EP0814172B1 - 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 that are identical chemical composition the properties of melt metallurgical manufactured steels are superior. In particular This is what distinguishes steels manufactured using powder metallurgy from that they are in all dimensions across their entire Cross-section have the same structural condition. So are also the mechanical properties across the entire cross-section essentially the same.

It is also already known, the hot work steel X40CrMoV51 to be produced by powder metallurgy by hot isostatic pressing. In this regard, the archive for the iron and steel industry 55 (1984), pages 169-176, that the hot-work steel mentioned Carbon 0.37-0.41%, silicon 1.0 - 1.07%, manganese from 0.38 - 0.42%, chromium from 5.3 - 5.5%, molybdenum of 1.37-1.41%, vanadium of 1.0-1.27 and negligible Nitrogen, oxygen, sulfur and phosphorus levels having.

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 aforementioned hot-work steel produced by powder metallurgy has a satisfactory hardness, is but because of its insufficient warm hardness, temper resistance and its tendency to experience temperature change cracks not suitable for heavy duty hot work tools like for mandrels, dies and block receivers for the metal pipe and extrusion, not for hot extrusion tools, Tools for hollow body production, tools for screw, nut, rivet and bolt products, Die casting tools, molding dies, die inserts and Hot shear blades. In short, the stability of the Steel in question for highly stressed hot work tools not satisfy.

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

The invention is also based on the object of a method for the production of an improved powder metallurgy Specify hot work steel.

With regard to the steel to be created, this task solved by the subject matter of claim 1. Regarding the This task is to be created by the object of claim 4 solved.

The invention further relates to the use of powder metallurgy manufactured hot work steel as a material for Manufacture of press mandrels, press dies and block sensors for extrusion and for the production of forging presses and die casting molds.

The technical progress achievable with the help of the invention results primarily from the fact that as a result of the invention composition containing cobalt, synergistic reinforced by the special compression according to the invention, a powder metallurgy hot work tool for Is provided, which is essentially as good Toughness properties like a known cobalt-free Hot-work steel, but also with high hot hardness, Temper and hot fire crack resistance values.

There are vigorous concerns about inclusion in the professional world from cobalt into hot-work steel. In particular there is a notion among experts that alloying in no way the toughness properties of cobalt, especially the heat toughness properties of a powder metallurgy manufactured hot work steel received or even have it improved.

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

Both in the conventional powder metallurgical production as well as in the manufacture according to the invention is used as a raw material from precious scrap and ferro alloys. While the prior art, however, cobalt in powder metallurgy avoids hot-work steels are invented Cobalt contents provided. According to the state of the Technology as well as according to the invention takes place the melting of Starting alloys preferably held in the induction furnace.

Induction heat is used to produce the steels according to the invention and used accurate temperature control until the Slag content is correct. Then under a protective gas atmosphere (preferably high purity nitrogen). The APM Calidus system has proven particularly suitable for this emphasized because with its help inclusions in the manufactured Powder can be avoided.

Efforts have become known in the prior art to: to achieve a high degree of purity of the melt by the melt through a slag lid with the help of electrodes heated.

In the conventional process, the melt is fed directly into the capsule to be compressed, which increases the risk of undesirable Inclusions enlarged.

In the production of the steel according to the invention, what is obtained Alloy powder filled into capsules, which are designed in this way are that the final product is in its intended form receives the greatest possible material yield. Thus, according to the invention Capsules used that are the product to be manufactured should at least largely give the desired shape.

After filling, the capsules are shaken to get the largest possible To achieve bulk density. Then they are like this filled capsules pumped air-free and then sealed gas-tight.

In the conventional method, as already mentioned, is direct atomized in capsules, which are then sealed gas-tight. The state of the art basically knows only a single standard capsule size with a diameter of 465 mm and one Length of 1600 mm.

In the conventional method, as mentioned above, pretreated capsule at a pressure of about 3.5 Kbar cold isostatic pressed to this way the thermal conductivity to improve the powder batch contained in the capsule.

For the production of the hot-work steel according to the invention Such cold pressing is not necessary because the powder batch such a high filling density as a result of the shaking has the desired thermal conductivity properties are given in the powder batch.

In the conventional method, those as described above are used Capsules in a preheating oven without overpressure the temperature of the hot isostatic pressing (HIP temperature) heated and then transported to the hot press system. There Thermal conductivity even after conventional cold pressing the powder batch is only low, arises at the beginning of the preheating treatment a steep temperature gradient in the powder batch, which leads to segregations of oxygen, sulfur and Carbon leads. These segregations are considerable Extent of what is indicated by deep etching or chemical analysis can be demonstrated. Furthermore, the steep temperature gradient leads to some carbide growth.

In the manufacture of hot-work steels according to the invention there is no preheating of the capsules and - as already mentioned - also no cold pressing.

In the production according to the invention, the capsules are under simultaneous pressurization heated. In particular the first step is to apply pressure about 200 bar with the help of compressed argon. Subsequently the heating takes place in the HIP system, the pressure of the the compressors supplying the compressed argon essentially is kept constant. With increasing temperature the grows Pressure on continuously without the pressure of the argon compressors should be increased. The powder batch is compacted under pressure at a relatively low temperature, even before Oxygen, sulfur and carbon transport occurs. Consequently, the hot-work steel according to the invention is free of Segregations.

Once the intended HIP pressure has been reached, use the appropriate one Regulation and control measures a further increase in pressure and temperature prevented.

The HIP temperature is 1000 to 1230 ° C, one temperature of 1150 ° C is preferred. The HIP pressure is 0.8 to 3.5 kbar, with a HIP pressure of 1 kbar currently being extremely has proven advantageous. When pressing less than 0.8 Kbar does not result in a sufficient compression of the material and especially the risk of gas inclusions in residual pores remain. HIP pressures of more than 3.5 kbar are possible with modern HIP systems, but does not lead to one quality increase justifying the effort.

In the steel production according to the invention, the holding time is at the target HIP temperature and at the target HIP pressure at least 3 h. This time period applies to small dimensions to be manufactured. Larger dimensions to be manufactured need longer compaction times. conventional Procedures usually work with holding times of only a single hour. Since in the method according to the invention filled capsules simultaneously high temperatures and high Exposed to pressure will result in a homogeneous material high density achieved.

The powder metallurgy produced in a conventional manner Hot work steel requires final forging or rolling treatments. Such processing measures in the heat carried out lead to an undesirable carbide growth and also to undesirably round off the carbides.

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

As far as quality control is concerned, it should be mentioned that the conventional methods a control, for example Inclusions, only after removing the powder batch from the deformed Capsule is done. In contrast, the invention Steel material already in the powder state of critical quality control subjected.

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. It has a degree of purity K1 <10.

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

The PM hot-work steel according to the invention has the in the following surprisingly good values with increased Temperatures (guidelines).

1. heat resistance

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

Second hot hardness

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

Third 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

4th Resistance to fatigue due to temperature changes

The resistance of the material according to the invention against that Cracks appear as a result of repeated temperature changes was determined in the usual way in the laboratory. there the material is cyclically heated to a test temperature and cooled again in an emulsion. Subsequently the cracks that have occurred are on a predetermined measuring length counted. The fire crack number determined in this way permits Statements about the behavior of the investigated material in the 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 with their material numbers Inscribed "steel key". It is these Comparative materials for melt-metallurgically manufactured Steels. Result for the hot work steel according to the invention the cheapest 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 indicates all three Test conditions a) to c) significantly higher fire crack numbers. For test condition a) they are due to the reference material 1.2365 + Co values even increased by almost 100%.

5th hot ductility

The excellent heat toughness values of the invention Material are graphically in Fig. 2 to the specified Comparative materials compared values determined. in the investigated temperature range from about 600 to about 800 ° C. the material of the invention has excellent Einschnürungsergebnisse. The reference material also containing cobalt with the material number 1.2365 + Co are clearly inferior in terms of toughness.

Claims (5)

1. A powder-metallurgically produced hot-work steel consisting (in weight percent) of: 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 the balance being iron and possibly impurities resulting from production,
   and having a purity degree K1 < 10.
2. A PM hot-work steel according to claim 1, which can be produced by taking the following steps:

   producing a steel melt with the desired chemical composition,

   atomizing the melt under a high-purity nitrogen atmosphere,

   filling the resulting powder into capsules which are designed such that the final product is given its intended shape at a maximum material yield,

   shaking the filled capsules for achieving a maximum filling density,

   evacuating the filled capsules and closing the same in a gas-tight manner,

   introducing the capsules into a hot isostatic press and simultaneously subjecting the capsules to pressure and heat until a pressure of 0.8 to 3.5 kbar and a temperature of 1000°C to 1230°C are reached, and

   maintaining pressure and temperature for a period of at least 3 h.
3. A PM hot-work steel according to claim 2, characterized in that the powder charge has been subjected in the hot isostatic press to a pressure of 1 kbar.
4. A method for the powder-metallurgical production of a hot-work steel, comprising the following steps:

   producing a steel melt with 0.25-0.45% carbon, 2.40-4.25% chromium, 2.50-4.40% molybdenum, 0.20-0.95% vanadium, 2.10-3.90% cobalt, 0.10-0.80% silicon, 0.15-0.65% manganese, the balance being iron and unavoidable accompanying elements, and with a purity degree K1 < 10,

   atomizing the melt under a high-purity nitrogen atmosphere,

   filling the resulting powder into capsules which are designed in such a manner that the final product is given its intended shape at a maximum material yield,

   shaking the filled capsules for achieving a maximum filling density,

   evacuating the filled capsules and closing the same in a gas-tight manner,

   introducing the capsules into a hot isostatic press and heating the capsules under simultaneous pressurization to a temperature of 1000°C to 1230°C and at a pressure of 0.8 to 3.5 kbar, advantageously 1 kbar, and

   holding the charge at the selected temperature and the selected pressure for a period of at least 3 h.
5. Use of a steel according to at least one of claims 1 to 3 or produced according to claim 4 for producing pressing mandrels, pressing dies and containers for extrusion, and also for producing forging presses and die-casting dies.

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