Classifications

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  • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
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  • 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
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  • 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
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • 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
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • 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
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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  • C22C—ALLOYS
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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  • C22C—ALLOYS
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
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  • 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
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  • 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
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  • 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
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  • 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
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  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • 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
  • C21D2211/002—Bainite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to the application of tough fully and partially bainitic heat treatments on certain steels, often alloyed tool steels or steels that can be used for tools and in particular hot work tool steels.
• This heat treatment strategy allows obtaining a fairly homogeneous distribution of properties through heavy sections.
• the resulting microstructures present high toughness.
• the present invention is also often applied to high toughness plastic injection moulding and structural steels and even to cold work and highspeed steels.
• Tool steels often require a combination of different properties which are considered opposed.
• a typical example can be the yield strength and toughness.
• the best compromise of such properties is believed to be obtainable when performing a purely martensitic heat treatment followed by the adequate tempering, to attain the desired hardness.
• hardness i.e mechanical resistance or yield strength
• toughness resilience or fracture toughness
• properties can be: resistance to working conditions (corrosion resistance, wear resistance, oxidation resistance at high temperatures,...), thermal properties (thermal diffusivity, thermal conductivity, specific heat, heat expansion coefficient,%), magnetic and/or electric properties, temperature resistance and many others. Often these properties are microstructure dependent and thus will be modified during heat treatment. So heat treatment is optimized to render the best property compromise for a given application.
• Wear in material shaping processes is, primarily, abrasive and adhesive, although sometimes other wear mechanisms, like erosive and cavitative, are also present.
• hard particles are generally required in tool steels, these are normally ceramic particles like carbides, nitrides, borides or some combination of them.
• the volumetric fraction, hardness and morphology of the named hard particles will determine the material wear resistance for a given application.
• the use hardness of the tool material is of great importance to determine the material durability under abrasive wear conditions.
• the hard particles morphology determines their adherence to the matrix and the size of the abrasive exogenous particle that can be counteracted without detaching itself from the tool material matrix.
• FGM materials functionally graded materials
• the tool material must be hard and have hard particles.
• the resistance to the working environment is more focused on corrosion or oxidation resistance than wear although both often co-exist.
• oxidation resistance at the working temperature or corrosion resistance against the aggressive agent are desirable.
• corrosion resistance tool steels are often employed, at different hardness levels and with different wear resistances depending on the application.
• Thermal gradients are the cause of thermal shock and thermal fatigue. In many applications steady transmission states are not achieve due to low exposure times or limited amounts of energy from the source that causes a temperature gradient.
• the magnitude of thermal gradient for tool materials is also a function of their thermal conductivity (inverse proportionality applies to all cases with a sufficiently small Biot number).
• a material with a superior thermal conductivity is subject to a lower surface loading, since the resultant thermal gradient is lower.
• the thermal expansion coefficient is lower and the Young's modulus is lower.
• plastic injection molding is preferably executed with tools having a hardness around 50-54 HRc, but for big plastic injection moulds often 30-45 HRc pre-hardened materials areused, die casting of zink alloys is often performed with tools presenting a hardness in the 47-52 HRc range, while brass and aluminium are more often cast in dies with 35-49 HRc, hot stamping of coated sheet is mostly performed with tools presenting a hardness of 48-54 HRc and for uncoated sheets 54-58 HRc.
• hot stamping of coated sheet is mostly performed with tools presenting a hardness of 48-54 HRc and for uncoated sheets 54-58 HRc.
• the most widely used hardness lies in the 56-66 HRc range.
• 64-69 HRc For some fine cutting applications even higher hardness are used in the 64-69 HRc. In almost all instances of the different applications described in this paragraph, either resilience, fracture toughness or both are of great significance.
• bainitic heat treatments can be attained with a less abrupt quenching rate. Also for some tool steels they can deliver a similar microstructure trough a thicker section. For some tool steels with a retarded bainitic transformation it is possible to attain a perfectly homogeneous bainitic microstructure trough an extremely heavy section.
• bainite can be very fine and deliver high hardness and toughness if the transformation occurs at low enough temperatures.
• Many applications require high toughness, whether resilience or fracture toughness.
• plastic injection applications often thin walls (in terms of resistant cross-section) are subjected to high pressures. When those walls are tall a big moment is generated on the base that often has a small radius, and thus high levels of fracture toughness are required.
• hot working applications the steels are often subjected to severe thermal cycling, leading to cracks on corners or heat checking on the surface. To avoid the fast propagation of such cracks it is also important for those steels to have as high as possible fracture toughness at the working temperature.
• the inventors have observed that a possible way for attaining uniform high toughness values in tooling requiring heavy sections and high mechanical properties is through the achievement of an at least 70% bainitic microstructure (preferably more than 80% and even more than 90%) on tool steels, or likely highly alloyed steels, with a low enough martensite start of transformation temperature and attaining most of the bainitic transformation at a temperature close enough to the martensite start of transformation temperature as to have a fine bainitic microstructure.
• the problem can be solved with the presence of enough alloying elements and the proper tempering strategy to replace most Fe3C with other carbides and thus attaining high toughness even for coarser bainite.
• Super-bainitic or high strength bainitic steels are low alloy steels developed by H.K.D.H. Bhadeshia et al. where low temperature bainitic transformations are used to attain high mechanical properties (as an example can be taken: Very strong low temperature bainite, F.G. Caballero, H.K.D.H. Bhadeshia et al., in: Materials Science and Technology, March 2002, Vol. 18, Pg. 279-284 . DOI 10.1179/026708301225000725). They are steels with low martensite transformation start temperature mostly due to their high carbon contents, and with slow transformation kinetics for equilibrium phases (especially ferrite/perlite and upper bainite).
• the tool steels of the present invention rely on higher alloying for the attaining of the desirable mechanical properties, and normally lower %Ceq contents. As a consequence the transformation temperatures for the present invention are often higher leading to lower mechanical strength in the "as quenched" condition, which is not normally the condition of usage.
• the present invention is based on a combination of alloying and heat treatments and how those heat treatments are applied.
• the preferred microstructure is predominantly bainitic since is normally the type of microstructure easier to attain in heavy sections and also because it can be very tough if properly attained.
• For some applications having some ferrite and or perlite is not too detrimental, but for most applications no ferrite/perlite will be desirable or at the most 2% or eventually 5%.
• the applications more tolerant to ferrite/perlite can allow up to 10% or even 18%.
• a bainitic microstructure In a bainitic microstructure generally the presence of martensite leads to a decrease in fracture toughness, for applications where fracture toughness is not so important there is no restrictions on the fraction of bainite and martensite, but the applications where fracture toughness matters on predominantly bainitic microestructures will prefer the absence of martensite or at most its presence up to 2% or eventually 4%. For some compositions 8% or even 17% of martensite might be tolerable and yet maintaining a high fracture toughness level. If high fracture toughness at lower temperatures is desirable, in heavy cross sections, there are two possible strategies to be followed for the steels of the present invention within the predominantly bainitic heat treatments.
• Either alloy the steel to assure the martensitic transformation temperature is low enough (normally lower than 400°C, preferably lower than 340°C, more preferably lower than 290°C and even lower than 240°C.
• the transformation temperature should be below 220°C, preferably below 180°C and even below 140°C), and all transformation kinetics to stable not so desirable structures (ferrite/perlite, upper bainite) slowly enough (at least 600 seconds for 10% ferrite/perlite transformation, preferably more than 1200 seconds for 10% ferrite/perlite transformation, more preferably more than 2200 seconds for 10% ferrite/perlite transformation and even more than 7000 seconds for 10% ferrite/perlite transformation.
• the alloying content regarding elements with higher propensity than Fe to alloy with %C, %N and %B has to be chosen to be high enough. In this sense, most significant are the presence of %Moeq, %V, %Nb, %Zr, %Ta, %Hf, to a lesser extend %Cr and all other carbide formers.
• thermo-mechanical treatment leading to a refining of the final grain size is advantageous, especially for predominantly bainitic heat treatments because then the effect is not only in the improvement of toughness but also in the increase of hardenability, the same can be said for treatments avoiding carbide precipitation on grain boundaries.
• Such a treatment can be, for example, a first step at high temperatures above 1.020 °C to coarsen the austenite grain size (since it is a diffusion process the higher the temperature the lower the time required, strain can also be introduced trough mechanical deformation but recrystallization avoided at this point).
• the steel is cooled fast enough to avoid transformation into stable microstructures (ferrite/perlite, and also bainite as much as possible) and also to minimize carbide precipitation. Finally the steel is stress released at a temperature close to Ac1. This will promote the nucleation of very fine grains in the final heat treatment, especially if it is predominantly bainitic.
• the present invention is especially well suited to obtain steels for the hot stamping tooling applications.
• the steels of the present invention perform especially well when used for plastic injection tooling. They are also well fitted as tooling for die casting applications.
• Another field of interest for the steels of the present document is the drawing and cutting of sheets or other abrasive components.
• Also forging applications are very interesting for the steels of the present invention, especially for closed die forging.
• Also for medical, alimentary and pharmaceutical tooling applications the steels of the present invention are of especial interest.
• the present invention suits especially well when using steels presenting high thermal conductivity (thermal conductivity above 35 W/mK, preferably 42 W/mK, more preferably 48 W/mK and even 52 W/mK), since their heat treatment is often complicated especially for large or complex in geometry dies. In such cases the usage of the present invention can lead to very significant costs savings, due to the levels of toughness not attainable in any other way, at least at high hardness levels and for heavy sections.
• compositional ranges are of special significance for certain applications. For example when it comes to the %Ceq content it is preferably to have a minimum value of 0.22% or even 0.33%. On the other hand for very high conductivity applications it is better to keep %C below 1.5% and preferably below 0.9%. %Ceq has a strong effect in reducing the temperature at which martensitic transformation starts, thus higher values of %Ceq will be desirable for either high wear resistance applications or applications where a fine bainite is desirable. In such cases it is desirable to have a minimum of 0.4% of Ceq often more than 0.5% and even more than 0.8%.
• %Moeq %Mo + 1 ⁇ 2 ⁇ %W
• thermal conductivity normally above 3.0% often above 3.5%, preferably above 4% or even 4.5%.
• thermal conductivity needs to be maximized is better to do so within a compositional range with lower %Cr, normally less than 2.8% preferably less than 1.8% and even less than 0.3%.
• %Zr+%Hf+%Nb+%Ta should be above 0.2%, preferably 0.8% and even 1.2%.
• %V is good carbide former that tends to form quite fine colonies but has a higher incidence on thermal conductivity than some of the former, but in applications where thermal conductivity should be high but is not required to be extremely high and wear resistance and toughness are both important, it will generally be used with a content above 0.1%, preferably 0.3% and even more than 0.55%. For very high wear resistance applications it can be used with a content higher than 1.2% or even 2.2%.
• the present invention can be particularly interesting for applications requiring a steel with improved ambient resistance, especially when high levels of mechanical characteristics are desirable for big cross sections.
• compositional ranges within the above mentioned compositional range are of special significance for certain applications.
• %Ceq content it is preferably to have a minimum value of 0.22%, preferably 0.28% more preferably 0.34% and when wear resistance is important preferably 0.42% and even more preferably 0.56%.
• Very high levels of %Ceq are interesting due to the low temperature at which martensite transformation starts, such applications favor %Ceq maximum levels of 1.2%, preferably 1.8% and even 2.8%.
• Applications where toughness is very important favor lower %Ceq contents, and thus maximum levels should remain under 0.9% preferably 0.7% and for very high toughness under 0.57%.
• %Cr %Cr
• %Mo present in the steel
• %Zr+%Hf+%Nb+%Ta should be above 0.2%, preferably 0.8% and even 1.2%.
• %V is good carbide former that tends to form quite fine colonies but has a higher incidence on thermal conductivity than some of the former, but in applications where thermal conductivity should be high but is not required to be extremely high and wear resistance and toughness are both important, it will generally be used with a content above 0.1%, preferably 0.54% and even more than 1.15%. For very high wear resistance applications it can be used with content higher than 6.2% or even 8.2%.
• the present invention can be particularly interesting for applications requiring a steel with corrosion or oxidation resistance, especially when high levels of mechanical characteristics are desirable throughout heavy sections.
• compositional ranges within the above mentioned compositional range are of special significance for certain applications.
• %Ceq content it is preferably to have a minimum value of 0.22%, preferably 0.38% more preferably 0.54% and when wear resistance is important preferably 0.82%, more preferably 1.06% and even more than 1.44%.
• Very high levels of %Ceq are interesting due to the low temperature at which martensite transformation starts, such applications favor %Ceq maximum levels of 0.8%, preferably 1.4% and even 1.8%.
• Applications where toughness is very important favor lower %Ceq contents, and thus maximum levels should remain under 0.9% preferably 0.7% and for very high toughness under 0.57%.
• %Cr corrosion resistance for martensitic microstructure
• 11 % Cr usually higher levels of %Cr are recommendable, normally more than 12% or even more than 16%.
• %Moeq present in the steel, often more than 0.4%, preferably more than 1.2% and even more than 2.2% offer a significant effect in this sense.
• %Zr+%Hf+%Nb+%Ta should be above 0.1%, preferably 0.3% and even 1.2%.
• %V is good carbide former that tends to form quite fine colonies but has a higher incidence on thermal conductivity than some of the former, but in applications where thermal conductivity should be high but is not required to be extremely high and wear resistance and toughness are both important, it will generally be used with a content above 0.1%, preferably 0.24% and even more than 1.15%. For very high wear resistance applications it can be used with content higher than 4.2% or even 8.2%.
• the present invention can be particularly interesting for applications requiring a steel with very high wear resistance, especially when high levels of hardness are desirable throughout heavy sections.
• compositional range is of special significance for certain applications. For example when it comes to the %Ceq content it is preferably to have a minimum value of 0.62%, preferably 0.83% more preferably 1.04% and when extreme wear resistance is important preferably 1.22%, more preferably 1.46% and even more than 1.64%. Very high levels of %Ceq are interesting due to the low temperature at which martensite transformation starts, such applications favor %Ceq maximum levels of 1.8%, preferably 2.4% and even 2.8%. %Cr has two ranges of particular interest: 3.2%-5.5% and 5.7%-9.4%.
• %Moeq present in the steel, often more than 2.4%, preferably more than 4.2% and even more than 10.2% offer a significant effect in this sense.
• %Zr+%Hf+%Nb+%Ta should be above 0.1%, preferably 1.3% and even 3.2%.
• %V is good carbide former that tends to form quite fine colonies of very hard carbides, thus when wear resistance and toughness are both important, it will generally be used with a content above 1.2%, preferably 2.24% and even more than 3.15%. For very high wear resistance applications it can be used with content higher than 6.2% or even 10.2%.
• the present invention can be also applied for the manufacturing of big plastic injection tools particularly interesting for applications requiring very low cost steel with high mechanical resistance and toughness. This particular application of the present invention is also interesting for other applications requiring inexpensive steels with high toughness and considerable yield strength throughout heavy sections.
• %Moeq present in the steel, often more than 0.4%, preferably more than 1.2%, more preferably more than 1.6% and even more than 2.2% offer a significant effect in this sense.
• the elements that mostly remain in solid solution the most representative being %Mn, %Si and %Ni are very critical. It is desirable to have the sum of all elements which primarily remain in solid solution exceed 0.8%, preferably exceed 1.2%, more preferably 1.8% and even 2.6%. As can be seen both %Mn and %Si need to be present. %Mn is often present in an amount exceeding 0.4%, preferably 0.6% and even 1.2%.
• %Si is even more critical since when present in significant amounts it strongly contributes to the retarding of cementite coarsening. Therefore %Si will often be present in amounts exceeding 0.4%, preferably 0.6% and even 0.8%. When the effect on cementite is pursuit then the contents are even bigger, often exceeding 1.2%, preferably 1.4% and even 1.65%. Also for applications where wear resistance or thermal conductivity are important it is advantageous to use strong carbide formers, then %Zr+%Hf+%Nb+%Ta should be above 0.1%, preferably 1.3% and even 2.2%.
• %V is good carbide former that tends to form quite fine colonies of very hard carbides, thus when wear resistance and toughness are both important, it will generally be used with a content above 0.2%, preferably 0.4% and even more than 0.8%. For very high wear resistance applications it can be used with content higher than 1.2% or even 2.2%.
• Other elements may be present, especially those with little effect on the objective of the present invention. In general it is expected to have less than 2% of other elements (elements not specifically cited), preferably 1%, more preferably 0.45% and even 0.2%.
• the critical elements for attaining the mechanical properties desired for such applications need to be present and thus it has to be %Si+%Mn+%Ni+%Cr greater than 2.0%, preferably greater than 2.2%, more preferably greater than 2.6% and even greater than 3.2%.
• %Cr for %Mo
• the presence of %Mo can be dealt alone when present in an amount exceeding 1.2%, preferably exceeding 1.6%, and even exceeding 2.2%.
• a contribution to the increase in toughness in the bainitic microstructures of the present invention can be made through the dissolution of the cementite and the carbon that goes into solid solution can contribute to the separation or precipitation of carbides containing carbide-forming elements.
• carbides containing carbide-forming elements Cr, Mo, W, V, Nb, Zr, Ta, Hf
• Cr, Mo, W, V, Nb, Zr, Ta, Hf often mixed carbides containing those elements and others like for example iron.
• Those carbides often precipitate as M7C3, M4C3, MC, M6C, M2C and others carbides.
• the temperature at which this happens is often above 400 °C, preferably 450 °C, more preferably 480 °C and even 540 °C.
• Small secondary hard particles are those with a maximum equivalent diameter (diameter of a circle with equivalent surface as the cross section with maximum surface on the hard particle) below 7.5 nm. It is then desirable to have a volume fraction of small secondary hard particles for such applications above 0.5%. It is believed that a saturation of mechanical properties for hot work applications occurs at around 0.6%, but ithas been observed by the inventors that for some applications requiring high plastic deformation resistance at somewhat lower temperatures it is advantageous to have higher amounts than these 0.6%, often more than 0.8% and even more than 0.94%. Since the morphology (including size) and volume fraction of secondary carbides change with heat treatment, the values presented here describe attainable values with proper heat treatment.
• Cobalt has often been used in hot work tool steels principally due to the increase in mechanical strength, and in particular the increase of yield strength maintained up to quite high temperatures. This increase in yield strength is attained trough solid solution and thus it has a quite negative effect in the toughness.
• the common amounts of Co used for this propose is 3%. Besides the negative effect in toughness it is also well known the negative effect in the thermal conductivity. The inventors have seen that within the compositional ranges of the present invention it is possible to use Co, and attain an improved yield strength/ toughness relation since Co can promote the nucleation of secondary hard particles and thus keep their size small.
• Heat treatment has to be selected with a rather high austenitization temperature and an abnormally high tempering temperatures, actually more than 55 HRc commonly achieved with at least one tempering cycle at 630 °C or even above, 50 HRc can be maintained even with one tempering cycle at 660 °C or more.
• Proper thermo-mechanical processing together with the compositional rules just explained have to be implemented to minimize scattering at high temperatures, the optimized arrangements is characterized by providing diffusivities of more than 5.8 mm2/s, often more than 6.1mm2/s and even more than 6.5mm2/s at measuring temperatures as high as 600 °C.
• Te, Bi or even Pb, Ca, Cu, Se, Sb or others can be used, with a maximum content of 1%, with the exception of Cu, than can even be of 2%.
• the most common substance, sulfur has, in comparison, a light negative effect on the matrix thermal conductivity in the normally used levels to increase machinability.
• its presence must be balanced with Mn, in an attempt to have everything in the form of spherical manganese bisulphide, less detrimental for toughness, as well as the least possible amount of the remaining two elements in solid solution in case that thermal conductivity needs to be maximized.
• Another hardening mechanism can be used in order to search for some specific combination of mechanical properties or environmental degradation resistance. It is always the intention to maximize the desired property, but trying to have minimal possible adverse impact on thermal conductivity.
• Solid solution with Cu, Mn, Ni, Co, Si, etc... including some carbide formers with less affinity to carbon, like Cr) and interstitial solid solution (mainly with C, N and B).
• precipitation can also be used, with an intermetallic formation like Ni 3 Mo, NiAl, Ni 3 Ti... (also ofNi and Mo, small quantities of Al and Ti can be added, but special care must be taken for Ti, since it dissolves in M 3 Fe 3 C carbides and a 2% should be used as a maximum).
• atomic mass and the formed type of carbide determine if the quantity of a used element should be big or small. So, for instance, 2%V is much more than 4%W. V tends to form MC carbides, unless it dissolves in other existing carbides. Thus, to form a carbide unit only a unit of V is needed, and the atomic mass is 50.9415. W tends to form M 3 Fe 3 C carbides in hot work steels. So three units of W are needed to form a carbide unit, and the atomic mass is 183.85. Therefore, 5.4 more times carbide units can be formed with 2%V than with 4%W.
• Tool steel of the present invention can be manufactured with any metallurgical process, among which the most common are sand casting, lost wax casting, continuous casting, melting in electric furnace, vacuum induction melting. Powder metallurgy processes can also be used along with any type of atomization and eventually subsequent compacting as the HIP, CIP, cold or hot pressing, sintering (with or without a liquid phase and regardless of the way the sintering process takes place, whether simultaneously in the whole material, layer by layer or localized), laser cusing, spray forming, thermal spray or heat coating, cold spray to name a few of them.
• the alloy can be directly obtained with the desired shape or can be improved by other metallurgical processes.
• Tool steel of the present invention can be obtained in the form of bar, wire or powder (amongst others to be used as solder or welding alloy). Even, a low-cost alloy steel matrix can be manufactured and applying steel of the present invention in critical parts of the matrix by welding rod or wire made from steel of the present invention. Also laser, plasma or electron beam welding can be conducted using powder or wire made of steel of the present invention.
• the steel of the present invention could also be used with a thermal spraying technique to apply in parts of the surface of another material.
• the steel of the present invention can be used as part of a composite material, for example when embedded as a separate phase, or obtained as one of the phases in a multiphase material. Also when used as a matrix in which other phases or particles are embedded whatever the method of conducting the mixture (for instance, mechanical mixing, attrition, projection with two or more hoppers of different materials).
• Tool steel of the present invention can also be used for the manufacturing of parts under high thermo-mechanical loads and wear resistance or, basically, of any part susceptible to failure due to wear and thermal fatigue, or with requirements for high wear resistance and which takes advantage of its high thermal conductivity.
• the advantage is a faster heat transport or a reduced working temperature.
• components for combustion engines such as rings of the engine block
• reactors also in the chemical industry
• heat exchange devices generators or, in general, any power processing machine.
• Dies for plastic forming of thermoplastics and thermosets in all of its forms In general, any matrix, tool or part can benefit from increased wear resistance and thermal fatigue.
• dies, tools or parts that benefit from better thermal management as is the case of material forming or cutting dies with release of large amounts of energy (such as stainless steel or TRIP steels) or working at high temperatures (hot cutting, hot forming of sheet).

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