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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • 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
• • 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
  • 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
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of 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
  • C22C38/02—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
  • C22C38/40—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
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite

Definitions

• a novel group of hot work tool steels which exhibit exceptionally high values of thermal conductivity, typically above 45W/mK at the commonly used range work hardness (44-51HRC) has been developed. These steels are characterized by their tendency of largely retaining or even increasing their thermal conductivity at elevated temperatures, with a peak of thermal conductivity typically in the range between 400°C and 500°C.
• the impact toughness and other mechanical properties remain in compliance with the most common toughness requirements and standard specifications, for hot work tool steels (i.e. H13), while retaining sufficient hardenability to ensure homogenous properties within thick sections.
• This steel exhibits high tempering stability, low distortion during heat treatment and a very high resistance to heat checking.
• the newly developed steel is especially suitable for production of die casting dies which operate between 400°C and 650°C, and tools for hot stamping of high strength steel and aluminum sheets.
• this steel is also suitable for applications of die forging and extrusion as well as other tools for hot working and plastic molding which operate at temperatures above 200°C.
• the present invention refers to a novel low-chromium high thermal conductivity hot-work tool steel that is suitable for production of die casting dies and hot stamping tools and to a method of producing said steel.
• the new type of hot-work tool steel retains very high values of thermal conductivity at elevated temperatures.
• a novel hot-work tool steel was developed having the following chemical composition (all percentages being in weight percent): C 0.25-0.45% Si max 0.15% Cr max 0.20% Mn 0.05- 0.25% Mo 1.5-4.0% W 0.0-2.0% Co 0.2-1.0% as well as optional elements such as Ni 0.0-2.5% or Cu 0.0-1.0% and the intentional micro additions of Ti, Nb, B, Ta, Ce, the remainder being essentially iron except for unavoidable impurities.
• Said hot-work tool steel is of predominantly bainitic microstructure.
• the current invention is feasible without any special manufacturing requirements using the conventional equipment. However, as with all hot-work tool steels used in demanding applications such as die casting/forging, a secondary refining process such as ESR or VAR re melting is highly recommended.
• Hot-work tool steels are a group of ferrous alloys used for production of tools which operate at elevated temperatures in applications of die forging, squeezing and extrusion of light metals (Al, Mg), die casting of light metals (Al, Mg), hot stamping as well as plastic molding and continuous casting of Al and its alloys.
• the most common critical temperature range of operation in the above mentioned applications is generally considered to be between SOOT and 650°C, the requirements are significantly different though, depending on process.
• the proposed steel of the invention is suitable for all fields of applications, particularly for the production of die casting and hot stamping tools.
• thermal fatigue is the most common failure mode of dies for die casting of non-ferrous metals and alloys, predominantly based on aluminum and magnesium.
• the resistance of materials to the occurrence of thermal fatigue is proportional to the surface stress induced due to thermal loading which can be expressed with equation (1) as follows [1]:
• Hot stamping is the technology of choice in development of sheet metal parts from martensitic high strength steels and aluminum alloys.
• the process includes the forming of hot metal sheet in the desired shape during stamping and subsequent heat treatment - quenching, within the closed cavity of the forming tool. Mechanical properties of the final part and tools performance are therefore largely dependent on the cooling rate of sheet metal.
• the steels currently in use for die casting applications vary significantly in terms of achievable work hardness and toughness, whereby thermal conductivities are severely limited due to their alloy design which commonly includes additions of Cr (3.0-6.0 wt. %), V (0.5-1.6 wt. %), Si (0.25-2.0 wt. %) and Mn (0.4- 0.6 wt. %); the values in brackets serve as orientation for the most commonly used standardized grades of Wr. Nr 1.2343, 1.2344, 1.2367 and so forth.
• the additions of afore mentioned elements are known to be beneficial for the obtainment of desired mechanical properties of hardness, toughness, hot strength and tempering resistance.
• Such alloying principles are sufficient to reach adequate work hardness (45-50HRC) however they do not provide all the necessary properties such as hardenability.
• the latter is solved via addition of Ni in varying amounts between 1.0-3.0 wt. %, typically around 2 wt. %, which is used to increase hardenability, while being the least detrimental element to the steel's thermal conductivity.
• Figure 1 Comparison of thermal conductivity of standard tool steel grades and the steel of the invention, quenched in oil and tempered to a hardness range of 44-46HRC.
• FIG. 5 A continuous cooling transformation (CCT) curve of hot-work tool steel of the invention - steel S, with recommended cooling rates.
• Figure 9 Annealed microstructure of hot-work tool steel of the invention - steel S. While a lot of attention was put on the type, amount and distribution of carbides, the microstructure of tempered martensite is predominantly considered as being the most suitable. As previously mentioned, heat within steels is transferred both via electrons and lattice vibrations; and as the content of alloying elements increases - the latter become increasingly influential, whereby the contribution of electrons in highly alloyed cast iron can be essentially neglected [3]. However, cast iron exhibits thermal conductivity which is comparable or even higher than that of steels. This indicates that neither the purity of the lattice nor the character of imbedded carbides but rather the microstructure itself should be the main focus in designing steels with high thermal conductivity. The phononic thermal conductivity is expressed by equation 3 as [4]:
• thermoelectrics the microstructure which inhibits the phonon movement the most, is the one that consists of mesoscale grains with nanoscale precipitates, i.e. with a high dislocation density and a high content of solid solution elements in the matrix [5].
• the microstructure of bainite is much more uniform in scale, as the width of the individual subunits is mostly influenced by the strength of austenite matrix. Bainite is also much less supersaturated with respect to carbon which is also a major factor that determines thermal conductivity of steel, as illustrated from tempering data [6]
• the steel of the invention is alloyed with Co in concentrations between 0.2 and 1.0 wt. % preferably between 0.75 and 0.85 wt. %.
• Elements detrimental to thermal conductivity of steels such as Cr, Si, Mn and V are preferably kept at a minimum concentration, below 0.2 wt. %, preferably below 0.1 wt. %.
• V is the least detrimental; therefore V content up to 1.6 wt. % may be acceptable in order to improve abrasive wear resistance.
• Ti may also be added in the amount up to 0.2 wt. % .
• Alloying with Mo may be used independently or in combination with W.
• V max. 0.15% the rest consisting of Fe and unavoidable impurities.
• the amount of Mn is dependent upon the S content in a ratio of 15 : 1. Within said ratio the whole sulfur present in the steel binds in Mn and forms deformable MnS excrements which are substantially more favorable as if the sulfur does not form inclusions, but stays at the boundaries of crystalline grains of steel where it causes fragility.
• additions of Ni may be added to the composition in amounts between 1.0 and 2.4 wt. %.
• hardenability may be increased through a combined use of Cu+B, preferably in combination with other micro-additions, such as Ti and Nb.
• Ni is added to steel, Cu additions are to be limited to max. 0.15 wt. % preferably around 0.1 wt. %, as the co-alloying of these elements results in a higher amount of Cu remaining in a solid solution which is in turn detrimental to thermal conductivity.
• a modified steel alloy denoted R yielding increased hardenability due to Ni with the following composition is proposed, all percentages being in weight percent:
• V max 0.15% the remainder being Fe except for unavoidable impurities.
• Additions of Co are the most critical to obtain the desired thermal conductivity characteristics.
• the effect of Co on physical metallurgy is appreciative from dilatometry measurements.
• Mf transformation finish temperature
• Co is often added to hot-work tool steels in similar amounts in order to improve tempering stability [7], however, in the context of the steels of the invention this is regarded as merely a secondary effect and is not the main reason for alloying with Co, as tempering stability is ensured by the high Mo and/or W content.
• microstructural bands The formation of micro-banding is in practice very difficult to prevent even if secondary refining (re-melting) processes such as ESR and VAR are applied - which are nevertheless, the preferred technological route for production of hot- work tool steels.
• the formation of banding is mitigated by additions of Co, which affects the microstructure as seen in Fig. 2.
• the steel contains at least 0.2% wt. Co, preferably from 0.6-0.9 wt. % Co, more preferably from 0.75-0.85 wt. %, and it has been henceforth named S for the sake or distinction, which is presented in greater detail.
• the most recommended amount is 0.8 wt. % Co (designated as SITHERM S140R).
• SITHERM S140R A higher amount of Co additions must be balanced with Ni content as Co is known to increase the thermodynamic driving force for ferrite formation whereby, the presence of grain boundary ferrite deteriorates the toughness.
• Table 2 Influence of Co addition on the retained austenite content of novel hot-work tool steel
• steels of the invention contain very low amounts of Cr due to the detrimental effect that Cr exerts on thermal conductivity. There are, however, sound arguments to minimize the Cr content also from the microstructural point of view. It has been argued that Cr is detrimental for the properties of hot-work tool steels, as Cr rich carbides tend to coarsen more rapidly compared to other (Mo, W) carbides [8] In steels of the invention, the effect of Cr additions is very pronounced and it results in an appreciative grain growth under identical heat treatment conditions, as shown in Fig. 3. If the amount of Cr is higher the microhomogeneity of the steel decreases.
• the minimal interval of continuous cooling rates lies between 120 and 10°K/min, preferably 120 and 30°K/min, and is as such compatible with the current industry practice, as most heat treatments of hot work tools are performed in vacuum furnaces at slower cooling rates compared to oil quenching.
• the importance of cooling rate is best illustrated on the example where steel S is cooled at two different cooling rates corresponding to 120°K/min and water quenching in order to ensure a fully bainitic and martensitic microstructure respectively.
• the corresponding thermal conductivities are shown in the graph in Fig. 6.
• the newly developed steels are characterized by the fact that they form negligible retained austenite contents within predominantly bainitic microstructures, which is in a sharp contrast to conventional high chromium hot-work tool steels. These form more retained austenite when cooled through the bainitic region of the CCT diagram.
• Optimal heat treatment involves rapid cooling into the bainite region and a short isothermal holding at 400°C.This serves to fully complete the bainite formation and to equalize the temperatures of surface and central region of tool. Quenching rates can be high as bainite produces distortions which are up to an order of magnitude smaller compared to martensite. The volume change from the initial spheroidized pearlite is considered to be negligible [9]
• steel is further cooled to 200°C or lower as shown in Fig. 7.
• the Bf temperature as indicated in the CCT diagram is at approx. 350°C, however one should keep in mind that all such data are essentially dependent on the resolution of the measurement equipment. Accordingly, a higher undercooling is recommended to achieve a retained austenite content below 1% which is common for this steel, after which we proceed with tempering.
• the first temper is to be performed at temperatures 580°C and 590°C for small and large tools respectively, for duration of 5h adjusted by common additions accounting for the tool cross sections of l/2h per inch.
• a prolonged first temper is due to the initially sluggish tempering kinetics.
• steel achieves hardness between 47-51HRC depending on quenching rate, whereby a slower cooling rate results in a lower obtainable hardness.
• steel is suitable for applications where high toughness is not required, such as for instance die inserts prone to washout or hot stamping tools with simple geometries, exposed primarily to abrasive and adhesive wear.
• the achieved hardness is higher compared to H13, when tempering above 590°C.
• the B parameter is generally below 0.74.
• Newly developed steels as proposed in the present invention show such behavior that, in conventional heat treatment, a bainite microstructure is formed instead of the conventional martensite.
• This microstructure is further characterized as being structurally homogeneous, that is, other microstructural constituents, such as martensite, ferrite are not present within the segregation bands.
• steels of the invention develop a predominantly bainite microstructure.

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• 2019
  • 2019-05-10 WO PCT/SI2019/050008 patent/WO2020231346A1/en unknown
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