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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  • 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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  • C21D6/00—Heat treatment of ferrous alloys
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  • 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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  • 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
  • C22C38/24—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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  • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C—ALLOYS
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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
  • 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
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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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  • 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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  • 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
  • C21D2211/002—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 high-speed 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 are used, die casting of zinc 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.
• the most widely used hardness lies in the 56-66 HRc range.
• 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.
• the traditional way can be used in certain instances, consisting in avoiding coarse Fe3C and/or its precipitation on grain boundaries with the additions of elements that promote its nucleation like Al, Si . . . . It is also advantageous for most applications to use thermo-mechanical treatments leading to the refining of the final grain size.
• 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, at least 52% vol %, preferably 75% vol %, more preferably 86% vol % and even more preferably more than 92% vol %, since is normally the type of microstructure easier to attain in heavy sections and also because it can be very tough when following the indicated steps.
• High Temperature bainite will be preferred since it is the first bainite to form when cooling the steel after austenitization.
• High Temperature bainite refers to any microstructure formed at temperatures above the temperature corresponding to the bainite nose in the TTT diagram but below the temperature where the ferritic/perlitic transformation ends.
• High temperature bainite just refers to upper bainite, in the present invention both upper and lower bainite are referred, the last one which can occasionally form in small amounts also in isothermal treatments at temperatures above the one of the bainitic nose.
• the high temperature bainite should be the majoritary type of bainite and thus from all bainite is preferred at least 50% vol %, preferably 65% vol %, more preferably 75% vol % and even more preferably more than 85% vol % to be High Temperature Bainite.
• bainite is one of the decomposition products when austenite is not cooled under thermodinamical equilibrium. It consists on a fine non-lamellar structure of cementite and dislocation-rich ferrite plates as it is a non-diffusion process. The high concentration of dislocations in the ferrite present in the bainite makes this ferrite harder than it would normally be.
• Upper bainite refers to the coarser bainite microstructure formed at the higher temperatures range within the bainite region, to be seen in the TTT temperature-time-transformation diagram, which in its turn, depends on steels composition.
• Fine bainite refers qualitatively to the size of the plates or laths of ferrite, which in this case mean small; On the contrary, for big lath sizes bainite is known as coarse bainite.
• stable phases like ferrite or perlite are in general terms not very desirable structures to achieve during heat treating.
• the inventors have seen that a way to increase the toughness of the High Temperature Bainite, including the Upper and lower Bainite is to reduce the grain size, and thus for the present invention when tough upper bainite is required grain sizes of ASTM 8 or more, preferably 10 or more and more preferably 13 or more are advantageous.
• the inventors have also seen that surprisingly high values of toughness can be attained with High Temperature Bainite when using microstructures where cementite has been suppressed, strongly reduced and/or its morphology altered to finer lamella or even more so when the cementite is globulized.
• bainites including retained austenite the same applies for the morphology of the retained austenite phase.
• Tough High Temperature Bainite small grain size high temperature bainite and/or low cementite bainite and/or fine lamella or globular morphology high temperature bainite.
• the high temperature bainite being tough high temperature bainite more than a 60%, preferably more than a 78%, and even more preferably more than a 88% in volume percent.
• more than 400 seconds for 20% transformation into bainite preferably more than 800 seconds for 20% bainite, more preferably more than 2100 seconds for 20% bainite and even more than 6200 seconds for 20% bainite) to be able to make a predominantly fine bainite (at least 50% vol %, preferably 55% vol %, more preferably 60% vol % and even more preferably more than 70% vol %) heat treatment.
• 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.
• Elements having an affinity for carbon higher than iron are Hf, Ti, Zr, Nb, V, W, Cr, Mo as most important ones and will be referred in this document as strong carbide formers (special attention has to be played since this definition does not coincide with the most common one in the literature where often Cr, W and even Mo and V are often not referred as strong carbide formers).
• Elements with higher carbon affinity than Fe will form their respective carbides or a combination of them before the iron carbide can form, from now on referred to as alloyed carbides. Depending on the carbide itself, properties can vary. Special cases are later on and depending on the particular properties sought, properly described.
• 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). Then 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.
• composition of steels is normally given in terms of Ceq, which is defined as carbon upon the structure considering not only carbon itself, or nominal carbon, but also all elements which have a similar effect on the cubic structures of the steel, normally being B, N. During last description, carbon was meant to be just carbon content, or nominal carbon.
• Martensitic or bainitic microstructures are often rather brittle right after quenching and one way to recover some ductility and/or toughness is by tempering them.
• references are made to tempered martensite and tempered bainite, with this terminology in this text is referred a martensite and/or bainite that has undergone any type of heating after forming (during the quenching process). This heating leads at first to a relaxation of the structure, followed by a migration of the carbon atoms (often the resulting microstructures are given particular names in the literature: Troostite, Sorbite . . . .
• the austenitization temperature for tool steels is normally well above Ac3 since it is often convenient to bring most carbides into dissolution before quenching. Depending on final application, it will be more interesting to austenitise at lower temperatures, even between AC1 and AC3 (where an incomplete austenitization takes place). Typically the austenitization temperatures will be above AC3, but often below the temperature of complete carbide dissolution, even in absence of primary carbides, as grain growth is directly proportional to temperature. Small grains sizes are normally accompanied by higher strength so lower temperatures are more convenient for this purpose. If it is accompanied with short time, once the core has reached the temperature, even better. For some applications, these values are below 1040° C., preferably below 1020° C. and even below 990° C.
• the heating up and austenitization can be carried out in any type of furnace, atmospheric, protected atmosphere, salt bath, vacuum . . . . Uneven heating or overheating from specified temperature should be avoided. Heating rate must be controlled, specially around the AC3 range because contraction of the body centered cubic structure from ferrite transforms to the face centered cubic structure of austenite may produce micro cracks that can grow afterwards.
• the material is subjected to a rapid cooling to an intermediate temperature or transformation temperature T int .
• This cooling has to be fast enough so that no massive ferrite transformation occurs during the process.
• Normally less than 20% ferrite or stable phases is desirable, more preferably less than 12% and most preferred is less than 2% or even none.
• the holding at this temperature has to be long enough to minimize the transformation of austenite to martensite.
• T int has to be in a range where lite or no martensite is able to form and most of final microstructure consists on bainitic microstructure with fine carbide-like constituients. Therefore, Tint has to be below the martensite start of transformation (Ms)+300° C. and above Ms ⁇ 50° C.
• Desired final structure has to be at least 70% vol % bainitic microstructure, preferably 75% vol %, more preferably 86% vol % and even more preferably more than 92% vol %. It is very advantageous when the bainitic transformation is done at a temperature Tint below 400° C., attaining a final hardness above 45 HRc.
• T int will be renamed as T int1 and T int2 .
• Concerning upper limit for T int1 it is desirable to be below 750°, more preferably below 620° C., more preferably below 590° C. and even more preferably below 560° C.
• lower limit is desirable to be above 460° C., preferably above 495° C., more preferably above 512° C. and even more preferably 523° C.
• T int1 ⁇ T int2 The holding time at this temperature range (T int1 ⁇ T int2 ) can vitiate from several minutes to several hours depending on the heat treated piece size and the lack of transformation at T int1 , desirably at least half an hour, preferably at least 1 h, more preferably at least 2 h and in some cases even more than 5 h.
• T int2 upper limit is desirably below 450° C., preferably below 420° C., more preferably below 320° C. and even more preferably below 360° C.
• the lower limit is desirable to be below 350° C., preferably below 320° C., more preferably below 250 and even more preferably below 200° C.
• the present invention is a method to manufacture a steel, casting die or tool, comprising providing a steel with both a bainite and a martensite domain of existence, characterized in that the steel is subjected to a thermal treatment comprising the following steps:
• the present invention is well suited for steels which have a martensite start (Ms) of transformation equal or lower than 540° C., preferably lower than 480° C., more preferably lower than 440 and even more preferably lower than 360° C.
• Ms martensite start
• the present invention is advantageous when thermal treatment is followed by at least one tempering cycle desirably above 500° C., preferably above 550° C., more preferably above 600° C. and even more preferably above 620° C. Often more than one cycle is desirable. more preferably more than one cycle to separate the alloy cementite to dissolve the cementite in solid solution and to separate the carbide formers stronger than iron.
• 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.
• the steel is tempered with at least one tempering cycle at a temperature above 500° C. to ensure that a significant portion of the cementite is replaced by carbide-like structures containing carbide formers stronger than iron.
• the traditional way can be used in certain instances, consisting in avoiding coarse Fe3C and/or its precipitation on grain boundaries with the additions of elements that promote its nucleation like Al, Si . . . .
• At least 70% of the bainitic transformation is made at temperatures below 400° C. and/or the thermal treatment includes at least one tempering cycle at a temperature above 500° C. to ensure separation of stronger carbide formers carbides, so that most of the attained microstructure, with the exception of the eventual presence of primary carbides, is characterized by the minimization of rough secondary carbides, in particular at least 60% in volume of the secondary carbides has a size of 250 nm or less, such that a toughness of 10 J CVN or more is attained.
• the composition and tempering strategy is chosen so that high temperature separation secondary carbide types such as types MC, MC-like type as M4C3, M6C and M2C are formed, in such a manner that a hardness above 47 HRc is obtainable even after holding the material for 2 h at a temperature of 600° C. or more.
• the steel has a composition within the following range:
• tempering strategy is chosen to minimize carriers scattering, such that a low scattering structure characterized by a diffusivity of 8 mm 2 /s or more is obtainable even for a hardness of 45 HRc or more.
• the steel has the following composition:
• the steel produced according to the method of the invention presents at least two of the following features:
• 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.
• the present invention is well indicated in particular when using high thermal conductivity steels, with the following composition all percentages being indicated in weight percent:
• composition as such forms an invention without the restrictions of claim 1 .
• trace elements refer to any element, otherwise indicated, in a quantity less than 2%.
• trace elements are preferable to be less than 1.4%, more preferable less than 0.9% and sometimes even more preferable to be less than 0.4%.
• Possible elements considered to be trace elements are H, He, Xe, Be, O, F, Ne, Na, Mg, P, S, Cl, Ar, K, Ca, Sc, Fe, Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Re, Os, Ir, Pt, Au, Hg, Ti, Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Np, Pu, Am
• trace elements or even trace elements in general can be quite detrimental for a particular relevant property (like it can be the case sometimes for thermal conductivity and toughness), for such applications it will be desirable to keep trace elements below a 0.4%, preferably below a 0.2%, more preferably below 0.14% or even below 0.06%.
• 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%.
• % 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%.
• hardness above 60 HRc with low scattering structures characterized by thermal diffusivity above 8 mm 2 /s and generally more than 9 mm 2 /s are possible in the present invention.
• the present invention is also especially good indicated when using steels with the following composition range, all percentages being indicated in weight percent:
• 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 Although a noticeable ambient resistance can be attained with 4% Cr, usually higher levels of % Cr are recommendable, normally more than 8% or even more than 10%. For some special attacks like those of chlorides it is highly recommendable to have % Mo present in the steel, normally more than 2% and even more than 3.4% offer a significant effect in this sense. Also for applications where wear resistance is important it is advantageous to use strong carbide formers, then % 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 steels described above can be particularly interesting for applications requiring a steel with improved ambient resistance, especially when high levels of mechanical characteristics are desirable and the cost associated to heat treatment (both in terms of time and money) for its execution or associated distortions, are significant.
• the present invention is also especially good indicated when using steels with the following composition range, all percentages being indicated in weight percent:
• 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
• % Cr 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 steels described above can be particularly interesting for applications requiring a steel with corrosion or oxidation resistance, especially when high levels of mechanical characteristics are desirable and the cost associated to heat treatment (both in terms of time and money) for its execution or associated distortions, are significant
• the present invention is then especially good indicated when using steels with the following composition range, all percentages being indicated in weight percent:
• 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 0.62%, 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 steels described above can be particularly interesting for applications requiring a steel with very high wear resistance, especially when high levels of hardness are desirable and the cost associated to heat treatment (both in terms of time and money) for its execution or associated distortions, are significant.
• the present invention can be applied to low cost steels with the following composition, all percentages being indicated in weight percent:
• composition as such forms an invention without the restrictions of claim 1 .
• % 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%.
• 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%. For particular applications, Mn is interesting to be even 1.5%.
• % 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 exceeding 0.6% and even exceeding 0.8%. When the effect on cementite is pursuit then the contents are even bigger, often exceeding 1.2%, preferably 1.5% and even 1.65%.
• Al can also be used, at least exceeding 0.4%, preferably exceeding 0.5 and even exceeding 0.8%.
• % 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%.
• % Si+% Mn+% Ni+% Mo greater than 2.0% . . .
• 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%.
• % Si+% Mn+% Ni+% Cr replaced by % Si+% Mn and then the same preferential limits can apply, but in presence of other alloying elements, also lower limits can be used like % Si+% Mn>1.1%, preferably 1.4% or even 1.8%.
• % Ni is desirable to be at least 1%.
• tough bainite treatments at temperatures close to martensite start of transformation (Ms) are very interesting (often at least 60%, preferably 70% and more, even more preferably at least 82% of the transformation of austenite should take place below 520° C., preferably 440° C., more preferably 410° C. or even 380° C., but not below 50° C. below martensite start of transformation [Ms]).
• the steels described above 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. It is particularly advantageous when the steel requires a harder surface for the application and the nitriding or coating step is made coincide with the hardening step.
• 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. Therefore, the present invention is well suited for steels which contain at least 3% carbide formers stronger than iron and thermal treatment is followed by at least one tempering cycle above 500° C. to separate the alloy cementite, to dissolve the cementite in solid solution and to separate the carbide formers stronger than iron.
• 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 it has 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 6.1 mm2/s and even more than 6.5 mm2/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 of Ni 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).
• Samples with compositions specified in Table 2 were austenitised at a temperature between 1000-1150° C. for about 45 minutes (once the core of the pieces had reached the temperature).
• T int,1 was chosen to be in the 500-600° C. range and T int,2 was chosen to be in the 320-450° C. range.
• the rapid cooling to T int,1 was made by means of changing the piece to another furnace running at the T int,1 temperature and samples were soaked at that temperature for one hour. Afterwards, the temperature further decreased to T int,2 in approximately 2 to 8 hours and afterwards was cooled in air.
• T int,1 was chosen to be in the 500-600° C. range and T int,2 was chosen to be in the 320-450° C. range.
• the rapid cooling to T int,1 was made in two steps: a first step comprising rapid cooling to the range 500-600° C., a second step of heating to a temperature within the same temperature range but higher and cooling again to the first temperature within the same range 500-600° C. by means of changing the piece to another furnace running at the T int,1 temperature and samples were soaked at that temperature for one hour. Afterwards, the temperature further decreased to T int,2 in approximately 3 to 10 hours and afterwards was cooled in air.

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