EP1471160B1 - Cold-worked Steel Object - Google Patents

Description

The invention relates to a cold work tool steel article. More precisely, the invention relates to a cold work tool steel article with improved property profile, in particular with high strength and high ductility.

For a cold massive forming, for example, with extrusion dies and punches for the production of components and also for cutting tools with additional high demands on the material toughness, such as taps and the like, in modern art objects with a high overall material property level are needed. Such cold work steels are known, for example, from JP 7316739. This also results from the expenses incurred for the manufacture of tools, because a complicated geometry of a component to be manufactured usually entail high costs for tool production.

This requirement is primarily to be seen in terms of improved economics in a large number of components or components. In order to keep the overall costs low, a selection of materials for the part should therefore be made for the particular case of need, which can achieve the highest possible service life of the same due to the material properties.

To improve the service life of a cold work tool in use with high overall stress, the material properties ductility to prevent tool breakage and strength to ensure dimensional stability are equally set to a high level and to minimize wear.

Increased resistance to abrasive wear is found in iron-based materials with a high carbide content, in particular with a high monocarbide content in a hard matrix. Such steels usually have a high carbon content up to 2.5 wt .-% at a concentration of monocarbide-forming elements to 15 wt .-%, ie a high primary carbide content, but have a low material toughness in the thermally tempered state. Through a powder metallurgical Production, the microstructure, in particular the carbide size and the carbide distribution in the material of the article can be improved, many times, however, a required material toughness can not be achieved.

Improved toughness properties can be achieved in typical high-alloyed high-speed steel materials, for example according to DIN Werkstoff no. 1.3351 be achieved in powder metallurgical production of the parts, but this toughening of the material for particularly stressed items is not enough, so that in long-term operation often a failure by breaking the same occurs.

The object of the invention is therefore to provide a cold work tool article whose material, with high wear resistance and hardness, has increased toughness as well as a similar compressive strength and has improved fatigue strength. In other words, it is the object of the invention to characterize a cold work tool steel article with simultaneously high strength and ductility values, which article, especially in one embodiment as dies and punches, provides high economy in a large number of parts.

This object is achieved according to the invention in a cold work tool steel article with a chemical composition of the material in wt .-%: Carbon (C) more than 0.6 and less than 1.0 Silicon (Si) more than 0.3 and less than 0.85 Manganese (Mn) more than 0.2 and less than 1.5 Phosphorus (P) MAX 0.03 Sulfur (S) less than 0.5 Chrome (Cr) more than 4.0 and less than 6.2 Molybdenum (Mo) more than 1.9 and less than 3.8 Nickel (Ni) less than 0.9 Vanadin (V) more than 1.0 and less than 2.9 Tungsten (W) more than 1.8 and less than 3.4 Copper (Cu) less than 0.7 Cobalt (Co) more than 3.8 and less than 5.8 Aluminum (Al) less than 0.045 Nitrogen (N) less than 0.2 Oxygen (O) MAX 0,012 Iron (Fe) as well as melting-related accompanying and impurity elements as a remainder, wherein the material is produced by a powder metallurgical process and after a thermal quenching to a hardness of 64 HRC has a bending work at room temperature of greater than 40.0 Joule (J) achieved.

The advantages achieved with the invention consist essentially in the fact that a material composition within narrow limits as well as a powder metallurgical production synergetically create the conditions for a cold work tool which has a desired property profile after thermal quenching.

In the chemical composition of the material, the activities of the alloying elements with respect to a microstructure in the quenched state and on required material properties are coordinated with each other kinetic kinetics.

The carbon content is aligned with the sum of the carbide formers in the alloy to both form carbides and to determine the hardenability and desired properties of the matrix. Concentrations of carbon of more than 0.6% by weight are required to achieve high hardness values of the matrix at the intended maximum contents of the carbide-forming elements during tempering, whereas contents of less than 1.0% by weight are important in order to obtain a to set the desired amount of carbide and carbide morphology.

The carbide-forming elements chromium (Cr), molybdenum (Mo), vanadium (V) and tungsten (W) are to be regarded as alloying together because their carbon activity, as it turned out, totals the composition of the austenite or cubic face-centered atomic structure at curing temperature and subsequently the matrix properties and the secondary Determine carbide precipitations after at least one annealing.

It is important that the vanadium content of the alloy in wt .-% is greater than 1.0, but less than 2.9, on the one hand to represent sufficient monocarbides and on the other hand enough secondary hardness potential. The secondary hardness potential must be seen with a residual vanadium and the contents of the elements molybdenum (Mo) and tungsten (W), because by concentrations in wt .-% of 3.8 molybdenum (Mo), and 3.4 tungsten (W ) and larger already causes a deterioration of the matrix toughness, whereas larger contents than 1.9 molybdenum (Mo) and 1.8 tungsten (W) are required for an advantageous vanadium masking in order to avoid large sharp-edged monocarbides.

For this interaction of the elements, it may also be important that the molybdenum content is at most 10% greater than that of tungsten (W).

The elements chromium (Cr), silicon (Si), manganese (Mn) and, to a lesser extent, nickel (Ni) and cobalt (Co) are important for the hardening and hardenability of the material.

Silicon contents of more than 0.3% by weight are required to ensure low levels of oxygen in the material. Less than 0.85% by weight of silicon should be included in the alloy to counteract a ferrite stabilizing effect and a reduction in the cure of the matrix by this element.

Manganese as an important, a required cooling rate in the curing of the article controlling element should, according to the invention have contents in the wt .-% of less than 1.5. However, since low manganese concentrations are also required for binding residual sulfur in the alloy, a minimum value of greater than 0.2% by weight must be provided.

To avoid martensite formation on cooling from a hardening temperature To influence undesirable, nickel contents in the material of less than 0.9 wt .-% can be adjusted.

Although cobalt is also effective with regard to the applicable tempering technology, according to the invention, this effect has been taken into account by alloying technology. To obtain a high hardness by solid solution strengthening of the material, a concentration in the matrix of more than 3.8 and less than 5.8 wt% of cobalt is meaningful. In the limits according to the invention, the kinetics and the size of secondary carbide precipitates are favorably influenced by cobalt with regard to the material properties. Very fine carbides representing the secondary hardness are formed and their coarsening tendency is reduced, whereby a significantly delayed softening of the quenched alloy by elevated temperatures takes place. Lower cobalt contents than 3.8% by weight lower the hardness and the fatigue strength of the material. Cobalt values of 5.8% by weight and higher, in turn, particularly reduce the toughness of the material.

It is known that aluminum can partially act as a substitution element for cobalt and increases the cutting performance of high speed steels. Due to a nitriding tendency as well as a simple atomization technology and a low nitrogen concentration in the metal of less than 0.2 wt%, the aluminum content in the alloy should be less than 0.045 wt%.

Lower oxygen concentrations of greater than 0.012 wt .-%, as was found, even in PM production, the mechanical properties of the composite material according to the invention.

By phosphorus contents of over 0.03 wt .-%, the manufacturability is deteriorated.

For achieving particularly advantageous mechanical material properties, in particular of high strength and ductility, powder-metallurgical production of the cold-work tool article is important in accordance with the invention. Due to an alloying effect caused by im Essentially round primary carbides with a small diameter and a high degree of purity with favorable microstructural formation of the material, it was possible to avoid a crack initiation usually caused by sharp-edged carbide and contaminant particles. In this way, with high material hardness, a high impact bending work of the material as well as a favorable fatigue strength of the steel object can be achieved in use.

The performance characteristics of a cold work tool article according to the invention can be further enhanced if one or more elements in the material are present in a concentration in wt% of: Carbon (C) more than 0.75 and less than 0.94 in particular more than 0.8 and less than 0.9 Silicon (Si) more than 0.35 and less than 0.7 in particular more than 0.4 and less than 0.65 Manganese (Mn) more than 0.25 and less than 0.9 in particular more than 0.3 and less than 0.5 Phosphorus (P) MAX 0.025 Sulfur (S) less than 0.34 in particular MAX 0.025 Chrome (Cr) more than 0.4 and less than 5.9 in particular more than 4.1 and less than 4.5 Molybdenum (Mo) more than 2.2 and less than 3.4 in particular more than 2.5 and less than 3.0 Nickel (Ni) less than 0.5 Vanadin (V) more than 1.5 and less than 2.6 in particular more than 1.8 and less than 2.4 Tungsten (W) more than 2.0 and less than 3.0 Copper (Cu) less than 0.45 in particular MAX 0.3 Cobalt (Co) more than 4.0 and less than 5.0 especially more than 4.2 and less than 4.8 Aluminum (Al) less than 0.065 in particular more than 0.01 and less than 0.05 Nitrogen (N) more than 0.01 and less than 0.1 in particular more than 0.05 and less than 0.08 Oxygen (O) MAX 0.01 in particular MAX 0.09

Of particular advantage for high toughness values and good fatigue properties of the article is when one or more impurity element (s) in the material have a concentration in wt .-% of: Tin (Sn) MAX 0.02 Antimony (Sb) MAX 0.022 Arsenic (As) MAX 0.03 Selenium (Se) MAX 0.012 Bismuth (bi) MAX 0.01

The purity and thus also the mechanical properties of the material, in particular the toughness, can be promoted if the powder metallurgy process atomizing the melt with high-purity nitrogen to metal powder having a powder grain size of at most 500 microns and subsequently substantially introducing the powder Prevention of oxygen access into a vessel and a hot isostatic pressing of the metal powder in the closed vessel to create a blank comprises.

For an economical production of a cold work tool article, but also because of the material properties, it may be favorable if the hot isostatically pressed blank is further processed by hot forming.

If, as may be provided, the cold work tool article has a pressure flow limit of more than 2700 MPa, measured at a hardness of 61 HRC, highly reliable extrusion dies with complex, delicate moldings can be produced, which exhibit low surface wear and the risk of cracking even in long-term operation ,

Of advantage for a hard embossing insert with shock-like load in long-term operation can be provided according to the invention that the cold work tool steel after thermal quenching to a hardness of 64 HRC an impact work at room temperature of greater than 80.0 Joule (J), preferably larger 100 joules (J), owns.

In the following, the invention will be explained on the basis of scientific tests and test results in comparison and conclusions.

To characterize the article according to the invention, the impact bending work at room temperature according to DIN 51222 of unnotched samples 7 x 10 x 55 mm was used, because such values allow the accurate assessment of the toughness behavior.

For a determination of the elongation at break and the plastic work from the static uniaxial tensile test, special tensile specimens with diameter-increasing enlarged clamping heads in spherical form were used, whereby the clamping device in the testing machine took into account the ball head geometry. Such investigations are reported in the literature (6th Intemational Tooling Conference, The Use of Tool Steels: Experience and Research, Karlstad University 10-13 September 2002, Material Behavior of Powder-Metallurgically Processed Tool Steels in Tensile and Bending Tests, pages 169-178). described.

The 0.2% compression limit of the material was determined in a pressure test according to DIN 50106 at room temperature.

Abrasion wear was tested with SiC abrasive paper P 120.

Materials testing uses different methods to characterize the strength and ductility of metallic materials. The most important experiment is the uniaxial tensile test. With this test, essential strength and ductility characteristics can be determined. In addition, this experiment allows statements about the solidification behavior of the materials under uniaxial Tensile stress.

In Fig. 1, the elongation at break of the material according to the invention in comparison with a HS-6-5-4 high speed steel is shown as a function of a set with a thermal hardness material hardness, wherein the test was carried out using the samples described above.

The elongation at break of the alloy according to the invention is higher than that of the comparative steel in the entire hardness range of the materials and, in particular in the upper hardness range from 58 HRC to 62 HRC, has an elongation at break 4 times higher.

The property combination of high strength and high ductility of the material according to the invention, which is advantageous over the prior art, is particularly significant in comparison with the plastic work which is determined from the static uniaxial tensile test. At substantially the same temper, the material according to the invention at room temperature was determined to be about 20% higher plastic work in the tensile test at a material hardness of 63 HRC. With a material hardness of 61.5 HRC, an increase in plastic work of about 50% was found, using the powder-metallurgically produced high-speed steels HS-10-2-5-8-PM and HS-6-5-3-PM as reference material ,

In addition to the outstanding property combination of strength and ductility shown above, the alloy according to the invention has a very good abrasive wear resistance, which was determined in the SiC abrasive paper test. This property was achieved despite a reduced primary carbide content compared to standard PM alloys used in this field of application.

The mean wear value for the specified alloys is 7g ^-1 at a hardness of 61 HRC.

Claims (6)

1. Thermally hardened cold-worked steel object comprising a chemical composition of the material in % by weight of: carbon (C) more than 0.6 and less than 1.0 silicon (Si) more than 0.3 and less than 0.85 manganese (Mn) more than 0.2 and less than 1.5 phosphorus (P) 0.03 in maximum sulphur (S) less than 0.5 chromium (Cr) more than 4.0 and less than 6.2 molybdenum (Mo) more than 1.9 and less than 3.8 nickel (Ni) less than 0.9 vanadium (V) more than 1.0 and less than 2.9 tungsten (W) more than 1.8 and less than 3.4 copper (Cu) less than 0.7 cobalt (Co) more than 3.8 and less than 5.8 aluminium (Al) less than 0.045 nitrogen (N) less than 0.2 oxygen (O) 0.012 in maximum, and optionally tin (Sn) 0.02 in maximum antimony (Sb) 0.022 in maximum arsenic (As) 0.03 in maximum selenium (Se) 0.012 in maximum bismuth (Bi) 0.01 in maximum, the rest being iron (Fe)
   as well as smelting conditioned accompanying and impurity elements, the material being produced according to a powder metallurgical process and has, after thermal hardening up to a hardness of 64 HRC, an impact flexure energy at ambient temperature of more than 40.0 Joule (J).
2. Cold-worked steel object according to claim 1, wherein one or more element(s) in the material is (are) present in a concentration in % by weight of: carbon (C) more than 0.75 and less than 0.94 particularly more than 0.8 and less than 0.9 silicon (Si) more than 0.35 and less than 0.7 particularly more than 0.4 and less than 0.65 manganese (Mn) more than 0.25 and less than 0.9 particularly more than 0.3 and less than 0.5 phosphorus (P) 0.025 in maximum sulphur (S) less than 0.34 particularly 0.025 in maximum chromium (Cr) more than 0.4 and less than 5.9 particularly more than 4.1 and less than 4.5 molybdenum (Mo) more than 2.2 and less than 3.4 particularly more than 2.5 and less than 3.0 nickel (Ni) less than 0.5 vanadium (V) more than 1.5 and less than 2.6 particularly more than 1.8 and less than 2.4 tungsten (W) more than 2.0 and less than 3.0 copper (Cu) less than 0.45 particularly 0.3 in maximum aluminium (Al) less than 0.065 particularly more than 0.01 and less than 0.05 nitrogen (N) more than 0.01 and less than 0.1 particularly more than 0.05 and less than 0.08 oxygen (O) 0.01 in maximum particularly 0.009 in maximum.
3. Cold-worked steel object according to claim 1 or 2, wherein the metallurgical process comprises atomising the melt with nitrogen to a metal powder having a powder grain size of 500 µm in maximum, and subsequently substantially introducing the powder, while avoiding the access of oxygen, into a receptacle, and high-temperature isostatic compressing the metal powder in the closed receptacle to produce a blank.
4. Cold-worked steel object according to any of claims 1 to 3, wherein the high-temperature isostatically compressed blank is further processed by hot deformation.
5. Cold-worked steel object according to any of claims 1 to 4, which has a pressure yield point of more than 2700 MPa, when measured at a hardness of 61 HRC.
6. Cold-worked steel object according to any of claims 1 to 5, which has an impact flexure energy at ambient temperature of more than 80.0 Joule (J), preferably more than 100 Joule (J).

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