EP1445339B1 - Alloy and article with high heat resistance and high heat stability - Google Patents

Description

The invention relates to an alloy for the production of articles with high heat resistance and toughness.

In particular, the invention relates to a hot work tool steel article having high hardness, high heat resistance and high thermal stability.

In general, hot work tool steels can be referred to as thermally recoverable iron-based alloys whose elevated mechanical properties after heat treatment, in particular their high strength and hardness, are maintained up to temperatures of 500 ° C. and above.

In accordance with the increasing demands of technical development, there is a general requirement for hot working materials to further improve their quality and, in particular, to increase their heat resistance at high thermal stability and to increase their toughness.

Conventional hot working steels are carbon-containing iron-based alloys containing 0.3 to 0.4% by weight of carbon (C), the hardness of which is increased as required by quench hardening due to martensite formation in the microstructure and tempering. An addition of alloying elements usually in wt .-%: Silicon (Si) to 1.5 Chrome (Cr) 2.5 to 5.5 Molybdenum (Mo) to 3.0 Vanadin (V) to 1.0 to the iron base material and an application of specially designed heat treatment processes makes it possible to produce from this an article which has high values for desired mechanical properties at a temperature of use up to about 500 ° C. By alloying tungsten (W) to 9 wt .-% and cobalt (Co) to 3.0 wt .-%, the use temperature can be slightly increased.

In essence, the hot hardness of such steels is given by a precipitation mechanism, referred to by those skilled in the art as secondary hardening where the finest chromium-molybdenum-tungsten-vanadium carbides are formed in the martensite lattice , which is, for example, the documents JP 07228945A and US-A-3453151 reveal.

Another increase in the strength of a material, which differs in its nature for quench hardening, can be achieved by precipitation hardening. The prerequisite for a precipitation hardening is a decreasing with the temperature solubility of an alloy addition or of alloying elements in the base metal.

In precipitation hardening, an alloyed material is first subjected to a solution annealing treatment followed by increased cooling, with which an alloying additive or phase is completely or partially solubilized and maintained in supersaturated solution. Subsequent heating to a temperature below the solution annealing temperature causes the supersaturation fraction of the element (s) or phase (s) to precipitate, causing a change in material properties, typically a material hardness increase.

Precipitation-hardenable iron base materials generally have alloy contents in% by weight of: Carbon (C) to 0.05 Manganese (Mn) to 2.0 Chrome (Cr) to 16.0 Molybdenum (Mo) to 6.0 Nickel (Ni) to 26.0 Vanadin (V) to 0.4 Cobalt (Co) to 10.0 Titanium (Ti) to 3.0 Aluminum (Al) to 0.3

Both the iron-base alloys with a martensite formation in a quench hardening, as well as those experiencing a change in their mechanical properties by precipitation of elements and phases, have the common disadvantage that in each area of the alloy composition and / or by a heat treatment technology only individual properties, such as for example, the hardness and strength or the temperature resistance, be improved but a drop of other property values, such as material toughness, thermal stability and the like, is connected.

The aim of the invention is to provide an alloy which makes it possible to improve the overall property profile of an object made therefrom. According to the object of the invention, a hot work tool article with simultaneously high hardness and high toughness, high heat resistance and high thermal stability can be provided.

The object of the invention mentioned above is with an alloy consisting of in wt .-%: Carbon (C) 0.15 to 0.44 Silicon (Si) 0.04 to 0.3 Manganese (Mn) 0.06 to 0.4 Chrome (Cr) 1.2 to 5.0 Molybdenum (Mo) 0.8 to 6.5 Nickel (Ni) 3.4 to 9.8 Vanadin (V) 0.2 to 0.8 Cobalt (Co) 0.1 to 9.8 Aluminum (Al) 1.4 to 3.0 Copper (Cu) under 1.3 Niobium (Nb) under 0.35 Iron (Fe) rest as well as production-related impurities.

The advantages resulting from the invention are essentially to be seen in the fact that a material was created by alloying measures, in which the quenching or Martensithärtung a precipitation hardening is superimposed. The activities of the alloying elements compared to the carbon and those in connection or phase formation are chosen so low that even at comparatively low Austenitisierungstemperaturen curing by ultrafine, secondary Karbideausscheidungen, especially chromium-molybdenum-vanadium carbides, and curing by a Elimination of intermetallic phases, in particular of Al Fe ₂ Ni occur simultaneously in the remuneration and a high hot hardness with high toughness of the material is achieved.

According to the invention, a hardenability of large parts is improved possible because alloying a corresponding thermal conversion behavior of the material is set. Likewise, the tempering resistance and thus the thermal stability of the tempered material at high hardness are substantially improved.

In an iron-based alloy according to the invention, a carbon content of at least 0.15 wt .-% is provided so that a sufficient amount of carbide for a desired secondary increase in hardness can be eliminated. Higher carbon concentrations than 0.44 wt .-% can interfere with the proposed carbide-forming elements, the toughening reduce primary carbides, so that the content of carbon should be between 0.15 and 0.44 wt .-%.

The content of silicon must be an advantageous composition of a deoxidation product because of at least 0.04 wt .-%, on the other hand, however, should not be higher than 0.3 wt .-%, because higher silicon values adversely affect the material toughness.

Manganese is provided according to the invention in the steel with a concentration of between 0.06 and 0.4% by weight. Lower levels can cause thermoforming and higher levels of brittleness to harden the material.

The contents of chromium, molybdenum and vanadium are important for a desired secondary hardness formation of the material in the coating and should be considered together. Chromium contents below 1.2% by weight have a disadvantageous effect on the hardenability of the material; those of more than 5.0% by weight impair the thermal stability of the same, because this suppresses the activity of the molybdenum.

At molybdenum concentrations below 0.8 wt%, too little of this element is solubilized during the heat treatment, resulting in low secondary hardness levels. Over 6.5% by weight of molybdenum in the steel can cause too high a carbide content, which can lead to reduced toughness of the material and economic disadvantages.

The strong carbide Vanadin is provided according to the invention with a minimum content of 0.2 wt .-% to ensure sufficient, stable secondary hardening of the steel sure. Higher contents than 0.8% by weight of vanadium, especially at carbon contents in the upper range of the intended concentration range, can lead to the precipitation of primary carbides, whereby the toughness properties of the material are abruptly worsened.

Although the effect of niobium is similar to that of vanadium, it is characterized by the formation of very stable carbides, so that the content of niobium should advantageously be below 0.35 wt .-%.

To ensure a desired secondary hardness increase during tempering of the martensite structure of the alloy according to the invention, it thus has a content of chromium of from 1.2 to 5.0, molybdenum, at a carbon concentration of 0.15 to 0.44% by weight 0.8 to 6.5 and on vanadium from 0.2 to 0.8.

The nickel concentration of the steel and its aluminum content are to be seen in terms of the precipitation kinetics of the phase of Al Fe ₂ Ni for hardness increase in a proposed heat treatment technology. At nickel contents below 3.4 wt .-% and at an aluminum concentration of less than 1.4 wt .-% precipitation hardening is pushed back, so the additive increase in hardness as a material during tempering low.

Higher contents than 9.8 wt.% Nickel shift the γ / α conversion to lower temperatures, which can lead to problems in the soft-annealing treatment of the steel, a high processing hardness and the disruption of the precipitation kinetics.

Contents above 3.0% by weight of aluminum disadvantageously promote a high DELTA (δ) ferrite range in the conversion behavior, nitride formation and lower the material toughness of the alloy.

According to the invention, therefore, the nickel content and the aluminum content of the steel in wt .-% in the ranges 3.4 to 9.8 nickel and 1.4 to 3.0 aluminum.

Copper can form undesirable intermetallic phases and should be contained in the steel at a low concentration of less than 1.3% by weight.

To further improve the property profile of the alloy according to the invention, it may be provided that it has one or more of the elements in the following concentrations in% by weight: Carbon (C) 0.25 to 0.4 preferably 0.31 to 0.36 Silicon (Si) 0.1 to 0.25, preferably 0.15 to 0.19 Manganese (Mn) 0.15 to 0.3 preferably 0.2 to 0.29 Chrome (Cr) 1.9 to 2.9 preferably 2.2 to 2.8 Molybdenum (Mo) 1.2 to 2.9 preferably 2.1 to 2.9 Nickel (Ni) 5.0 to 7.6 preferably 5.6 to 7.1 Vanadin (V) 0.24 to 0.6 preferably 0.25 to 0.4 Cobalt (Co) 1.4 to 7.9 preferably 1.6 to 2.9 Aluminum (Al) 1.6 to 2.9 preferably 2.1 to 2.8

By means of these narrower content ranges of elements in the chemical composition of the steel, a further improvement in the properties of the articles produced therefrom can be achieved.

Of particular importance for overall high mechanical steel values, but especially for high toughness properties of the material is a limited amount of admixtures.

In an advantageous embodiment of the invention, an alloy is provided which contains one or more of the impurity elements with the following MAXIMUM concentrations in% by weight: Phosphorus (P) 0.02, preferably 0.005 Sulfur (S) 0,008, preferably 0,003 Copper (Cu) 0.15 preferably 0.06 Titanium (Ti) 0.01, preferably 0.005 Niobium (Nb) 0.001, preferably 0.0005 Nitrogen (N) 0,025, preferably 0,015 Oxygen (O) 0.009, preferably 0,002 Calcium (Ca) 0,003, preferably 0.001 Magnesium (Mg) 0,003, preferably 0.001 Tin (Sn) 0.01, preferably 0.005 Tantalum (Ta) 0.001, preferably 0.0005

In order to achieve a particularly pronounced precipitation hardenability of the alloy superimposed on the secondary hardening by means of carbides, it can be advantageous if the value of nickel content refracted by aluminum content in each case in wt .-% between 1.8 and 4.2, preferably between 2.1 and 3.9. As a result, an overhang of an element forming the precipitate is avoided.

The object of the invention is achieved according to an improved property profile of a hot work tool article when a hot-working and machining primary material prepared by a melt metallurgy or powder metallurgy process having a previously given chemical composition is molded, which molded article becomes secondary after a thermosetting heat treatment precipitated carbides, as well as intermetallic precipitates.

The total hardness of the material is advantageously achieved by a superposition of the secondary hardness increase by carbide precipitations and precipitation hardening. As a result, high material hardness values can be achieved, although the tempering technology is directed to obtaining high material toughness and using lower hardening temperatures compared to prior art hot work tool steel. This lower austenitizing temperature may also have significant advantages in terms of low distortion in a tempering treatment of complicated shaped parts.

If, however, the hardening temperatures are set to a high level, then, with otherwise good material toughness, extremely high hardness values of the steel object result.

If a ratio of intermetallic precipitates broken by secondary precipitated carbides is given in each case in the volume of less than 3.0, preferably 1.0 and less, but more than 0.38, in the microstructure of the hot-work tool, the values for high hardness values are Toughness is particularly high and the thermal stability is shifted by up to 50 ° C and more to higher temperatures.

A hot work tool according to the invention, which secondarily precipitated chromium-molybdenum-vanadium mixed carbides and substantially has intermetallic phases of the type Al Fe ₂ Ni in the structure, has a particularly preferred property profile and can be produced economically in conventional hardening equipment at relatively low curing temperatures.

A pronounced thermal stability of the article can be achieved if the alloy has a ratio of chromium + molybdenum + vanadium broken by carbon, each in wt% greater than 12 but less than 19.
On the basis of some investigation results and representations, the invention will be explained in more detail by way of example.

From an alloy A according to the invention, from a conventional hot-work steel B and from a precipitation-hardening steel C (maraging steel), samples were produced, thermally tempered and their material properties investigated. The alloys have the chemical compositions given in Tab. 1: Tab. 1 element Alloy A Alloy B Alloy C C 0.32 0.36 12:13 Si 12:18 0.40 <0.05 Mn 0.25 12:33 <0.02 Cr 2.45 4.79 0.11 Not a word 2:43 2.78 5.26 Ni 6.46 0.18 18.01 V 0.28 0.62 0.02 Co 1.97 <0.05 8.71 Al ^: 2.46 0.016 0.13 Cu 0.06 0.07 0.06 Nb <0.005 <0.005 <0.005 Fe bal. bal. bal. P 0,008 0,015 <0005 S 0.001 0.001 0.009 Ti <0 , 005 <0.005 0.79 N 0.0048 0.0068 0.0017 O 0.0022 0.0023 0.0007 Ca mg sn <0.005 <0.005 0.009 Ta

For the time being, a measurement of the thermal expansion α [10 ^-6 / K] as a function of the temperature at an initial hardness of the material of 50 to 52 HRC was carried out on the sample material. The values which can be taken from Table 2 show that, in comparison with a conventional hot-work steel B, the alloy according to the invention has a lower expansion, which also indicates a better dimensional stability during a heat treatment. Tab. 2 Temperature [° C] A B C 100 10.8 11.2 9 200 11.2 11.61 9.5 300 11.7 12 9.95 400 12.2 12.5 10.44 500 12.7 12.9 10.9

After hardening to approximately 55 HRC each of samples of the inventive alloy A and the conventional hot-work steel B, the hardness profile of the materials was determined as a function of the temperature. It is essential that the alloy A according to the invention required an austenitizing temperature of 990 ° C. in order to achieve this hardness, but that of 1050 ° C. was required for the conventional hot-rolled steel B, however. Depending on the temperature, as shown in Tab. 3A and Tab. 3B, increased in the range between 500 ° C and 600 ° C, the hardness of the inventively assembled sample A to values around 60 HRC, whereas in the conventional hot-rolled steel B a maximum Hardness value of 56 HRC at 500 ° C was determined. Tab. 3A A temperature Hardness In HRC 25 54 100 54 200 50 300 51 400 54 500 60 530 60 560 60 590 59 620 55 650 49 680 43 B temperature Hardness in HRC 25 55 300 52 400 53 500 54 530 53 560 52 590 50 620 47 650 43

In graphical representation is in Fig. 1 the respective hardness curve depending on the temperature of the material A according to the invention and the hot working steel alloy B according to the prior art comparatively shown. Starting from the same hardness, which however is achieved with an optionally advantageous lower austenitizing temperature, takes place in the alloy A according to the invention by a superimposed precipitation mechanism, in which Al Fe ₂ Ni precipitates are formed in the finest form in the microstructure, a much greater increase in the hot hardness of Subject, whereby this is maintained even at higher temperatures.

Based on a hardening specification according to Vickers, the softening behavior of the materials was investigated as a function of time at a temperature of 650 ° C.

A hardness determination on the test specimen at the test temperature was carried out according to the rebound hardness method (Shore hardness), for which return values hitherto only a conversion into Vickers hardness values is present.

Starting from approximately the same hardness at room temperature, namely from 50 to 52 HRC, which were achieved for the alloys A, B and C with a composition according to Tab. 1 by different thermal treatment methods given in the test result Sheet 1, a hardness test was carried out on the time at 650 ° C.

In comparison with a conventional hot-work steel B and a maraging steel C, the alloy A according to the invention with the same initial hardness at 650 ° C. for a time of up to 1000 minutes had the highest material hardness. After this time, maraging steel C had a higher hardness with high thermal stability, whereas hot work steel A according to the invention lost about 10% of its hardness up to about 2000 minutes. The thermal stability of the conventional hot-work steel B was low; the hardness difference in comparison with the alloy A according to the invention increased steadily up to 1000 minutes.

Claims (12)

1. Alloy for the production of articles having high heat resistance and toughness consisting of, in wt.-%: carbon (C) 0.15 to 0.44 silicon (Si) 0.04 to 0.3 manganese (Mn) 0.06 to 0.4 chromium (Cr) 1.2 to 5.0 molybdenum (Mo) 0.8 to 6.5 nickel (Ni) 3.4 to 9.8 vanadium (V) 0.2 to 0.8 cobalt (Co) 0.1 to 9.8 aluminium (Al) 1.4 to 3.0 copper(Cu) below 1.3 niobium (Nb) below 0.35 iron (Fe) rest and accompanying elements and impurities.
2. Alloy according to claim 1, containing one or more of the elements with the following concentrations in wt.-%: carbon (C) 0.25 to 0.40 silicon (Si) 0.10 to 0.25 manganese (Mn) 0.15 to 0.30 chromium (Cr) 1.9 to 2.9 molybdenum (Mo) 1.2 to 2.9 nickel (Ni) 5.0 to 7.6 vanadium (V) 0.24 to 0.6 cobalt (Co) 1.4 to 7.9 aluminium (Al) 1.6 to 2.9.
3. Alloy according to claim 1, containing one or more of the elements with the following concentrations in wt.-%: carbon (C) 0.31 to 0.36 silicon (Si) 0.15 to 0.19 manganese (Mn) 0.20 to 0.29 chromium (Cr) 2.2 to 2.8 molybdenum (Mo) 2.1 to 2.99 nickel (Ni) 5.6 to 7.1 vanadium (V) 0.25 to 0.4 cobalt (Co) 1.6 to 2.9 aluminium (Al) 2.1 to 2.8.
4. Alloy according to claim 1 to 3, containing one or more of the elements of impurities with the following maximum concentrations in wt.-% : phosphor(P) 0.02 sulfur (S) 0.008 copper(Cu) 0.15 titanium (Ti) 0.01 niobium (Nb) 0.001 nitrogen (N) 0.025 oxygen (O) 0.009 calcium (Ca) 0.003 magnesium (Mg) 0.003 tin (Sn) 0.01 tantalum (Ta) 0.001.
5. Alloy according to claim 1 to 3, containing one or more of the elements of impurities with the following maximum concentrations in wt.-% : phosphor(P) 0.005 sulfur (S) 0.003 copper(Cu) 0.06 titanium (Ti) 0.005 niobium (Nb) 0.0005 nitrogen (N) 0.015 oxygen (O) 0.002 calcium (Ca) 0.001 magnesium (Mg) 0.001 tin (Sn) 0.005 tantalum (Ta) 0.0005.
6. Alloy according to one of claims 1 to 5, wherein the value nickel content divided by aluminium content, respectively in wt.-%, is between 1.8 and 4.2: $$\frac{\mathrm{Ni}}{\mathrm{Al}}=1.8\mathit{to}\mathrm{4.2.}$$

   
7. Alloy according to one of claims 1 to 5, wherein the value nickel content divided by aluminium content, respectively in wt.-%, is preferably between 2.1 and 3.9: $$\frac{\mathrm{Ni}}{\mathrm{Al}}=2.1\mathit{to}\mathrm{3.9.}$$

   
8. Alloy according to one of claims 1 to 7, which, in its chemical composition, is a ratio value of chromium + molybdenum + vanadium divided by carbon, respectively in wt.-% , of higher than 12, but lower than 19 : $$\mathrm{12}<\frac{\mathrm{Cr}+\mathrm{Mo}+\mathrm{V}}{\mathrm{C}}<\mathrm{19.}$$

   
9. Hot-working steel article with high hardness, high heat resistance and high thermal stability, wherein a starting material, produced by a melt metallurgical or powder metallurgical process, with a chemical composition characterized in the preceding claims, was brought into shape by hot forming and machining, which formed article comprises, following a hardening heat treatment, secondary precipitated carbides in the structure, and intermetallic precipitates.
10. Hot-working steel article according to claim 9, which in the structure comprises a ratio of intermetallic precipitates divided by secondary precipitated carbides, respectively in vol.-%, of less than 3.0.
11. Hot-working steel article according to claim 9, which in the structure comprises a ratio of intermetallic precipitates divided by secondary precipitated carbides, respectively in vol.-%, of 1.0 and less, but above 0.38.
12. Hot-working steel article according to claim 9 to 11, which comprises chromium-molybdenum-vanadium mixed carbides secondarily precipitated in the structure and intermetallic phases of the AlFe₂Ni type in the structure.

Images