N, de Cr + 16 N - 5 C et de 17Cr + 500C + 500N pour chaque échantillon.

Figure imgb0005
Figure imgb0006
Figure imgb0007
 The compositions of the various steel samples tested are shown in Table 1, expressed in% by weight. The values underlined are those which are not in accordance with the invention. The values of C + N, Cr + 16 N - 5 C and 17Cr + 500C + 500N were also reported for each sample.
Figure imgb0005
Figure imgb0006
Figure imgb0007

Après coulée, ces aciers ont été réchauffés à une température supérieure à 1100°C, laminés à chaud jusqu'à une épaisseur de 3mm, recuits à une température de 800°C, puis décapés et laminés à froid jusqu'à une épaisseur de 1,5mm.After casting, these steels were reheated to a temperature above 1100 ° C, hot rolled to a thickness of 3mm, annealed at a temperature of 800 ° C, then pickled and cold rolled to a thickness of 1 , 5mm.

Les tôles d'acier ont été ensuite recuites à une température de 800°C.The steel sheets were then annealed at a temperature of 800 ° C.

Les tôles d'acier recuites ont subi ensuite un traitement d'austénitisation de 15 minutes à 1050°C suivi d'une trempe à l'eau jusqu'à la température de 20°C.The annealed steel sheets were then subjected to an austenitization treatment for 15 minutes at 1050 ° C. followed by quenching with water up to the temperature of 20 ° C.

Après découpe des tôles en deux parties, l'une des parties a été ensuite plongée pendant 10mn dans un bain thermostaté à -80°C, de façon à pouvoir évaluer les effets d'un traitement cryogénique qui s'ajouterait à la simple trempe à l'eau.After cutting the sheets into two parts, one of the parts was then immersed for 10 minutes in a bath thermostatically controlled at -80 ° C, so as to be able to evaluate the effects of a cryogenic treatment which would be added to the simple quenching at the water.

Un revenu de 1h à 200°C a ensuite été effectué sur chaque partie de tôle.An income of 1 hour at 200 ° C. was then carried out on each part of the sheet.

Le tableau 2 présente le résultat d'essais et observations effectués sur ces aciers. Les valeurs soulignées correspondent à des performances jugées insuffisantes.Table 2 presents the results of tests and observations carried out on these steels. The underlined values correspond to performances considered insufficient.

La santé interne est évaluée sur un état brut de solidification après coulée, sachant que les opérations de transformation ultérieures ne la dégraderont pas.The internal health is evaluated on a raw solidification state after casting, knowing that the subsequent transformation operations will not degrade it.

Le taux de martensite est mesuré après une trempe à l'eau à 20°C et après un traitement cryogénique par trempe à -80°C, cette trempe, ou la deuxième de ces trempes, ayant été suivie par un revenu à 200°C. Lorsque le taux de martensite est supérieur ou égale à 75% après trempe à l'eau à 20°C, les autres résultats donnés dans le tableau 2 concernent l'état trempé à 20°C suivi du revenu à 200°C. Lorsque le taux de martensite est inférieur à 75% après la trempe à l'eau à 20°C, les autres résultats donnés dans le tableau 2 concernent l'état après un traitement cryogénique (trempe jusqu'à une très basse température, effectuée par exemple dans de la neige carbonique) à -80°C, suivi du revenu à 200°C.The rate of martensite 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 quenchings, having been followed by tempering at 200 ° C. . When the rate of martensite 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 tempering at 200 ° C. When the rate of martensite is less than 75% after quenching with water at 20 ° C., the other results given in table 2 relate to the state after a cryogenic treatment (quenching to a very low temperature, carried out by example in dry ice) at -80 ° C, followed by tempering at 200 ° C.

La tenue à la corrosion est évaluée par un test électrochimique de corrosion par piqûre dans un milieu composé de NaCl 0.02M, à 23°C et à pH de 6.6. Le test électrochimique réalisé sur 24 échantillons permet de déterminer le potentiel E0.1 pour lequel la probabilité élémentaire de piqûration est égale à 0.1 cm-2. La tenue à la corrosion est considérée comme non satisfaisante si le potentiel E0.1 est inférieur à 350 mV, mesuré par rapport à l'électrode au calomel saturée au KCI (350 mV/ECS). Elle est considérée comme satisfaisante si le potentiel E0.1 est compris entre 350 mV/ECS et 450 mV/ ECS. Elle est considérée comme très satisfaisante si le potentiel E0.1 est supérieur à 450 mV/ECS.The corrosion resistance is evaluated by an electrochemical pitting corrosion test in a medium composed of 0.02M NaCl, at 23 ° C. and at pH 6.6. The electrochemical test carried out on 24 samples makes it possible to determine the potential E 0.1 for which the elementary probability of pitting is equal to 0.1 cm -2 . The corrosion resistance is considered unsatisfactory if the potential E 0.1 is less than 350 mV, measured with respect to the calomel electrode saturated with KCI (350 mV / DHW). It is considered satisfactory if the potential E 0.1 is between 350 mV / DHW and 450 mV / DHW. It is considered very satisfactory if the potential E 0.1 is greater than 450 mV / DHW.

La dureté Vickers est mesurée dans l'épaisseur sur une coupe polie miroir, sous une charge de 1kg avec une pointe pyramidale en diamant de base carrée, suivant la norme EN ISO 6507. La moyenne des duretés obtenues est calculée en réalisant 10 empreintes. La dureté est considérée comme insuffisante si la dureté moyenne est inférieure à 500 HV. Elle est considérée comme satisfaisante si la dureté moyenne est comprise entre 500 HV et 550 HV. Elle est considérée comme très satisfaisante si la dureté moyenne est comprise entre 551 et 600 HV. Elle est considérée comme excellente si la dureté moyenne est supérieure à 600 HV.Vickers hardness is measured in thickness on a mirror polished cut, under a load of 1 kg with a pyramidal point in square base diamond, according to standard EN ISO 6507. The average hardness obtained is calculated by making 10 impressions. The 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.

La polissabilité est évaluée en effectuant un polissage à plat jusqu'à la mi-épaisseur de l'échantillon, en utilisant successivement les papiers SiC 180, 320, 500, 800 et 1200 sous une force de 30 N, puis un polissage sur drap imbibé de pâte diamantée de granulométrie 3 µm puis 1 µm sous une force de 20 N. La surface est ensuite observée au microscope optique au grandissement de x100. La polissabilité est considérée comme non satisfaisante si la densité de défauts appelés classiquement « queues de comète » est supérieure ou égale à 100/cm2. La polissabilité est considérée comme satisfaisante si cette densité est comprise entre 10/cm2 et 99/cm2. La polissabilité est considérée comme très satisfaisante si cette densité est comprise entre 1 et 9/cm2. La polissabilité est considérée comme excellente si cette densité est inférieure à 1/cm2.The polishability is evaluated by performing a flat polishing up to the mid-thickness of the sample, successively using SiC papers 180, 320, 500, 800 and 1200 under a force of 30 N, then polishing on a cloth soaked of diamond paste with a particle size of 3 µm then 1 µm under a force of 20 N. The surface is then observed under an optical microscope at a magnification of x100. The 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 to be 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 .

La santé interne est évaluée en observant l'acier brut de solidification en coupe par métallographie optique au grandissement x25. La santé interne n'est pas satisfaisante et indiquée par la valeur « 0 » dans le tableau 2 si des cavités globulaires (soufflures) traduisant la formation de bulles d'azote à la solidification sont observées. Dans le cas contraire la santé interne est considérée comme satisfaisante et indiquée par la valeur « 1 » dans le tableau 2.Internal health is evaluated by observing the solidification solid steel in section by optical metallography at x25 magnification. Internal health is not satisfactory and indicated by the value "0" in Table 2 if globular cavities (blisters) reflecting the formation of nitrogen bubbles upon solidification are observed. Otherwise internal health is considered satisfactory and indicated by the value "1" in Table 2.

Le taux de martensite est déterminé par diffraction des rayons X en mesurant l'intensité des raies caractéristiques de la martensite comparativement à l'intensité des raies caractéristiques de l'austénite sachant que, dans tous les échantillons examinés, ce sont les deux seules phases en présence. De manière générale, il ne serait pas exclu que l'on observe marginalement d'autres phases dans des échantillons selon l'invention. C'est le taux de martensite qui est avant tout à considérer dans le cadre de l'invention.The rate of martensite 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 marginally in samples according to the invention. It is the rate of martensite which is above all to be considered in the context of the invention.

Un taux de martensite supérieur ou égal à 75% après trempe à 20°C et revenu à 200°C, ou supérieur ou égal à 75% après une trempe à 20°C, un traitement cryogénique à -80°C et un revenu à 200°C, est satisfaisant. Si un taux de martensite de 75% ou plus ne peut être obtenu par l'un de ces traitements, l'échantillon est considéré comme non satisfaisant. Tableau 2 : résultats des essais réalisés sur les échantillons du tableau 1 E0,1 (mV/ECS) Dureté HV Polissabilité (queues de comète/cm2) Santé interne Martensite (%) trempe 20°C Martensite (%) trempe -80°C Invention I1 610 554 0 1 100 100 I2 695 536 0 1 97 100 I3 570 650 0 1 95 100 I4 510 698 47 1 88 95 I5 510 689 36 1 78 86 I6 610 648 43 1 76 85 I7 660 687 51 1 69 81 I8 515 700 0,8 1 97 100 I9 565 690 0,6 1 96 100 I10 580 689 0,5 1 94 97 I11 690 680 0,5 1 90 94 I12 565 689 8 1 92 98 I13 510 670 0,4 1 95 100 I14 540 628 49 1 76 86 I15 535 580 0 1 98 100 I16 520 682 3 1 90 97 I17 580 653 2 1 93 100 I18 565 520 0 1 100 100 I19 585 621 0 1 96 100 Références R1 240 488 0 1 100 100 R2 240 566 0,2 1 100 100 R3 190 683 124 1 97 100 R4 330 693 18 1 95 100 R5 490 680 0,2 0 93 99 R6 400 686 109 1 93 98 R7 630 650 24 0 0 63 79 R8 550 699 215 1 48 70 R9 705 615 56 1 50 72 R10 510 479 0 1 100 100 R11 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 A rate of martensite greater than or equal to 75% after quenching at 20 ° C and tempering 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 level of 75% or more cannot be obtained by one of these treatments, the sample is considered unsatisfactory. Table 2: results of the tests carried out on the samples in Table 1 E 0.1 (mV / DHW) HV hardness Polishability (comet tails / cm 2 ) Internal health Martensite (%) quenching 20 ° C Martensite (%) quenching -80 ° C Invention I1 610 554 0 1 100 100 I2 695 536 0 1 97 100 I3 570 650 0 1 95 100 I4 510 698 47 1 88 95 I5 510 689 36 1 78 86 I6 610 648 43 1 76 85 I7 660 687 51 1 69 81 I8 515 700 0.8 1 97 100 I9 565 690 0.6 1 96 100 I10 580 689 0.5 1 94 97 I11 690 680 0.5 1 90 94 I12 565 689 8 1 92 98 I13 510 670 0.4 1 95 100 I14 540 628 49 1 76 86 I15 535 580 0 1 98 100 I16 520 682 3 1 90 97 I17 580 653 2 1 93 100 I18 565 520 0 1 100 100 I19 585 621 0 1 96 100 References R1 240 488 0 1 100 100 R2 240 566 0.2 1 100 100 R3 190 683 124 1 97 100 R4 330 693 18 1 95 100 R5 490 680 0.2 0 93 99 R6 400 686 109 1 93 98 R7 630 650 24 0 0 63 79 R8 550 699 215 1 48 70 R9 705 615 56 1 50 72 R10 510 479 0 1 100 100 R11 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

Les aciers selon l'invention I1 à I6, ainsi que les aciers I8 à I9, combinent de bonnes propriétés de tenue à la corrosion, de dureté et de polissabilité, et présentent une bonne santé interne, ainsi qu'un taux de martensite supérieur ou égal à 75% dès après une trempe à 20°C.The steels according to the invention I1 to I6, as well as the steels I8 to I9, combine good properties of resistance to corrosion, hardness and polishability, and have good internal health, as well as a higher rate of martensite or equal to 75% immediately after quenching at 20 ° C.

L'acier selon l'invention I7 combine de bonnes propriétés de tenue à la corrosion, de dureté et de polissabilité, et présente une bonne santé interne ainsi qu'un taux de martensite supérieur ou égal à 75%, mais à condition d'effectuer un traitement cryogénique à -80°C. En effet à l'issue d'une simple trempe à l'eau à 20°C, le taux de martensite n'est pas encore suffisant, ce qui est à relier à la présence de Cr à un niveau supérieur à celui des autres échantillons selon l'invention.The steel according to the invention I7 combines good properties of resistance to corrosion, hardness and polishability, and has good internal health as well as a rate of martensite greater than or equal to 75%, but on condition that it performs cryogenic treatment at -80 ° C. Indeed after a simple water quenching at 20 ° C, the rate of Martensite is not yet sufficient, which is to be linked to the presence of Cr at a level higher than that of the other samples according to the invention.

A niveau de N comparable, on voit que la dureté augmente entre d'une part les échantillons I1, I2 où C est entre 0,10 et 0,20%, et d'autre part les échantillons I3 où C est entre 0,20 et 0,30% et surtout I8, I9, I10 où C est entre 0,30 et 0,35%.At comparable level of N, it is seen that the hardness increases between on the one hand the samples I1, I2 where C is between 0.10 and 0.20%, and on the other hand the samples I3 where C is between 0.20 and 0.30% and especially I8, I9, I10 where C is between 0.30 and 0.35%.

I14, où C est encore plus élevé et N est du même niveau que les précédents, a une dureté moindre qu'eux, car la fraction de martensite après trempe commence à baisser de par la diminution de la température Mf en relation avec une valeur élevée de l'expression 17Cr + 500C + 500N (voir le tableau 1). Egalement à niveaux de N et des autres éléments essentiels comparables, on voit que l'augmentation de Cr permet d'améliorer la résistance à la corrosion, voir les échantillons I8 et I9. Inversement, l'augmentation de la teneur en Cr tend à diminuer la dureté, voir les échantillons I8, I10 et I11 dont les compositions ne diffèrent significativement que sur Cr. Aller au-delà de 18% de Cr pourrait augmenter la résistance à la corrosion, mais conduirait à diminuer les teneurs en C et N pour conserver une Ms satisfaisante, et une dureté correcte ne serait plus assurée.I14, where C is even higher and N is of the same level as the previous ones, has a lower hardness than them, because the fraction of martensite after quenching begins to decrease due to the decrease in temperature Mf in relation to a high value of the expression 17Cr + 500C + 500N (see Table 1). Also at levels of N and of the other comparable essential elements, it is seen that the increase in Cr makes it possible to improve the resistance to corrosion, see samples I8 and I9. Conversely, the increase in the Cr content tends to decrease the hardness, see samples I8, I10 and I11 whose compositions differ significantly only on Cr. Going beyond 18% of Cr could increase the corrosion resistance, but would lead to lowering the C and N contents to maintain a satisfactory Ms, and a correct hardness would no longer be ensured.

Les aciers de référence R1 à R3 ont des teneurs en Cr et N, ainsi que des sommes C + N et/ou Cr + 16 N - 5 C insuffisantes, ce qui ne permet pas une tenue à la corrosion satisfaisante.The reference steels R1 to R3 have insufficient Cr and N contents, as well as insufficient C + N and / or Cr + 16 N - 5 C sums, which does not allow satisfactory corrosion resistance.

Les aciers de référence R4 et R5 ont des teneurs en Cr insuffisantes. Sans compensation par un ajout de N, l'acier R4 présente aussi une combinaison Cr + 16 N - 5 C insuffisante conduisant à une tenue à la corrosion non satisfaisante. Pour l'acier R5, la compensation du manque de Cr par un ajout de N rétablit une tenue à la corrosion satisfaisante, mais ne permet plus d'assurer une bonne santé interne car la teneur en Cr n'est plus suffisante pour permettre une dissolution complète de N dans le métal liquide.The reference steels R4 and R5 have insufficient Cr contents. Without compensation by adding N, steel R4 also has an insufficient Cr + 16 N - 5 C combination, leading to unsatisfactory corrosion resistance. For R5 steel, compensating for the lack of Cr by adding N restores satisfactory corrosion resistance, but no longer ensures good internal health since the Cr content is no longer sufficient to allow dissolution. complete with N in the liquid metal.

L'acier de référence R6 a une teneur en C trop élevée et une teneur en N insuffisante. La teneur en C trop élevée ne permet pas une aptitude au polissage suffisante du fait de la formation de carbures trop importante.The reference steel R6 has too high a C content and an insufficient N content. The excessively high C content does not allow sufficient polishability due to the formation of too large carbides.

L'acier de référence R7 a une teneur en N trop élevée, ce qui dégrade la santé interne. Il en est de même pour l'acier de référence R14. L'acier de référence R8 à une teneur excessive en C, ce qui conduit à une mauvaise polissabilité et à un taux de martensite trop faible même après une trempe cryogénique à -80°C. L'acier de référence R9 contient trop de Cr, ce qui conduit à un taux de martensite insuffisant même après une trempe cryogénique à -80°C.The reference steel R7 has too high an N content, which degrades internal health. The same is true for the reference steel R14. The reference steel R8 has an excessive content of C, which leads to poor polishability and to a rate of martensite which is too low even after a cryogenic quenching at -80 ° C. The reference steel R9 contains too much Cr, which leads to an insufficient rate of martensite even after a cryogenic quenching at -80 ° C.

Les aciers de référence R10 et R11 ont des teneurs en C trop faibles ainsi que des sommes C + N insuffisantes, conduisant à des duretés trop faibles. Les aciers de référence R12 et R13 auraient des compositions conformes à l'invention sur les teneurs individuelles de chaque élément, mais leur somme Cr + 16 N - 5 C, qui est inférieure à 16,0%, est insuffisante pour garantir une résistance à la corrosion aussi élevée que celle des aciers qui sont en tous points conformes à l'invention, y compris de ceux qui ne dépassent que de peu la valeur 16,0% pour cette somme Cr + 16 N - 5 C.The reference steels R10 and R11 have too low C contents as well as insufficient C + N sums, leading to too low hardnesses. Steels of reference R12 and R13 would have compositions in accordance with 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 corrosion resistance also higher than that of steels which are in all respects in accordance with the invention, including those which only slightly exceed the value 16.0% for this sum Cr + 16 N - 5 C.

Les aciers selon l'invention sont utilisés avec profit pour la fabrication d'outils de coupe, comme par exemple des scalpels, des ciseaux, des lames de couteaux ou des lames circulaires de robots ménagers.The steels according to the invention are used with profit for the manufacture of cutting tools, such as for example scalpels, scissors, knife blades or circular blades of household robots.

Claims (9)

  1. 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 ≤ Si ≤ 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%; optimally 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 ≤ 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 17Cr + 500C + 500N ≤ 570%;
    the rest being iron and impurities resulting from the development.
  2. The steel according to claim 1, characterized in that its microstructure includes at least 75% martensite.
  3. A method for producing a semi-finished product made from martensitic stainless steel, characterized in that:
    - a semi-finished product is developed and cast from a steel having the composition according to claim 1;
    - said semi-finished product is heated to a temperature greater than or equal to 1000°C;
    - it is hot rolled to obtain a sheet, bar or wire rod;
    - said sheet, bar or wire rod is annealed at a temperature comprised between 700 and 900°C; and
    - a shaping operation is carried out on said sheet, bar or wire rod.
  4. The method according to claim 3, characterized in that said semi-finished product is a sheet, and said shaping operation is a cold rolling.
  5. The method according to claim 3, characterized in that said semi-finished product is a bar or wire rod, and said shaping operation is a forging.
  6. The 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 semi-finished product is next austenitized between 950 and 1150°C, then cooled at a speed of at least 15°C/s to a temperature of less than or equal to 20°C, then undergoes annealing at a temperature comprised between 100 and 300°C.
  7. The 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 shaped semi-finished product is next austenitized between 950 and 1150°C, then cooled at a speed 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 from -220 to -50°C, then tempering at a temperature comprised between 100 and 300°C.
  8. A cutting tool, characterized in that it has been made from a semi-finished product prepared according to the method of one of claims 3 to 7.
  9. The cutting tool according to claim 8, characterized in that it is a cutlery items such as a knife blade, a food processor blade, a scalpel, or a scissor blade.
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