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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0264—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements the maximum content of each alloying element not exceeding 5%
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the present invention relates to a low-chromium hot-work tool steel and a process for making a low-chromium hot-work tool steel article.
• the term 'hot-work tools' is applied to a great number of different kinds of tools for the working or forming of metals at comparatively high temperatures, for example tools for die casting, such as dies, inserts and cores, inlet parts, nozzles, ejector elements, pistons, pressure chambers, etc.; tools for extrusion tooling, such as dies, die holders, liners, pressure pads and stems, spindles, etc.; tools for hot-pressing, such as tools for hot- pressing of aluminium, magnesium, copper, copper alloys and steel; moulds for plastics, such as moulds for injection moulding, compression moulding and extrusion; together with various other kinds of tools such as tools for hot shearing, shrink-rings/collars and wearing parts intended for use in work at high temperatures.
• tools for die casting such as dies, inserts and cores, inlet parts, nozzles, ejector elements, pistons, pressure chambers, etc.
• tools for extrusion tooling such as dies, die holders
• Low-alloyed hot-work tool steel is used in small to medium sized tools in applications where the demands on tempering resistance and thermal fatigue are high.
• Tempering resistance is the ability of a hot-work tool steel to keep its hardness at an elevated temperature for prolonged time.
• Hot-work tool steels are developed for strength and hardness during prolonged exposure to elevated temperatures and generally use a substantial amount of carbide forming alloys.
• High speed steels are used for cutting tools where strength and hardness must be retained at temperatures up to or exceeding 760 °C.
• tungsten and chromium e.g. 18 and 4 wt-%, respectively.
• molybdenum 5-10 wt-% were developed.
• High speed steel differs from hot-work steel in composition and price and cannot be used as a substitute for hot- work steel.
• One object of the present invention is to provide a low-chromium hot-work tool steel having an improved property profile, in particular an improved tempering resistance.
• the steels of the present invention is particular suitable for small tools which do not require a steel composition having a high hardenability for their manufacturing.
• N 0.03-0.30 preferably 0.03-0.10
• V 1.0-1.3 preferably 1.15-1.25
• Ni ⁇ 1 preferably ⁇ 0.25
• Preferred embodiments of the low-chromium hot-work tool steel may fulfill one or more of the following conditions (in wt-%):
• N 0.04-0.30 preferably 0.04-0.10
• More preferred embodiments of the low-chromium hot-work tool steel may fulfill one or more of the following conditions (in wt-%):
• N 0.042-0.15 preferably 0.045-0.12
• low-chromium hot-work tool steel may fulfill one or more of the following conditions (in wt-%):
• N 0.042-0.12 preferably 0.045-0.12
• the low-chromium hot-work tool steel may have composition (in wt-%) according to the examples below:
• Another object is to provide a low-chromium hot-work tool steel article having an improved property profile, in particular an improved tempering resistance.
• this object is obtained by a process as defined in claim 1 1, ie. A process which comprises the steps of:
• step b) austenitizing the steel article obtained in step b) at a temperature of at most 1200 °C for a time on the order of half an hour followed by quenching;
• the total amount of carbon and nitrogen shall be regulated to 0.30 ⁇ (C+N) ⁇ 0.50, preferably 0,36 ⁇ (C+N) ⁇ 0.44.
• the nominal content shall be in the order of 0.40 wt-%.
• the preferred ranges are set out in the product claims.
• the nitrogen content preferably is on the order of 0.05 wt-%. This value gives a better performance than higher values.
• a nitrogen content on the order of 0.05 wt-% gives a higher potential for secondary hardening during quenching than higher contents do, thus giving the steel a high hardness.
• an amount in the order of 0.10 wt-% has shown to give a shift of the secondary hardening peak to somewhat higher tempering temperatures which is positive.
• the preferred ranges of are set out in the product claims.
• Chromium promotes the hardenability and corrosion resistance of steels. At too low contents the corrosion resistance will be adversely affected.
• a minimum chromium content in the steel therefore, is set to 1 wt-%.
• the maximum content is set to 4 wt-% in order to avoid undesired formation of chromium rich carbides/carbonitrides, e.g.
• the chromium content preferably shall not exceed 3 wt-%, and even more preferred preferably not exceed 2.6 wt-%. In one embodiment of the invention, the chromium content is 1.5-1.7 wt-%.
• the preferred ranges of are set out in the product claims. A low chromium content delays the precipitation of chromium carbides in the microstructure in favour of the more thermally stable vanadium-rich carbo-nitride. Thus the recovery is slowed down in the material and the tempering resistance becomes improved
• the steel shall contain vanadium in an amount of at least 0.8 wt-% in order to provide a sufficient precipitation potential and thus an adequate tempering resistance and desired high temperature strength properties.
• the upper limit of vanadium is 1.3 wt-%.
• vanadium is between 1.0 and 1.3 wt-%.
• the preferred ranges of are set out in the product claims.
• the ration Cr/V should preferably be less than 2, more preferably less than 1.8 in order to get the desired MC phase. The reason is that Cr can be considered as a poison for the MC phase.
• Silicon shall be present in the steel in an amount of between 0.1 - 0.5 wt-%, preferably 0.2 - 0.4 wt-%.
• Manganese is present in order to give the steel an adequate hardenability, particularly given the relatively low content of chromium and molybdenum in the steel.
• the content of manganese in the steel is between 0.5 and 2 wt-%, preferably between 1.0 and 2.0 wt-%).
• the preferred ranges of are set out in the product claims.
• Molybdenum shall be present in the steel in an amount of between 1.5 and 3 wt-%>, preferably 2.2 - 2.8 wt-%>, in order to provide a secondary hardening during tempering and to give a contribution to the hardenability.
• the preferred ranges of are set out in the product claims.
• Part of the molybdenum may be substituted for tungsten in a manner known per se but the steel shall preferably not contain any intentionally added amounts of tungsten, i.e. shall not contain tungsten in amounts exceeding impurity level, because of certain drawbacks related to the presence of that element.
• the ratio Mo/V should preferably lie in the range of 1.8 - 2.3, more preferably 1.9 -2.1 in order to get the desired precipitation sequence and precipitation potential of the secondary carbides. It is known that Mo stabilizes the M 2 C phase and by adjusting the contents of Mo and V to fall within the range of 1.8 -2.3 also the molybdenum rich M 2 C will form, which phase has a higher coarsening rate as compared to the vanadium rich MC phase.
• Nickel and cobalt are elements that may be included in the steel in amounts up to 3 wt- %> and 5 wt-%> respectively. Cobalt may increase the hardness at high temperatures which may be advantageous for some applications of the steel. If cobalt is added, an effective amount is about 4 wt. %>. Nickel may increase the corrosion resistance, hardenability and toughness of the steel. The preferred ranges of are set out in the product claims.
• austenitizing may be carried out at a temperature between the soft annealing temperature 820 °C and the maximum austenitizing temperature 1200 °C, but the austenitizing of the steel article preferably is carried out at a temperature on the order of 1050 - 1150 °C, preferably at 1080 - 1150 °C, typically at 1100 °C.
• higher austenitizing temperatures shift the tempering hardness to higher temperatures, i.e. the secondary hardening peak will be shifted to higher temperatures, which means that the desired hardness will be reached at a higher initial tempering temperature.
• the material will obtain an improved tempering resistance and the work temperature of the tools could be elevated.
• the tempering of the quenched steel article preferably is carried out at least twice at a retention time of 2 hours at a temperature between 500 and 700 °C, preferably 550 and 680 °C. In the most preferred embodiment of the steel composition, the tempering is carried out at a temperature between 600 and 650 °C, preferably between 625 and 650 °C.
• Nitrogen contents in the range of 0.05 - 0.10 wt-% may be obtained by incorporating the nitrogen by conventional casting methods to form a melt, casting the melt to form an ingot, and homogenizing the ingot by heat treatment. Nitrogen additions will produce large primary vanadium-rich M(C,N) precipitates, which in turn will give the material uneven hardness. However, the large primary carbo-nitrides will not occur if the nitrogen content is lowered and there is a homogenizing heat treatment prior to a subsequent forging.
• nitrogen may amount to up to 0.30 wt- %.
• conventional casting methods are insufficient.
• the nitrogen could be incorporated by first manufacturing a steel powder of essentially the desired composition, except for the nitrogen, then nitriding this powder in solid state by nitrogen containing fluid, e.g. nitrogen gas, thereafter hot pressing the powder isostatically at a temperature on the order of 1150 °C and a pressure on the order of 76 MPa to form an ingot.
• nitrogen containing fluid e.g. nitrogen gas
• the ingot is preferably forged at a temperature on the order of 1270 °C, and then soft annealed at a temperature on the order of 820 °C, followed by cooling at a rate of 10 °C per hour to a temperature of 650 °C and then free cooling in air to make it ready for austenitizing.
• the steel of the present inveintion has a much improved tempering resistance permitting a longer article life in hot-work applications.
• the nitrogen content preferably is on the order of 0.05 wt-% and the chromium content is preferably less than 3 wt-%, i.e 1.2 -2.6 or 1.3 - 2.3.
• the steel article of the present invention shall preferably also satisfy some of the following demands:
• Fig. 1 is a diagram showing hardness vs. tempering temperature of an exemplary prior art low-chromium hot-work tool steel containing no nitrogen.
• Fig. 2 is a diagram showing hardness of prior art steels (contents in wt-%) Cr 15, Mo 1, C 0.6 and Cr 15, Mo 1, C 0.29, N 0.35 at different tempering temperatures.
• Fig. 3 is a diagram illustrating the effect of low chromium content on the stability of M(C,N) in austenite.
• Fig. 4 is a diagram showing the mole fraction of M 6 C, M(C,N) and the bcc matrix as a function of temperature. (Balance phase: austenitic matrix.)
• Fig. 5 is a diagram showing the amount of M(C,N) phase and meta-stable M 2 C as
• Fig. 6 is a diagram showing hardness vs. tempering temperature curves for trial alloys N0.05, N0.10 and N0.30
• Fig. 7 is a back-scattered SEM image showing small undissolved M(C,N) precipitates and a globular mixed oxide-sulphide particle in NO.05.
• Fig. 8 is a back-scattered SEM image revealing undissolved, primary M(C,N) at former austenite grain boundaries in alloy NO.10.
• Fig. 9 is a back-scattered SEM image depicting primary particles in soft annealed
• Fig. 10 is a back-scattered SEM image revealing an even distribution of undissolved M(C,N) particles in N0.30.
• Fig. 11 is a back-scattered SEM image revealing some clusters of undissolved M(C,N) found in N0.30.
• Molybdenum and vanadium medium alloyed hot-work tool steels have good resistance to thermal fatigue, softening and high-temperature creep.
• An exemplary nominal chemical composition of such a prior art steel is presented in Table 1 (wt-%).
• FIG. 1 presents a tempering curve (hardness vs. tempering temperature) for the exemplary prior art tool steel.
• the samples were austenitized at 1030 °C, and then tempered two times at different temperatures; from 200 °C up to 700 °C for a tempering time of 2 + 2 hours.
• the interval 500 to 650 °C there is a pronounced secondary hardening peak at 550 °C.
• the desired hardness e.g. 44-46 HRC
• the material would get an improved tempering resistance, and the work temperature of the tools could be raised.
• the substitution of nitrogen for part of the carbon is used to achieve a higher hardness of the martensitic steel matrix.
• the nitrogen addition initially causes a larger amount of retained austenite.
• this austenite can later be transformed to martensite by cold work, and it is possible to achieve hardness as high as 68 HRC in this manner.
• a low chromium content appears to have a positive effect on the tempering resistance.
• a comparison of two different hot-work tools steels with 1.5 and 5.0 wt-% chromium shows that the lower chromium content delays the precipitation of chromium carbides in the microstructure in favour of the more thermally stable vanadium-rich MC. Thus the recovery is slowed down in the material and the tempering resistance becomes improved.
• a low-chromium hot-work tool steel article having increased tempering resistance is made by carrying out the following process steps: a) incorporating nitrogen in a low-chromium hot-work tool steel melt composition and thereby providing a steel composition as defined in any of the process claims;
• step b) austenitizing the steel article obtained in step b) at a temperature of at most 1200 °C for a time on the order of half an hour followed by quenching;
• vanadium carbo-nitrides when the nitrogen content is balanced to about 0.015 to 0.30 wt-% in a low-chromium steel, vanadium carbo-nitrides will form, which will be partly dissolved during the austenitizing step and then precipitated during the tempering step as particles of nanometer size.
• the particles are in the order of about 1 ⁇ to about 10 ⁇ .
• the average size of the particles are less than 1 ⁇ .
• the thermal stability of vanadium carbo-nitrides is better than that of vanadium carbides, and consequently the tempering resistance of the low-chromium hot-work tool steel article will be much improved. Further, by tempering at least twice, the tempering curve (showing hardness as a function of tempering temperature) will have a higher, secondary peak.
• the nitrogen content preferably is on the order of 0.05 percent by weight. This value gives a better performance than higher values. A nitrogen content on the order of 0.05 percent by weight gives a higher potential for secondary hardening during quenching than higher contents do.
• the chromium content preferably is 1.5-1.7 percent by weight.
• a low chromium content delays the precipitation of chromium carbides in the microstructure in favour of the more thermally stable vanadium-rich carbo-nitrides.
• austenitizing may be carried out at a temperature between the soft annealing temperature 820 °C and the maximum austenitizing temperature 1200 °C.
• the austenitizing of the steel article preferably is carried out at a temperature on the order of 1050 - 1150 °C, preferably at 1100 °C.
• higher austenitizing temperatures shift the tempering hardness to higher temperatures, i.e. the secondary hardening peak will be shifted to higher temperatures, which means that the desired hardness will be reached at a higher initial tempering temperature.
• the material will get an improved tempering resistance and the work temperature of the tools will be raised.
• the tempering of the quenched steel article preferably is carried out at least twice at a retention time of 2 hours at a temperature between 500 and 700 °C, preferably550 and 680 °C. In the most preferred embodiment of the steel composition, the tempering is carried out at a temperature between 600 and 650 °C, preferably between 625 and 650 °C.
• Nitrogen contents in the range of 0.05 - 0.10 percent by weight may be obtained by incorporating the nitrogen by conventional casting methods to form a melt, casting the melt to form an ingot, and homogenizing the ingot by heat treatment. Nitrogen additions will produce large primary vanadium-rich M(C,N) precipitates, which in turn will give the material uneven hardness. However, large primary carbo-nitrides will not occur if the nitrogen content is lowered and there is a homogenizing heat treatment prior to a subsequent forging.
• the nitrogen content preferably is on the order of 0.05 wt-%. This value gives a better performance than higher values.
• a nitrogen content on the order of 0.05 wt-% gives a higher potential for secondary hardening during quenching than higher contents do, thus giving the steel a high hardness.
• an amount in the order of 0.10 wt-% has shown to give a shift of the secondary hardening peak to somewhat higher tempering temperatures which is positive. Additionally, the performed tests and modelling calculations indicate that an increased austenitizing temperature is required in connection with increased nitrogen contents.
• nitrogen may amount to up to 0.30 wt-
• the nitrogen then is incorporated preferably by first manufacturing a steel powder of essentially the desired composition, except for the nitrogen, then nitriding this powder in solid state by nitrogen gas, thereafter hot pressing the powder isostatically at a temperature on the order of 1150 °C and a pressure on the order of 76
• the ingot is preferably forged at a temperature on the order of 1270 °C, and then soft annealed at a temperature on the order of 820 °C, followed by cooling at a rate of 10 °C per hour to a temperature of 650 °C and then free cooling in air to make it ready for austenitizing.
• NO.05 designates a material having a nitrogen content of 0.05 wt-%, and so on. Note that these are the actual compositions of the trial ingots.
• the aim was to keep the level of all alloying elements except carbon and nitrogen constant. Compared to the standard low chromium hot-work tool steel of Table 1, chromium was also slightly decreased. There was a small decrease in molybdenum content and an increase in manganese content. For carbon and nitrogen, the aim was to have a constant sum of around 0.40 wt-% of these elements, and this was relatively well achieved.
• the tempering stage concerns mainly meta-stable phases, and previous electron microscopy work has shown that they exist in standard low chromium hot-work tool steel at tempering temperature intervals, i.e. 400 to 700 °C.
• These carbide phases are mainly vanadium-rich MC (FCC) and molybdenum-rich M 2 C (HCP).
• FCC vanadium-rich MC
• HCP molybdenum-rich M 2 C
• Some amount of chromium-rich M 7 C 3 has also been found in the standard low chromium hot-work tool steel.
• the following calculations were made in order to decide whether or not these nitrogen containing alloys were possible to harden, i.e. if enough alloying elements could be dissolved into the austenitic matrix at the austenitizing temperature, so that martensite would form during quenching.
• the interesting temperature interval thus was between the soft annealing temperature, 820 °C and the set practically usable maximum austenitizing temperature, 1200
• the amount of carbon, nitrogen and vanadium would be lower in the NO.30 matrix after austenitizing at 1100 °C than in the N0.05 matrix. Since the molybdenum-rich M 6 C phase only dissolves carbon and no nitrogen, it suffers from the lower carbon content in NO.10 and NO.30, thus the amount of M 6 C decreases with decreasing carbon content. It should also be noted that all M 6 C is dissolved at the austenitizing temperatures used. The calculations performed in the tempering temperature region were only done in order to estimate the potential for secondary precipitation in NO.05, NO.10 and NO.30. The equilibria found can at best show what phases would be present in the material after a sufficiently long time. Previous work has shown that in practice there is some auto- tempering in the standard low chromium hot-work tool steel. This means that M 3 C (cementite) will precipitate after the austenitizing process.
• the two alloys NO.05 and NO.10 were conventionally cast as small ingots of 50 kg.
• NO.10 was the first trial and there was no homogenizing treatment done on this ingot before the forging process.
• the third alloy, NO.30 had a too high nitrogen content to be manufactured by conventional casting. Therefore this alloy was produced using powder metallurgy. First the steel powder was manufactured and then this powder was nitrided in solid state by pressurized N 2 -gas. The powder was then hot isostatically pressed (HIP) at 1 150 °C with the pressure of 76 MPa.
• HIP hot isostatically pressed
• Fig. 6 The results from the hardness measurements are presented in Fig. 6. As can be seen, all three alloys have a secondary hardening peak in the temperature interval 500 to 650 °C. All tempering was done for 2 + 2 hours. NO.05 has the highest hardness in the as- quenched condition (53FIRC), while NO.10 and NO.30 had somewhat lower hardness. However, all three alloys are regarded as hardenable. The hardness curve of NO.05 is very similar to that of the standard low-chromium hot-work tool steel with a maximum of around 54 HRC as shown in Fig. 1.
• the secondary hardening peak of NO.10 seems to be somewhat shifted to a higher temperature with peak hardness at 600 °C.
• the peak hardness for both NO.05 and NO.30 was at 550 °C.
• Another phase that is easily found in NO.05 is the mixture of aluminium-oxide and manganese-sulphide, see Fig. 7, which is a SEM image (back-scattered) showing small undissolved M(C,N) precipitates 2 and a globular mixed oxide-sulphide particle 1 in NO.05.
• the sample was austenitized at 1 100 °C for 30 min and tempered at 625 °C for 2 + 2 hours.
• NO.05 (and NO.10) is that all trial ingots were manufactured and cast in open atmosphere.
• the most common size of the M(C,N) particles in NO.10 is between 5 and 10 ⁇ Equivalent Circle Diameter (ECD) after austenitizing at 1150 °C for 30 minutes and tempering at 625 °C for 2 + 2 hours. Larger, primary carbides 3 (precipitated in the melt) are frequently found in former austenite grain boundaries, see Fig. 8, which is a back-scattered SEM image revealing undissolved, primary M(C,N) at former austenite grain boundaries in alloy NO.10. The sample was austenitized at 1 150 °C for 30 min and tempered at 625 °C for 2 + 2 hours.
• Fig. 9 is a detail SEM micrograph of primary M(C,N) particles 4 in NO.10.
• the undissolved M(C,N) particles 6 had a size distribution (ECD) between 1 to 5 ⁇ with the most common size 2 ⁇ , thus the particles were small even though the nitrogen content was high.
• the particles were homogeneously distributed in the microstructure, see Fig. 10. However, as shown in Fig. 11, some clusters 7 of M(C,N) were found.
• the process and the low-chromium hot-work tool steel of the present invention are applicable where it is desired to get hot-work steel tools, which can be utilized at increased temperatures for an extended period of time.

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