Method for Manufacturing a Tool Steel as a Support for PVD Coatings and a Tool Steel

Abstract

A tool steel as well as a method for manufacturing a tool steel for cold-work and/or high-speed-work applications, in particular as an intermediate product for manufacturing cold-work and/or high-speed-work tools with a PVD coating, consisting of the following alloying elements: (all amounts expressed in wt %): C=0.55 to 0.75 Si=0.70 to 1.00 Mn=0.20 to 0.50 Cr=4.00 to 5.00 Mo=1.80 to 3.50 V=0.80 to 1.50 W=1.80 to 3.00 Co=3.00 to 5.00 N=0.02 to 0.10 and optionally one or more of Ni?1.5 Cu?1.0 Ti?1.5 Nb?1.5 Ta?1.5 Hf?1.5 Zr?1.5 Al?1.5 B?0.8 S?0.35 P?0.35 and residual iron and inevitable smelting-related impurities.

Claims

1. A method for manufacturing a tool steel for cold-work and/or high-speed-work applications, comprising melting and processing into a powder by atomization a steel material consisting of the following alloy elements: (all amounts expressed in wt %): C=0.55 to 0.75 Si=0.70 to 1.00 Mn=0.20 to 0.50 Cr=4.00 to 5.00 Mo=1.80 to 3.50 V=0.80 to 1.50 W=1.80 to 3.00 Co=3.00 to 5.00 N=0.02 to 0.10 and optionally one or more of Ni?1.5 Cu?1.0 Ti?1.5 Nb?1.5 Ta?1.5 Hf?1.5 Zr?1.5 Al?1.5 B?0.8 S?0.35 P?0.35 and residual iron and inevitable smelting-related impurities, the powder is then hot isostatic pressed, and the steel material thus produced is optionally subjected to a hot forming, wherein the steel material is subjected to heat treatment, the heat treatment being carried out in such a way that the steel material is first heated to a hardening temperature of 1100? C. to 1180? C., then kept at this hardening temperature for at most 2 to 20 minutes, and then cooled to a temperature ?60? C. at a cooling rate of ??3 for hardening purposes, and then tempered, wherein the tempering treatment comprises at least two cycles in which the steel material is heated to a temperature of 530? C. to 560? C., kept at this temperature of 530? C. to 560? C. for at least 1.5 hours, and then cooled to a temperature ?60? C.

2. The method according to claim 1, wherein the steel material, which contains at least one or more or all of the element(s) with the following concentration value(s) (all amounts expressed in wt %): C=0.58 to 0.68 Si=0.70 to 0.94 Mn=0.20 to 0.40 Cr=4.10 to 4.70 Mo=2.00 to 3.20 V=0.90 to 1.25 W=2.00 to 2.70 Co=3.50 to 4.30 N=0.03 to 0.08 is melted.

3. The method according to claim 1, wherein the steel material is heated to a hardening temperature selected from the group consisting of 1180? C., 1160? C., or 1100? C. and for a duration selected from the group consisting of at most 2 minutes, at most 3 minutes, or at most 20 minutes, and then cooled to a temperature ?60? C. for hardening purposes.

4. The method according to claim 1, wherein the steel material is tempered, wherein the tempering treatment is carried out at a temperature selected from the group consisting of 530? C., 550? C., or 560? C. for a duration selected from the group consisting of at least 1.5, 2, 2.5, 3, or 3.5 hours, wherein at least two tempering cycles are performed and the steel material is cooled to a temperature of ?60? C. after each tempering cycle.

5. The method according to claim 1, wherein the steel material is cooled to a temperature of ?30? C. after being heated to the hardening temperature and/or after each tempering step.

6. The method according to claim 1, wherein the heat treatment produces a steel material that has a compressive strength, measured as an offset yield point Rp0.2, of ?2700 MPa.

7. A tool steel for cold-work and/or high-speed-work applications, produced using the method according to claim 1, wherein the steel material consists of the following alloy elements (all amounts expressed in wt %): C=0.55 to 0.75 Si=0.70 to 1.00 Mn=0.20 to 0.60 Cr=4.00 to 5.00 Mo=1.80 to 3.50 V=0.80 to 1.50 W=1.80 to 3.00 Co=3.00 to 5.00 N=0.02 to 0.10 and optionally one or more of Ni?1.5 Cu?1.0 Ti?1.5 Nb?1.5 Ta?1.5 Hf?1.5 Zr?1.5 Al?1.5 B?0.8 S?0.35 P?0.35 and residual iron and inevitable smelting-related impurities.

8. The tool steel according to claim 7, wherein the steel material contains at least one or more or all of the element(s) with the following concentration value(s) (all amounts expressed in wt %): C=0.58 to 0.68 Si=0.70 to 0.94 Mn=0.20 to 0.40 Cr=4.10 to 4.70 Mo=2.00 to 3.20 V=0.90 to 1.25 W=2.00 to 2.70 Co=3.50 to 4.30 N=0.03 to 0.08.

9. The tool steel according to claim 7, wherein the carbon content in the steel alloy has an upper limit of 0.68 wt %, and a lower limit of 0.58 wt %.

10. The tool steel according to claim 7, wherein the vanadium content in the steel alloy has an upper limit of 1.25 wt %, and a lower limit of 0.90 wt %.

11. The tool steel according to claim 7, wherein the cobalt content in the steel alloy has an upper limit of 4.30 wt %, and a lower limit of 3.50 wt %.

12. The tool steel according to claim 7, wherein the steel material has a steel matrix comprising MC and M.sub.6C carbides to increase a compressive strength, wherein the MC carbides have an average diameter of 0.6 ?m and the M.sub.6C carbides have an average diameter of 0.9 ?m.

13. The tool steel according to claim 7, wherein the steel material comprises a steel matrix, and wherein a carbide density in the steel matrix is at most 27538 particles/mm.sup.2 for M.sub.6C carbides and at most 39845 particles/mm.sup.2 for MC carbides.

14. The tool steel according to claim 12, wherein the M.sub.6C carbides have an area fraction of at most 1.9% and the MC carbides have an area fraction of at most 1.3%.

15. The tool steel according to claim 7, wherein the steel material has a hardness of at least 62 HRC.

16. The tool steel according to claim 7, wherein the steel material has a toughness, measured as impact bending work at room temperature, of at least 73 J.

17. The tool steel according to claim 7, wherein the steel material has a compressive strength, measured as an offset yield point Rp0.2, of ?2700 MPa.

18. The tool steel according to claim 7, wherein the steel material satisfies the following formula: $0.005?0.8\left[\mathrm{Nb}\right]+\left[\mathrm{Ti}\right]+\left[\mathrm{Al}\right]?0.18$ where [Nb], [Ti], and [Al] represent the contents of Nb, Ti, and Al, respectively, in wt %.

19. The tool steel according to claim 7, wherein the steel material satisfies the following ratio: $0.5?\left[C\right]/\left[V\right]?0.6$ where [C] and [V] represent the contents of C and V in wt %.

20. The tool steel according to claim 7, wherein the steel material satisfies the following ratio: $8.2?\left(\mathrm{VM}*\mathrm{HM}\right)/\left(\mathrm{VS}*\mathrm{HS}\right)=13.5$ where VM is a volume fraction of the matrix, HM is a hardness of the matrix in HV (Vickers hardness), VS is a volume fraction of secondary carbides, and HS is a hardness of the secondary carbides.

21. A method of using the tool steel according to claim 7, comprising using the tool steel as a support for a PVD coating.

22. A method of using the tool steel according to claim 7, comprising using the tool steel for a stamping or fine blanking tool.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0141] The invention will be explained by way of example with the aid of the drawings. In the drawings:

[0142] FIG. 1: shows the possible steel compositions according to the invention;

[0143] FIG. 2: is a comparison table showing two known steel materials and the material according to the invention;

[0144] FIG. 3: is a very schematic depiction of a manufacturing route; the powder metallurgy PM route is according to the invention;

[0145] FIG. 4: shows a thermodynamic stability calculation for different carbide phases;

[0146] FIG. 5: shows SEM images of a cross-section of a steel material hardened at 1150? C. according to the invention;

[0147] FIG. 6: shows another SEM image of a cross-section showing the M.sub.6C and MC carbides of the material;

[0148] FIG. 7: shows a heat treatment according to the invention;

[0149] FIG. 8: shows the fractions of MC and M.sub.6C carbides at different hardening temperatures;

[0150] FIG. 9: shows the carbide contents at a hardening temperature of 1150? C.;

[0151] FIG. 10: shows the size distribution of M.sub.6C carbides;

[0152] FIG. 11: shows the size distribution of MC carbides;

[0153] FIG. 12: shows the hardness and impact bending work (IB) as a function of tempering temperature (not according to the invention) for a hardening temperature of 1030? C.:

[0154] FIG. 13: shows the hardness and impact bending work (IB) as a function of tempering temperature (not according to the invention) for a hardening temperature of 1070? C.;

[0155] FIG. 14: shows the hardness and impact bending work (IB) as a function of tempering temperature (according to the invention) for a hardening temperature of 1150? C.;

[0156] FIG. 15: shows the results for compressive strength;

[0157] FIG. 16: shows examples of steel compositions according to the invention, heat treatments, and the resulting hardness, toughness, and compressive strength;

[0158] FIG. 17: shows examples of steel compositions not according to the invention;

[0159] FIG. 18: shows a sample heat treatment consisting of hardening and tempering.

DETAILED DESCRIPTION OF THE INVENTION

[0160] FIG. 1 shows the analysis range within which the invention can be implemented and the effects according to the invention are achieved.

[0161] FIG. 2 shows the composition of the steel material according to the invention, which is in the range of the composition according to FIG. 1 and shows one embodiment of the steel material. Two other embodiments, namely REF 1 (EP3050986) and REF 2 (EP1469094), are compared to this steel material in which the silicon, molybdenum and cobalt contents are significantly increased compared with the known embodiments and the nickel content in particular differs considerably and in particular, is reduced.

[0162] Compared to known alloys in the prior art, a very narrow selection is pursued, which reliably ensures the effects according to the invention, particularly with the heat treatment according to the invention.

[0163] FIG. 3 shows a conventional melting metallurgy manufacturing route (not according to the invention), the possible powder metallurgy manufacturing route for producing the powder (according to the invention), and corresponding articles produced thereby.

[0164] After the feedstock is melted and the desired composition is set, a corresponding steel melt is atomized, particularly with nitrogen or other inert gases, to form a powder. If necessary, this powder is graded by screening or sieving, and the graded powder is then assembled into a desired grain band, dispensed into a corresponding capsule, which is welded and then compacted by hot isostatic pressing. Correspondingly, a material that is transformed in this way can then be fed into the hot forming process.

[0165] In particular, the dense and homogeneous material produced by the hot isostatic pressing can be rolled or forged to the required dimensions in a forming process. The thickness after the hot rolling can, for example, be 60 mm, which corresponds to a forming degree of 7 times the diameter reduction.

[0166] Segregation occurs in steel materials that contain segregation-active elements and are produced by the conventional casting process. In the segregation zones, there is often an imbalance of element concentrations. This can result in the formation of primary carbides even though primary carbide formation would not be expected based on the alloy composition in thermodynamic equilibrium. The powder metallurgy manufacturing route has the advantage of hindering the occurrence of segregation zones and thus the formation of primary carbides.

[0167] The production parameters during atomization of the molten steel have a significant influence on the powder grain size and thus on the carbide grain size. Fine adjustment of the setting parameters of temperature and pressures is also necessary in the HIP process to prevent carbide growth or the formation of carbide clusters. Especially in the case of such high-alloy steels as in the subject matter of the invention, high carbide contents are often present. Carbides have a positive effect on compressive strength and on hardness in general. But when it comes to toughness, compressive strength, and fatigue strength, carbides constitute imperfections that limit these properties. In this respect, it is particularly important to have small, round carbides that are homogeneously distributed over the cross-section. Due to the high number of carbides, it often happens with such high-alloy steels that the carbides conglomerate during the conventional casting process, which can severely limit the toughness and fatigue strength and, subsequently, also the service life of the tool that is produced from it. In the present subject matter according to the invention, fine singular carbides are present.

[0168] In particular, these are the so-called secondary hardening carbides of the MC and M.sub.6C types, which are produced from the solid phase during a tempering treatment. The secondary carbides usually have a smaller particle size compared to primary carbides precipitated from the melt.

[0169] A thermodynamic stability calculation using Thermo-Calc for various carbide phases is shown in FIG. 4. The calculation shows which carbide phases are in equilibrium or are thermodynamically stable at a certain temperature. This is necessary for establishing the hardening temperature at which sufficient solubility of the carbide phases is present. Carbides of the M.sub.23C.sub.6 and M.sub.7C.sub.3 types dissolve completely in the matrix during hardening; carbides of MC and M.sub.6C types dissolve to a large extent but not completely. Complete solubility of the carbides is not desired, however, so the maximum hardening temperature is limited to 1180? C. A certain amount of carbides should be retained in the structure during hardening to prevent a coarsening of the grains. This can be explained by the fact that carbides function like growth retardants and slow the unwanted grain growth.

[0170] It is clear that most carbide phases are not thermodynamically stable at 1100? C., particularly at 1150? C., and decompose. But if the temperature falls below 1100? C., too few alloying elements are dissolved in the matrix. This leads to a reduced hardness level. In addition, a hardness temperature that is too low results in an increased carbide content in the alloy composition according to the invention. In other words, at a lower hardening temperature, the secondary carbide volume is higher because fewer secondary carbides dissolve in the matrix. Higher temperatures and longer holding times result in a lower carbide volume. In the temperature range around 1100? C., the holding time is therefore 20 min.

[0171] The maximum hardening temperature at which the effects according to the invention can still be achieved is 1180? C. If the temperature is exceeded, then more carbon and carbide formers are dissolved in the matrix. This increases the hardness of the steel material, but leads to a significant reduction in toughness. In this connection, it is particularly important to adhere to the holding time, which must not exceed 2 min in the temperature range around 1180? C. Longer holding times increase the carbide growth at this temperature.

[0172] It has turned out that the temperature range around 1140-1160? C. is particularly advantageous and results in a balanced combination of hardness and toughness properties. The optimum holding time here is at most 3 min.

[0173] The lower limit for the hardening temperature according to the invention is therefore 1100? C., in particular 1150? C. The upper temperature limit at which the effects according to the invention can also be achieved is 1180? C.

[0174] This is clearly evident in FIG. 5. A steel surface hardened at 1150? C. and then heat-treated according to the invention exhibits fine, singular, finely distributed carbides. No segregations, carbide agglomerates, or inhomogeneities are discernible in the structure.

[0175] With the alloy composition according to the invention, particularly in combination with the heat treatment according to the invention, the carbide phase distribution is especially homogeneous (FIG. 6). The carbides, particularly of the MC and M.sub.6C types, are round and uniformly distributed in the steel matrix. No carbide conglomerates are present. No large primary carbides are present either.

[0176] A finely tuned heat treatment has significant influence on the size, the homogeneous distribution, and finally the area fraction of the carbides. Since the secondary carbides are precipitated from the solid phase, a fine adjustment of the holding time and subsequent tempering treatment to match the respective hardening temperature is required.

[0177] The temperature range between 530? C. and 560? C. has proved to be particularly advantageous for a tempering treatment in the case of the alloy composition according to the invention. But if the temperature of 560? C. is exceeded, the hardness level is reduced too much. If the temperature falls below 530? C., then the toughness is significantly reduced. In addition, this leads to an increased proportion of retained austenite, which cannot be completely eliminated even after a three-stage tempering treatment. Consequently, the upper limit for the tempering treatment is 560? C. and the lower limit is 530? C.

[0178] FIG. 7 shows hardening and tempering treatments according to the invention. In one embodiment according to the invention, the steel material and/or the tool made from it is hardened at a temperature of 1180? C. for at most 2 min and then rapidly cooled to ?30? C. at a cooling rate of ??3 (FIG. 7). Here, ? values are used to define cooling rates and denote the time required to cool a steel from 800? C. to 500? C., in units of hectoseconds. Thus, ?=3 means that cooling from 800 to 500? C. takes about 3 hs=300 s=5 min.

[0179] It is essential to keep the temperature below the 30? C. limit since this reduces the amount of retained austenite. Any remaining retained austenite can severely harm the mechanical properties. It can also result in tool failure. This can be explained by a structural transformation during operation, which is accompanied by a change in volume and dimensions. To prevent this, the steel is tempered two or three times at 560? C. for 120 min each time. After each tempering cycle, the steel material is preferably cooled to ?30? C.

[0180] In the course of this, the proportion of retained austenite is significantly reduced after each hardening and tempering cycle composed of heating, holding, and cooling. Depending on the desired minimum retained austenite content, up to three tempering cycles can be provided since with each additional tempering cycle, an additional percentage of the retained austenite flips over into the desired martensite. The lowest possible proportion of retained austenite is so advantageous because it transforms when subjected to a load and because the corresponding part, e.g. a punch, can then be susceptible to brittle fracture.

[0181] To ensure this transformation of the retained austenite into martensite, cooling to ?60? C., preferably ?30? C., is required after each hardening cycle and advantageously after each tempering cycle.

[0182] In a further embodiment, the steel material and/or the tool made from it is hardened at 1160? C. for at most 3 min. It is then cooled to ?30? C. during which ? values?3 are maintained. After the cooling, the steel material is tempered two or three times at 560? C. for 120 min each time. After each tempering cycle, the steel material is preferably cooled to ?30? C.

[0183] In a particularly advantageous embodiment, the steel and/or the tool made from it is hardened at a temperature of 1150? C. for at most 3 min and then cooled to ?30? C. Then the steel is tempered two or three times at 530? C. for 120 min each time. After each tempering cycle, the steel material is preferably cooled to ?30? C.

[0184] Advantageously, the steel and/or the tool made from it is hardened at 1140? C. for at most 3 min. It is then cooled to ?30? C. and tempered two or three times at 530? C. for 120 min each time. After each tempering cycle, the steel is preferably cooled to ?30? C.

[0185] It is also advantageous if the steel material and/or the tool made from it is hardened at 1100? C. for at most 20 min and then cooled to ?30? C. The steel material is then subjected to a tempering treatment of two or three tempering cycles at 530? C. for 120 min each time. After each tempering cycle, the steel is preferably cooled to ?30? C.

[0186] The tempering treatment according to the invention provides for the tempering to be carried out immediately after the hardening for at least 2 hours for each tempering cycle, with the furnace being set to the tempering temperature as the set point. Direct heating to this set point is carried out, this being done in a nitrogen atmosphere. In each cycle, heating to the set point temperature is performed for 2 hours and then the heating is switched off while the nitrogen atmosphere remains. The final temperature is below 30? C. and when it is reached the next cycle is started. Two or three tempering cycles are performed. It is of course possible to carry out each tempering cycle differently with regard to the tempering temperature or heating and cooling rates, but it can be perfectly advantageous to carry out each tempering cycle in an identical fashion.

[0187] The resulting carbide content varies depending on the heat treatment and in particular on the hardening temperature used because in the course of this, elements are dissolved that are required for the formation of the secondary hardening carbides that are produced later. But it is advantageous if a certain amount of secondary carbides is retained in the structure. This slows down grain growth and thus hinders a coarsening of the grains.

[0188] The invention will be further explained based on an example:

[0189] Three specimens of an alloy according to the invention are hardened at 1070? C. (FIG. 13, not according to the invention), 1150? C. (FIG. 14, according to the invention), and 1030? C. (FIG. 12, not according to the invention), quenched with ?=0.35, and tempered at 530? C. (according to the invention), 560? C. (according to the invention), or 590? C. (not according to the invention). The hardening not according to the invention results in lower hardness levels, and tempering not according to the invention results in reduced hardness and toughness. Then the area fraction of MC and M.sub.6C carbides is determined (FIG. 8). The specimens for which the hardening temperature according to the invention is not maintained contain a higher carbide content while the specimen hardened at 1150? C. has the lowest carbide content.

[0190] The specimen hardened at 1070? C. contains 1.59% MC carbides and 2.62% M.sub.6C carbides. Hardening at 1030? C. results in 1.51% MC carbides and 3.43% M.sub.6C carbides. The lowest carbide content is obtained for the specimen hardened at 1150? C. and correspondingly results in 1.33% for MC carbides and 2.45% for M.sub.6C carbides. The results demonstrate that the desired low carbide content can only be achieved in the narrow temperature window according to the invention. The carbide content is expressed as an area fraction.

[0191] According to the invention, the vanadium-rich MC carbides have a maximum size of 1.5 ?m and the tungsten-rich and molybdenum-rich M.sub.6C carbides have a maximum size of 2.1 ?m. The average diameter of the small MC carbides is 0.6 ?m, while the average diameter of the larger M.sub.6C carbides is 0.9 ?m (FIG. 9). The size distribution of the MC carbides and M.sub.6C carbides is shown in FIGS. 10 and 11. The carbide size is expressed as ECD (equivalent circle diameter).

[0192] It is advantageous if the carbide density in the matrix is at most 27538 particles/mm.sup.2 for M.sub.6C carbides and at most 39845 particles/mm.sup.2 for MC carbides. Accordingly, it is advantageous if the average area fraction of the large M.sub.6C carbides is at most 1.9% and the average area fraction of the small MC carbides at most 1.3%.

[0193] The alloy composition according to the invention and the heat treatment fine-tuned to it are used to create a steel material and/or a tool made from it, which has a high compressive strength. Offset yield points Rp0.2 of more than 2950 MPa can be achieved at a hardness level of 64 to 65 HRC. FIG. 15 shows the results of the uniaxial compression test with the modulus of elasticity (E), the 0.05% offset yield point at 0.05% deformation (Rp0.05), the 0.01% offset yield point at 0.1% deformation (Rp0.1), and the offset yield point at 0.2% deformation (Rp0.2). The specimens are measured at room temperature with a test speed of 0.00025 RPS. The specimens are of the LCF type with a shortened shank and have a diameter of 9 mm and an initial gauge length (L.sub.0) of 12 mm (measured with an Instron 8854 servo-hydraulic testing machine with a 250 kN load cell).

[0194] FIG. 16 shows various powder metallurgy-produced steels according to the invention, heat treatments and resulting hardness in HRC, toughness in the form of impact bending energy (IB) in joules, and offset yield point Rp0.2 in MPa

[0195] FIG. 17 shows various steel compositions not according to the invention which have been processed with a heat treatment consisting of hardening and tempering and the resulting hardness, toughness, and offset yield point.

[0196] FIG. 18 shows an exemplary heat treatment consisting of hardening and 3 tempering cycles. In this embodiment, before the hardening temperature of 1150? C. is reached, two holding points are established, the first at 690? C. and the second at 850? C. These ensure that the steel material is heated through.

[0197] With the alloy according to the invention and the heat treatment finely tuned to it, a 15 to 20% increase in fatigue strength can be expected compared with other powder metallurgy-produced cold-work steels of identical hardness (62-65 HRC, fatigue strength approx. 950 to 1050 MPa)

[0198] The alloy composition and heat treatment according to the invention succeed in creating a steel material with an outstanding combination of hardness and toughness. The material according to the invention possesses exceptionally good toughness at a very high hardness so that it has been possible to successfully reconcile two competing mechanical properties.

[0199] With the invention, it is advantageous that the hardness-toughness advantage can be achieved, particularly at the specified hardening temperature of approximately 1150? C., if the specified heat treatment cycle is adhered to. At the above-mentioned hardening temperature, a hardness of 65 HRC and toughness of 73 J can be achieved. It is true that even slight deviations in the hardening temperature downward or upward cannot be ruled out, but the significant hardness-toughness advantages compared to the prior art are no longer assured to the same extent. At temperatures above 1180? C., there is a risk that instances of initial melting can already occur in the material, which is also undesirable.

[0200] With the invention, it is advantageous that the method according to the invention makes it possible to very reliably achieve mechanical properties that were previously incompatible with one another in this form. In particular, very high hardness values of over 62 HRC are achieved with toughnesses of 70-90 J or more (measured as impact bending work at room temperature in accordance with SEP 1314), which were previously not reliably achievable in this range with these materials in this form. For this purpose, it is necessary to reliably adhere to this narrow selection.

[0201] In addition, a high compressive strength, which is measured as the offset yield point Rp0.2, of over 2700 MPa is achieved at a hardness level of 62-65 HRC. Such a steel material is outstandingly suitable as a support material for PVD coatings, in particular hard coatings, and for the manufacture of high-strength tools, especially stamping and fine blanking tools.