EP0814172A1 - Powder metallurgy hot-work tool steel, and process for its manufacture - Google Patents

Abstract

A powder metallurgically produced hot work steel has the composition (by wt.) 0.25-0.45% C, 2.40-4.25% Cr, 2.50-4.40% Mo, 0.20-0.95% V, 2.10-3.90% Co, 0.10-0.80% Si, 0.15-0.65% Mn, balance Fe and impurities. Preferably, the steel has a purity K1 of less than 10 mu m. Also claimed is powder metallurgical production of the above hot work steel by (a) atomising a steel melt of the above composition in a high purity nitrogen atmosphere; (b) filling the resulting powder into capsules designed such that the final product achieves its intended shape with maximum material yield; (c) shaking the filled capsules to achieve maximum filling density; (d) evacuating and then gas-tightly sealing the capsules; and (e) hot isostatically pressing the capsules at 1000-1230 degrees C and 0.8-3.5 (preferably 1) kbar pressure for ≥ 3 hrs.

Description

State of the art

It has long been known that powder metallurgy steels have properties which, with an identical chemical composition, are superior to the properties of melt metallurgy steels. In particular, steels manufactured using powder metallurgy are distinguished by the fact that they have the same structural state in all dimensional ranges over their entire cross section. The mechanical properties are therefore essentially the same over the entire cross section.

It is also already known to produce the hot work steel X40CrMoV51 by powder metallurgy by hot isostatic pressing. In this regard, it can be seen from the Archives for the Ironworks 55 (1984), pages 169-176, that the hot-work steel mentioned carbon of 0.37-0.41%, silicon of 1.0-1.07%, manganese of 0.38 - 0.42%, chromium from 5.3 - 5.5%, molybdenum from 1.37 - 1.41%, vanadium from 1.0 - 1.27 as well as negligible nitrogen, oxygen, sulfur and phosphorus contents .

A powder of the above composition prepared by nitrogen atomization from the melt is compacted in steel capsules which are evacuated to a vacuum of less than 10 ^-4 mbar before sealing. The compression takes place at temperatures of 1075 - 1225 ° C.

The above-mentioned powder-metallurgy hot-work steel has a satisfactory hardness, but because of its insufficient hot hardness, temper resistance and its tendency to show temperature change cracks, it is not suitable for highly stressed hot-work tools, such as for press mandrels, press dies and block receptacles for metal pipe and extrusion presses for hot extrusion tools, tools for hollow body production, tools for the screw, nut, rivet and bolt products, die casting tools, molding dies, die inserts and warm shear blades. In short, the stability of the steel in question cannot be satisfactory in the case of highly stressed hot work tools.

The invention is therefore based on the object of creating a powder-metallurgically produced hot-working steel which, in addition to having a sufficient toughness, has a high hot hardness and, in particular, good resistance to the occurrence of temperature change cracks. In particular, an object of the invention is to provide a powder-metallurgy hot-work steel which is particularly suitable for use in extrusion, in particular for mandrels, press dies and block receptacles, and also for use in forging presses and die-casting molds, especially in the case of large dimensions , suitable is.

The invention is also based on the object of specifying a method for producing an improved hot-work steel produced by powder metallurgy.

With regard to the steel to be created, this object is achieved by the subject matter of claim 1. With regard to the method to be created, this object is achieved by the subject matter of claim 5.

Furthermore, the invention relates to the use of the powder-metallurgically produced hot-work steel as a material for the production of press mandrels, press dies and block receptacles for the extrusion and for the production of forging presses and die casting molds.

The technical progress achievable with the aid of the invention results primarily from the fact that, as a result of the cobalt-containing composition according to the invention, synergistically enhanced by the special compression according to the invention, a powder metallurgy hot work steel is made available which has essentially as good hot toughness properties as a known cobalt-free hot work steel, but additionally has high hot hardness, tempering and hot fire crack resistance values.

There are lively concerns among experts about the inclusion of cobalt in hot-work steel. In particular, there is a notion among experts that by adding cobalt, the toughness properties, in particular the hot toughness properties, of a hot-work steel produced by powder metallurgy can in no way be maintained or even improved.

Preferred embodiments and further refinements of the invention are specified in the subclaims.

Both in the conventional powder metallurgical production and in the production according to the invention, the raw material is precious scrap and ferro alloys. However, while the prior art avoids cobalt in hot-work steel produced by powder metallurgy, cobalt contents are provided according to the invention. Both in the prior art and in accordance with the invention, the melting of the starting alloys preferably takes place in the induction furnace.

To produce the steels according to the invention, induction heat and precise temperature control are used until the slag content is correct. The mixture is then sprayed under a protective gas atmosphere (preferably high-purity nitrogen). The APM Calidus system has proven to be particularly suitable for this, since it helps to avoid inclusions in the powder produced.

Efforts have become known in the prior art to achieve a high degree of purity in the melt by heating the melt through a slag cover with the aid of electrodes.

In the conventional method, the melt is sprayed directly into the capsule to be compressed, which increases the risk of undesired inclusions.

In the production of the steel according to the invention, the alloy powder obtained is filled into capsules which are designed in such a way that the end product obtains its intended shape with the greatest possible material yield. Capsules are therefore used according to the invention, which should at least largely give the product to be manufactured the desired shape.

After filling, the capsules are shaken to achieve the greatest possible filling density. The capsules filled in this way are then pumped air-free and then sealed gas-tight.

In the conventional method, as already mentioned, atomization is carried out directly in capsules, which are then sealed in a gas-tight manner. The state of the art basically knows only a single standard capsule size with a diameter of 465 mm and a length of 1600 mm.

In the conventional method, the capsule pretreated, as mentioned above, is cold isostatically pressed at a pressure of approximately 3.5 Kbar in order to improve the thermal conductivity of the batch of powder contained in the capsule.

Such a cold pressing is not required for the production of the hot-work steel according to the invention, since the powder batch already has such a high filling density as a result of the shaking that the desired thermal conductivity properties are given in the powder batch.

In the conventional method, the capsules as described above are heated in a preheating oven to the temperature of the isostatic hot pressing (HIP temperature) without overpressure and then transported to the hot pressing system. There Even after conventional cold pressing, the thermal conductivity of the powder batch is only low, a steep temperature gradient arises in the powder batch at the beginning of the preheating treatment, which leads to segregations of oxygen, sulfur and carbon. These segregations are of considerable magnitude, which can be demonstrated by deep etching or chemical analysis. Furthermore, the steep temperature gradient leads to a certain carbide growth.

In the production of the hot-work steels according to the invention, the capsules are not preheated and, as already mentioned, there is also no cold pressing.

In the production according to the invention, the capsules are heated with simultaneous pressurization. In particular, in a first step, pressure is applied at about 200 bar using compressed argon. The heating is then carried out in the HIP system, the pressure of the compressors supplying the compressed argon being kept essentially constant. As the temperature rises, the pressure increases continuously without the pressure of the argon compressors having to be increased. The powder batch is compressed under pressure at a relatively low temperature before oxygen, sulfur and carbon transport occurs. Consequently, the hot-work steel according to the invention is free from segregation.

If the intended HIP pressure is reached, a further increase in pressure and temperature is prevented by suitable regulating and control measures.

The HIP temperature is 1000 to 1230 ° C, with a temperature of 1150 ° C being preferred. The HIP pressure is 0.8 to 3.5 kbar, although a HIP pressure of 1 kbar has currently proven to be extremely advantageous. At pressures of less than 0.8 Kbar, there is no sufficient compression of the material and, in particular, the risk that gas inclusions are retained in residual pores. HIP pressures of more than 3.5 kbar are possible with modern HIP systems, but do not lead to an increase in quality that justifies the effort.

In the steel production according to the invention, the holding time at the desired HIP temperature and at the desired HIP pressure is at least 3 hours. This time period applies to small dimensions to be manufactured. Larger dimensions to be manufactured require longer compression times. Conventional processes usually work with a holding time of just one hour. Since the filled capsules are simultaneously exposed to high temperatures and high pressures in the process according to the invention, a homogeneous, high-density material is obtained as a result.

The hot-work steel produced in a conventional manner by powder metallurgy requires final forging or rolling treatments. Such processing measures, which are carried out in the heat, lead to an undesirable carbide growth and also to an undesirable rounding of the carbides.

In contrast to the prior art, the hot work steel composed according to the invention and manufactured according to the invention is used in the hipped state, i.e. in the state in which it was released from the capsule after pressing. For economic reasons, however, round material according to the invention with diameters of less than 60 mm and flat material with a cross-sectional ratio is flat rolled or forged.

As far as quality control is concerned, it should be mentioned that, in the conventional method, a check, for example for inclusions, takes place only after the powder batch has been removed from the deformed capsule. In contrast, the steel material according to the invention is subjected to a critical quality control even in the powder state.

Kohlenstoff:

0,25 - 0,45

Chrom:

2,40 - 4,25

Molybdän:

2,50 - 4,40

Vanadium:

0,20 - 0,95

Kobalt:

2,10 - 3,90

Silicium:

0,10 - 0,80

Mangan:

0,15 - 0,65

The hot-work steel produced by powder metallurgy according to the invention has the following composition (in% by weight):

Carbon:

0.25-0.45

Chrome:

2.40 - 4.25

Molybdenum:

2.50 - 4.40

Vanadium:

0.20-0.95

Cobalt:

2.10 - 3.90

Silicon:

0.10 - 0.80

Manganese:

0.15-0.65

Balance iron and possibly production-related impurities. A degree of purity K 1 <10 μm is preferred.

For the steel according to the invention the hot forming temperature is 900 to 1100 ° C, the soft annealing temperature is 750 to 800 ° C, the stress relieving temperature is 600 to 650 ° C and the hardening temperature is 1000 to 1070 ° C. Oil in a warm bath (500 to 550 ° C) is preferably used as the hardening agent. After soft annealing, the hardness HB is a maximum of 229. After hardening, the Rockwell hardness amounts to 52 to 56 HRC.

The PM hot-work steel according to the invention has the surprisingly good values compiled below at elevated temperatures (guide values).

1. Heat resistance

Quality resistance 1600 N / mm ^2nd Tensile strength N / mm ^2nd 0.2 limit N / mm ^2nd 400 ° C 500 ° C 600 ° C 650 ° C 400 ° C 500 ° C 600 ° C 650 ° C 1380 1210 950 760 1150 1000 750 630

2. Warm hardness

Working hardness 46 HRc; Maintained at test temperature for 30 min 500 ° C 600 ° C 700 ° C 390 HV 330 HV 170 HV

3. Hardness (HRc) after tempering at different temperatures

Tempering temperature in ° C 100 200 300 400 500 550 600 650 700 Rockwell hardness HRc 54 53 50 52 52 53 52 47 46

4. Resistance to fatigue due to temperature changes

The resistance of the material according to the invention to the occurrence of cracks as a result of repeated temperature changes was determined in the usual way in the laboratory. The material is cyclically heated to a test temperature and cooled again in an emulsion. The cracks that have occurred are then counted over a predetermined measuring length. The fire crack number determined in this way allows statements to be made about the behavior of the investigated material in comparison with the behavior of a comparison material.

• a) bei einer Prüftemperatur von 700 °C und 10³-Temperaturwechseln,
• b) bei einer Prüftemperatur von 700 °C und 10⁴-Temperaturwechseln und
• c) bei einer Prüftemperatur von 750 °C bei 10³-Temperaturwechseln

am erfindungsgemäßen Werkstoff sowie an sechs Vergleichswerkstoffen ermittelt wurden. Die untersuchten Werkstoffe besaßen nach dem Anlassen eine Festigkeit von 47 HRc.Fig. 1 shows the results of such fire crack number determinations, which

• a) at a test temperature of 700 ° C and 10 ³ temperature changes,
• b) at a test temperature of 700 ° C and 10 ⁴ temperature changes and
• c) at a test temperature of 750 ° C with 10 ³ temperature changes

were determined on the material according to the invention and on six comparison materials. The materials examined had a strength of 47 HRc after tempering.

The comparison materials are identified with their material numbers "steel key". These comparative materials are steels produced by melt metallurgy. For the hot-work steel according to the invention, the most favorable results for all test conditions a) to c), i.e. the lowest fire crack numbers. The comparison steel containing cobalt with the material number 1.2365 + Co has significantly higher fire crack numbers under all three test conditions a) to c). For test condition a), the values determined on the comparison material 1.2365 + Co are even almost 100% higher.

5. Heat toughness

The excellent thermal toughness values of the material according to the invention are compared graphically in FIG. 2 with the values determined on the specified comparison materials. In the investigated temperature range from about 600 to about 800 ° C, the material according to the invention has excellent necking results. The comparison material, which also contains cobalt and has the material number 1.2365 + Co, is clearly inferior in terms of toughness.

Claims (6)

Hot-work steel produced by powder metallurgy, consisting of (in% by weight): Carbon: 0.25-0.45 Chromium: 2.40 - 4.25 Molybdenum: 2.50 - 4.40 Vanadium: 0.20-0.95 Cobalt: 2.10 - 3.90 Silicon: 0.10-0.80 Manganese: 0.15-0.65 Balance iron and possibly production-related impurities. PM hot-work steel according to claim 1, characterized by a degree of purity K1 of less than 10 µm. PM hot-work steel according to claim 1 or claim 2, producible by the following steps: - production of a molten steel with the desired chemical composition, Atomizing the melt under a high-purity nitrogen atmosphere, Filling the powder obtained into capsules which are designed in such a way that the end product obtains its intended shape with the greatest possible material yield, Shaking the filled capsules in order to achieve the highest possible filling density, Evacuating the filled capsules and sealing them gas-tight, - Introducing the capsules into a hot isostatic press and simultaneously applying pressure and Temperature up to a pressure of 0.8 to 3.5 kbar and a temperature of 1000 to 1230 ° C and - Maintain pressure and temperature for a period of at least 3 hours. PM hot-work steel according to claim 3, characterized in that the powder batch in the hot isostatic press has been subjected to a pressure of 1 kbar. Process for the powder metallurgical production of hot-work steel, comprising the following steps: - Making a steel melt with 0.25 to 0.45% carbon, 2.40 to 4.25% chromium, 2.50 to 4.40% molybdenum, 0.20 to 0.95% vanadium, 2.10 to 3.90% cobalt, 0.10 to 0.80% silicon, 0.15 to 0.65% manganese, Remainder iron and inevitable accompanying elements, Atomizing the melt under a high-purity nitrogen atmosphere, Filling the powder obtained into capsules which are designed in such a way that the end product obtains its intended shape with the greatest possible material yield, Shaking the filled capsules in order to achieve the highest possible filling density, Evacuating the filled capsules and sealing them gas-tight, - Introducing the capsules into a hot isostatic press and heating the capsules while applying pressure to a temperature of 1000 to 1230 ° C. and a pressure of 0.8 to 3.5 kbar, advantageously 1 kbar, and - Keep the batch at the selected temperature and pressure for a period of at least 3 hours. Use of a steel according to at least one of claims 1 to 4 or produced according to claim 5 for the production of press mandrels, press dies and block receptacles for extrusion, as well as for the production of forging presses and die casting molds.

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