Classifications

• • 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
  • C22C33/00—Making ferrous alloys
  • C22C33/006—Making ferrous alloys compositions used for making ferrous alloys
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working

Definitions

• the present invention relates to a steel with high mechanical strength and high wear resistance.
• high wear resistance steels are used. Examples are steels intended to manufacture equipment for the mineral industries and which must resist abrasion. They are also steels intended to manufacture tools for cold or medium-hot forming of metal parts and which must withstand wear by metal-to-metal friction. For these tooling applications, at least the steels must retain good properties despite heating at temperatures up to 500 ° C or 600 ° C. In addition to this wear resistance, the steels considered here must have properties adapted to be machined or welded. They must finally be able to withstand shocks or intense efforts.
• steels containing approximately between 0.3% and 1.5% of carbon, less than 2% of silicon and less than 2% of manganese are used. up to 3% nickel, between 1% and 12% chromium, between 0.5% and 5% molybdenum, with possible addition of vanadium or niobium.
• the wear resistance results mainly from the hardening caused by the secondary precipitation of molybdenum carbides. This resistance to wear can be improved, if necessary, by the presence of large léburitic carbides especially rich in chromium.
• the object of the present invention is to remedy this drawback by proposing a means for obtaining a steel whose properties are equivalent to those known steels, but the harmfulness of segregated veins is significantly reduced.
• the subject of the invention is a method for reducing the harmfulness of segregated veins of a steel with high mechanical strength and high wear resistance, the composition of which comprises, by weight:
• the amount of carbon added ⁇ C forming early titanium carbides and / or zirconium, is no longer available and therefore does not intervene in the hardening secondary precipitation of molybdenum carbides, tungsten, vanadium and secondarily chromium.
• the chromium content is between 2.5 and 3.5%, and if the carbon, titanium and zirconium contents are such that C ⁇ 0.51% before adjustment, the contents in W are preferably limited so that, after adjustment, W ⁇ 0.85% if Mo ⁇ 1, 21% and W / Mo ⁇ 0.7 if Mo> 1, 21%.
• the invention also relates to a steel with high mechanical strength and high wear resistance, possibly obtainable by the method according to the invention, whose chemical composition comprises, by weight: 0.35% ⁇ C ⁇ 1 , 47%, 0.05% ⁇ If ⁇ 1, 5%, Mn ⁇ 1, 95%, Ni ⁇ 2.9%, 1, 1% ⁇ Cr 7.9%, 0% ⁇ Mo ⁇ 4.29% , 0.21% ⁇ W ⁇ 4.9% 0.61% ⁇ Mo + W / 2 ⁇ 4.4% 0% ⁇ Ti ⁇ 1, 49% 0% ⁇ Zr ⁇ 2.9% 0.2% ⁇ Ti + Zr / 2 ⁇ 1, 49% - optionally one or more elements taken from vanadium, niobium and tantalum, in contents such that V ⁇ 1, 45%,
• Nb ⁇ 1, 45%, Ta ⁇ 1, 45% and V + Nb / 2 + Ta / 4 ⁇ 1, 45%, - optionally up to 0.1% boron, - optionally up to 0.19% of sulfur, up to 0.38% of selenium and up to 0.76% of tellurium, the sum S + Se / 2 + Te / 4 remaining less than or equal to 0.19%, optionally up to 0 , 01% of calcium, - optionally up to 0.5% of rare earths, - possibly up to 1% of aluminum, - possibly up to 1% of copper, the rest being iron and impurities resulting from the development, the composition satisfying the following conditions: (Ti + Zr / 2) / W ⁇ 0.20 (Ti + Zr / 2) x C ⁇ 0.07 0.3% ⁇ C * ⁇ 1.42%, and preferably ⁇ 1, 1% 800 D ⁇ 1150 with D 540 (C +) + 25 + 245 (Mo + W / 2 + 3V + 1.5 Nb + 0.
• the steel may further satisfy one or more of the following conditions: If ⁇ 0.45%, if preference is to be given to thermal conductivity, or Si> 0.45%, if preference is to be given to suitability for work hot, or:
• the steel may be such that: Ti + Zr / 2 ⁇ 0.7% in order to favor toughness, or such that: Ti + Zr / 2 ⁇ 0.7% in order to favor the wear resistance.
• the invention also relates to a method for manufacturing a steel part according to the invention, according to which: a liquid steel having the desired composition is produced by adjusting the titanium and / or zirconium contents in the molten steel bath , preferably by avoiding at any time the local over-concentrations of titanium and / or zirconium in the molten steel bath, - said steel is cast to obtain a semi-finished product; and then subjecting said semi-finished product to a shaping treatment by hot plastic deformation and, optionally, a heat treatment, to obtain said part.
• the addition of titanium and / or zirconium is made by progressively adding titanium and / or zirconium to a slag covering the liquid steel bath and allowing the titanium and / or zirconium to slowly diffuse into the liquid steel bath.
• the addition of titanium and / or zirconium can also be carried out by introducing a wire comprising titanium and / or zirconium into the bath of liquid steel, while stirring the bath.
• the invention finally relates to a steel piece according to the invention that can be obtained by the manufacturing method according to the invention.
• tungsten is an alloying element whose effects on the properties of steel are comparable to those of molybdenum.
• tungsten has effects of hardening and resistance to thermal softening comparable to those of molybdenum in the proportion of two parts of tungsten to one part molybdenum.
• tungsten is little used, except in some high-alloy steels not concerned by the present invention, and this, in particular, because it is much more expensive than molybdenum.
• tungsten like molybdenum, has the disadvantage of segregate very strongly and give rise to segregated veins very hard and very fragile.
• Titanium or zirconium also form carbides. But these carbides are relatively large, and therefore comparatively few in number and have no noticeable hardening effect on the metal matrix itself. the inventors have found in a new and unexpected way that, when the steel simultaneously contains titanium and / or zirconium on the one hand, and tungsten on the other hand, tungsten tends to precipitate together with titanium and / or zirconium to form large, non-hardening precipitates. Thus, in view of these observations, it may be thought that, in the presence of titanium and / or zirconium, the tungsten content and thus the density of fine carbide-hardening precipitates is decreased, and more particularly in the segregated veins.
• the process according to the invention applies to a steel which, before the process is used, contains mainly from 0.30% to 1.4% of carbon, from 0.05% to 1.5% of silicon, less than 1, 95% manganese, less than 2.9% nickel, 1.1% to 7.9% chromium, 0.61% to 4.4% molybdenum, up to 1, 45% vanadium, up to 1.45% niobium, less than 1.45% tantalum with V + Nb / 2 + Ta / 4 ⁇ 1, 45%.
• This steel has a hardness index D, which will be explained later, between 800 and 1150.
• It may contain, in addition, up to 0.1% of boron, up to 0.19% of sulfur, up to to 0.38% selenium, up to 0.79% tellurium, the sum S + Se / 2 + Te / 4 remaining less than 0.19%, optionally up to 0.01% calcium, up to at 0.5% rare earths, up to 1% aluminum and up to 1% copper.
• all or part of the molybdenum is replaced by a substantially double proportion of tungsten, titanium and / or zirconium are added so as to obtain sufficient quantities of titanium and / or zirconium taking into account the amounts of tungsten introduced into the steel, and the carbon content is adjusted so that, in particular, the hardness of the steel remains substantially unchanged.
• the target composition for steel without tungsten so to obtain the desired job characteristics, in particular the level of hardness.
• the target composition is then modified by choosing a tungsten content, thereby adjusting the molybdenum content and the titanium or zirconium and carbon contents, so that at least one of the main use characteristics, in particular the hardness, remains substantially unchanged.
• a steel is developed corresponding to the modified analysis.
• substantially unchanged is meant, for example, that the hardness of the steel after adjustment of the composition is equal to the hardness of the steel before adjustment of the composition to within 5%.
• the tolerance is introduced to take into account the practical difficulties of producing steel with precisely defined properties. However, it is desirable that the characteristics obtained are as close as possible to the characteristics targeted for the steel before adjustment of the composition. Also, it is preferable that the tolerance is only 2%, and, insofar as one is interested only in the characteristics concerned, it is still more preferable that the characteristic of hardness aimed after adjustment of the composition is equal to the hardness characteristic referred to before adjustment of the composition.
• the amount of tungsten added should be greater than or equal to 0.21%, preferably greater than 0.4%, more preferably greater than 0.7%, and more preferably greater than 1.05%. Indeed, the greater the substitution of molybdenum by tungsten, the greater the effect on segregations.
• the titanium and zirconium contents must be such that the sum Ti + Zr / 2 is greater than or equal to 0.2 ⁇ W, preferably greater than or equal to 0.4 ⁇ W, more preferably greater than or equal to 0.6 x W.
• the titanium and zirconium contents must be such that the sum Ti + Zr / 2 is greater than or equal to 0.2 ⁇ W, preferably greater than or equal to 0.4 ⁇ W, more preferably greater than or equal to 0.6 x W.
• the steel according to the invention contains more than 0.35% carbon, preferably more than 0.51%, and better still more than 0.65%, in order to be able to form enough carbides and reach the level of hardness that it is desired to obtain, but less than 1, 47% and preferably less than 1, 1% and more preferably less than 0.98% to avoid too much weaken the steel.
• the steel contains titanium and zirconium, and these elements combine at high temperatures with carbon to form primary carbides.
• C * C - Ti / 4 - Zr / 8 (C, Ti and Zr being the contents of carbon steel, titanium and zinconium, respectively, in the following C will also be called “total carbon content”).
• This amount of available carbon be sufficient to allow the precipitation of secondary carbides and in particular carbides of tungsten, molybdenum or other elements which are added to the steel, and from this point of view, this free carbon content C * must be greater or equal to 0.3%.
• this content should not exceed 1.42%, and preferably 1.1% or better 0.98%, or more preferably 0.79%, not to excessively impair the toughness of the matrix itself.
• Steel contains more than 0.05% silicon because this element is a deoxidizer. In addition, it contributes a bit to the hardening of steel. However, the silicon content must remain less than or equal to 1.5% and preferably less than or equal to 1.1%, better still 0.9%, and better still, less than or equal to 0.6%, in order to avoid excessively weaken the steel and too much reduce its ability to plastic deformation hot, for example by rolling. In addition, it may be desirable to impose a minimum silicon content of 0.45%, and better still 0.6%, in order to improve the machinability of the steel and also to improve the strength of the steel. oxidation.
• the improvement in oxidation resistance is particularly desirable when the steel is used to manufacture workpieces intended to work at relatively high temperatures of the order of 450 ° C to 600 ° C, which requires resistance to oxidation. sufficient softening.
• the content of Mo + W / 2 it is desirable for the content of Mo + W / 2 to be greater than or equal to 2.2%.
• the minimum values of silicon content, 0.45% or better still 0.6%, are more particularly advantageous when the contents of molybdenum and tungsten are such that the sum Mo + W / 2 is greater than or equal to 2 , 2%, without this nevertheless having an exclusive character.
• the silicon content remains below 0.45%, and preferably, as low as possible.
• the steel contains manganese up to 1.95% by weight in order to improve the hardenability of the steel, but this content should preferably remain less than or equal to 1, 5% and better still less than or equal to 0, 9% to limit segregations that would result in poor forgeability and insufficient toughness. It should be noted that the steel always contains a small amount of manganese, a few tenths of a percent, in particular to fix the sulfur and it is preferable that the Mn content is at least 0.4%. Steel contains up to 2.9% nickel to adjust quenchability and improve toughness. But this element, is very expensive. Also, one does not generally seek a nickel content exceeding 0.9% or even 0.7%.
• Steel may not contain nickel but when nickel is not added voluntarily, it is interesting that steel contains up to 0.2% or even up to 0.4% in the form of residuals resulting from development.
• the steel contains more than 1, 1% chromium and better still more than 2.1%, and even better still more than 3.1% and even more than 3.5%, in order to obtain sufficient quenchability and to increase hardening. to income, but less than 7.9%, and better still less than 5.9% or better still, less than 4.9% so as not to hinder the formation of secondary carbides, in particular containing Mo and / or W and, as such, more effective than the hardening chromium carbides.
• this element tends to form, especially in the segregated veins, type ledeburitic carbides that are coarse and more or less arranged in inter-dendritic networks. These carbides, despite a certain favorable effect on the wear resistance, contribute mainly to an embrittlement at least local matrix. So that when it is desired to favor hardness and wear resistance to the detriment of toughness, it is desirable to choose a chromium content greater than or equal to 3.5%, favoring the presence of ldeturitic type carbides. On the other hand, when seeking to promote the toughness of steel by accepting a slight reduction in wear resistance, it is preferable to choose a chromium content of less than or equal to 2.5%.
• this content is even desirable for this content to be greater than 2.2% in order to obtain a high degree of hardening and a better resistance to thermal softening, in particular when the use of the steel causes it to be heated up. at temperatures that may exceed about 450 ° C.
• This is the case, for example, of the steels used to produce mid-hot work tools for steel.
• the sum Mo + W / 2 can go up to 2.9%, even 3.4%, or even 3.9%, depending on the desired hardness and the temperature of income that one wishes to achieve on rooms.
• Mo + W / 2 can even go up to at 4.4%.
• This content may be up to 4.9% but will not usually exceed 1, 9%; In general, content is lower than or equal to 0.90% or even 0.79%.
• the molybdenum content may be at trace level, but preferably at least 0.51% and more preferably even at least 1.4%; better still, at least 2.05%.
• it will not be necessary to exceed the limiting levels of 4.29%, preferably 3.4% or, better, 2.9%, limitations which also allow to reduce accordingly the contributions of molybdenum to hardening segregation.
• the tungsten content is limited to not more than 0.85% when the molybdenum content is less than 1, 21%, and the tungsten / molybdenum ratio is limited to not more than 0,7 when the molybdenum content is greater than or equal to 1, 21%.
• the titanium and zirconium contents must be adjusted so that the sum Ti + Zr / 2 is at least 0.21% and preferably greater than or equal to 0.41% or better, greater than or equal to 0.61%, to obtain the desired effect of reducing the harmfulness of the segregated veins.
• these elements contribute to the formation of large carbides that improve wear resistance. However, this sum must remain less than 1.49% and preferably less than 1.19% or even less than 0.99% or even less than 0.79% so as not to deteriorate the toughness too much.
• the titanium and zirconium contents must be adjusted according to whether it is desired to favor the tenacity of the steel or its resistance to wear. From this point of view, when it is desired to favor the toughness of steel, the sum Ti + Zr / 2 should preferably remain below 0.7%. When it is desired to favor the wear resistance of the steel, the sum Ti + Zr / 2 must preferably be greater than or equal to 0.7%.
• the titanium and zirconium contents must be sufficient with respect to the total carbon content C.
• the product (Ti + Zr / 2) x C must be greater than or equal to 0.07, preferably greater than or equal to 0.12, and better still greater than or equal to 0.2.
• the minimum titanium content may be 0%, or traces, but it is preferable that it be at least 0.21%, and better 0.41%, better still, 0.61%; the minimum zirconium content may be 0%, or traces, but it is preferable that it be at least 0.06%, or better still at least 0.11%.
• the maximum content of titanium is 1.49% but can be reduced to 1.19%, or even 0.99%, better to 0.79% or even 0.7%, while the maximum content of Zirconium is 2.9%, preferably 0.9%, more preferably 0.49%.
• the steel optionally contains up to 1.45% of vanadium, up to 1.45% of niobium, up to 1.45% of tantalum, the sum V + Nb / 2 + Ta / 4 being less than 1, 45%, better below 0.95% and even below 0.45%.
• the minimum content is 0% or traces, but it is preferable that it be at least 0.11%, and more preferably at least 0.21%.
• the level of addition of V + Nb / 2 + Ta / 4 helps to determine resistance and income response as indicated in the D index formulation.
• These elements have the advantage of greatly improving the resistance to softening by the precipitation of carbides type MC.
• Niobium although it can be used, has the disadvantage of precipitating at a higher temperature than vanadium, which greatly reduces the forgeability of the steel. Therefore the presence of niobium is not recommended and, in any case, it is desirable that the niobium content remains less than 1% or even 0.5% or, better still, less than 0.05%.
• the steel optionally contains up to 0.095% or even up to
• 0.19% sulfur to improve machinability however, a content of less than 0.005% is preferable when looking for good toughness.
• a minimum sulfur content of 0.011% or better, 0.051% is desirable.
• the sulfur may be substituted, in whole or in part, by a double weight of selenium or quadruple tellurium; however, the more economical addition of sulfur will usually be preferred.
• it may be beneficial to enhance the favorable action of Sulfur on machinability by adding calcium content up to 0.010%, to promote the formation of mixed sulphides Mn and Ca, more effective against cutting tool.
• the steel may contain up to 0.38% of selenium, up to 0.76% of tellurium and up to 0.01% of calcium, the sum S + Se / 2 + Te / 4 remaining lower or equal to 0.19%.
• the steel optionally contains up to 0.5% rare earths to facilitate the germination of the carbides and refine the structure, and possibly up to 0.1% boron to improve the quenchability.
• Steel can also contain up to 1% copper. This element is not desired but can be provided by raw materials that would be too expensive to sort. Nevertheless, the copper content must be limited because this element has an adverse effect on hot ductility. In this respect, the presence of Ni in a content at least equal to that of copper is desired, at least when the copper content exceeds about 0.5%.
• the steel may contain aluminum which, like silicon, can contribute to the deoxidation of the liquid metal.
• the aluminum content will be at the trace level or better still, at least equal to 0.006%, better still, at least 0.020%.
• the content of this element must remain less than 1% to ensure sufficient cleanliness, and preferably will not exceed 0.100%, better still less than 0.050%.
• the rest of the composition consists of iron and impurities resulting from the elaboration.
• D is a hardness index which represents the hardening resulting from the income for standard income conditions (550 ° C for 1 hour).
• D the higher the value of D is, the higher the hardness after the temperature is determined at high temperature, or the higher the temperature that makes it possible to reach a given hardness level.
• the hardness varies as a function of temperature and tempering time as is known to those skilled in the art. Note that this formula applies both to the steel according to the invention or the steel obtained by the process according to the invention, than to the starting steel to which the method according to the invention is applied. In all cases, the grades to be taken into account are the actual grades of the steel for which the calculation is made.
• the coefficient D is between 800 and 1150.
• this interval can be decomposed into sub-intervals according to the level of hardness desired by the user and the expected temperature of income.
• the value of D will be in the following ranges: - between 800 and 900 - between 901 and 950 - between 951 and 1000 - between 1001 and 1075 - between 1076 and 1150 In these ranges, the typical hardness levels obtained after income at 550 ° C.
• compositions defined as follows, for the steel according to the invention: 0.55 ⁇ C ⁇ 1, 1% 0.21% ⁇ Ti ⁇ 1, 19 % Zr: 0% or traces 0.05% ⁇ Si ⁇ 0.9% Mn ⁇ 0.9% Ni ⁇ 0.9% 2.1% ⁇ Cr ⁇ 4.9% 2.05% ⁇ Mo ⁇ 2, 9% 0.21% ⁇ W ⁇ 0.79% 0.21% ⁇ V ⁇ 0.45% Nb: 0% or traces Within this domain, subdomains, or groups, defined by the carbon and titanium content ranges, which correspond to the fact that the toughness or wear resistance is more or less preferred.
• Group A 0.85% ⁇ C ⁇ 1, 1% 0.70% ⁇ Ti ⁇ 1, 19%
• Group B 0.65% ⁇ C ⁇ 1, 1% 0.61% ⁇ Ti ⁇ 0.99%
• Group C 0.65% ⁇ C ⁇ 0.98%
• Group D 0.51% ⁇ C ⁇ 0.85% 0.21% ⁇ Ti ⁇ 0.70%
• the hardness level can be adjusted taking into account the influences of the various alloying elements indicated by the expression of the hardness index D.
• the different groups in the order A, B, C, and D, are in the direction of a strengthening of the level of toughness at the cost of a reduction in wear resistance.
• titanium and zirconium be in the form of primary carbides and not in the form of nitrides which are likely to form in the liquid steel, especially when the transient supernoncentrations of titanium and zirconium in the liquid just after the addition are too high given the dissolved nitrogen contents that still exist in the liquid steel .
• a liquid steel is produced by melting all the elements of the grade according to the invention, with the exception of titanium and / or zirconium, - then titanium and zirconium are added to the molten steel bath, avoiding at any time the local overconcentrations of titanium and / or zirconium in the molten steel bath.
• a steel is poured in the form of a semi-finished product such as an ingot or a slab, it is shaped by hot plastic deformation and for example by rolling the semi-finished product, and then the product obtained is subjected to a possible heat treatment. .
• titanium and zirconium into the liquid steel, avoiding any local overconcentration, it is possible to proceed in various ways, and in particular it is possible: - to add titanium and / or zirconium in the slag covering the steel bath liquid, allowing titanium and zirconium to slowly diffuse into the steel bath. - Or add titanium and / or zirconium continuously through a wire composed of this or these elements while stirring the liquid steel bath with gas or by any other suitable method. - Or add titanium and / or zirconium by blowing a powder containing this or these elements in the liquid acid bath while stirring the bath with gas or by any other method. In the context of the present invention, it is preferred to use the various embodiments which have just been described.
• any method to avoid local overconcentration of titanium and / or zirconium may be used.
• This particular addition procedure of Ti and Zr is however not necessary for the preparation of the steel considered here but is an option.
• the heat treatments to which the manufactured part can be subjected are of conventional type for tool steels. Such a heat treatment may optionally comprise one or more anneals to facilitate cutting and machining, then austenitization followed by cooling in a mode adapted to the thickness, such as air or oil cooling. possibly followed by one or more income depending on the level of hardness you want to achieve.
• the contents of molybdenum and tungsten in (Mos and Ws) and out (Moh and Wh) segregated veins were measured by means of a microprobe by masking the large titanium carbides in order to take into account the levels of Molybdenum and Tungsten in the matrix, apart from what can be fixed in these large carbides of titanium and zinconium (which are themselves likely to contain molybdenum or tungsten, forming in fact mixed carbides (Ti Zr Mo W) C ). In this way we appreciate the hardening and weakening part of Mo and W with respect to the metal matrix.
• Mo + W / 2 criterion was chosen since it represents the cumulative hardening contribution of the Mo and W elements, both in segregated veins and outside them.
• Examples ai, bi, ci and di correspond to reference steels, that is to say steels whose composition is chosen before carrying out the process according to the invention.
• the other examples are deduced from these reference steels by the process according to the invention, except examples a 2 and b 2 for which the conditions relating to tungsten and titanium are not respected.
• Examples ai, a 2 and a 3 have the same hardness.
• Example 2 is deduced from Example 1a by replacing 0.20% molybdenum with 0.40% tungsten, without adding titanium. It can be seen that the segregation rate is not significantly modified.
• Example 3 in accordance with the invention, is deduced from Example 1a not only by the replacement of 0.20% of molybdenum with 0.40% of tungsten, but also by the addition of 0.40% by weight. % of titanium and the consequent adjustment of carbon.
• the segregation rate of this steel is very substantially reduced compared to that of examples a- and a 2 .
• examples bi, b 2 b and 3 show that the addition of titanium and zirconium tungsten without addition has no effect (compare b1, b2), while the desired effect appears presence of tungsten partially substituted for molybdenum (comparison b2, b3).
• Examples ci, c 2 and c 3 show that, with equal addition of tungsten, an increase in the addition of titanium has a favorable effect on the segregations. Similarly, examples di, d 2 , and d 3 show that an increase in the tungsten content has a favorable effect when the titanium or zirconium contents are sufficient.
• the ratio (Ti + Zr / 2) / W on the segregation of tungsten one can also consider the examples corresponding to the steels of the castings ref, 5, 7, 1, 9, 6, 2, 18, 13 , 17 and 3 which all correspond to the invention, except casting ref. The main element contents of these flows are reported in Table 2; the rest of the composition being iron and impurities resulting from the preparation.

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