EP0149210B1 - Process for manufacturing highly resistant ductile work pieces from iron based alloys rich in carbon - Google Patents

Description

The invention relates to a method for producing high-strength, ductile bodies from carbon-rich iron-based alloys, in which a molten iron-based alloy is quenched, atomized and thermomechanically compressed.

When manufacturing workpieces based on iron alloys, two fundamental requirements are always in the foreground. The material must be brought into the desired shape and the finished workpiece should have certain properties. The main focus is on strength, the important parameters of which are the yield strength, toughness and brittleness, which depend not only on the particular alloy, but also on the respective manufacturing process.

In most applications, end products are desired that have a high strength on the one hand, on the other hand, they are also characterized by favorable ductility parameters.

Various options are available for increasing the strength of low-carbon iron-based alloys. Most methods are designed to influence the ferrite structure or to increase the dislocation density in the ferrite.

At the forefront of the individual process variants is the heat treatment of the steel or iron or the workpieces made from it, i.e. the thermal treatment of the metal in the solid state. A grain refinement is achieved by annealing at approx. 800 - 950 ° C and then quenching, which requires a significant increase in strength, but at the same time also increases the brittleness of the workpiece. Subsequent quenching and tempering (for example by so-called tempering) will then cause the workpiece to lose some strength again, but favorable ductility and homogeneity properties can be achieved.

In addition, thermomechanical treatment processes, particularly for micro-alloyed structural steels, have recently come to the fore. This takes advantage of the fact that some metals that tend to form carbonitride have the property of forming carbonitride precipitates in the steel in the lower temperature region of the austenite and in the ferrite region, which dissolve during heat treatment in the upper temperature region of the austenite. The fact that these metals dissolve and, on the other hand, they can be eliminated again specifically, allows the effects of very fine carbonitride particles on the structure and the mechanical properties of the rolled products to be exploited. When the carbonitrides are precipitated in austenite in a relatively fine form during the subsequent austenite transformation, they act as germs and act as a brake against the migration of the phase and grain boundaries.

The previously known thermomechanical technologies, as described, for example, by Kaspar et. al. in "Stahl und Eisen" 101 (1981), 721 "Metallurgical processes during preheating and pre-rolling of microalloyed structural steels" all refer to weldable, i.e. low-carbon steels or iron alloys.

In contrast to low-carbon iron alloys, for example wrought alloys, unalloyed and alloyed cast iron, ie iron with a carbon content of more than 1.7% by weight, is particularly characterized by high brittleness. The plastic deformability of carbon-rich cast iron alloys is only 1 - 2%. The reason for this is in particular the relatively high volume fraction of carbides (V _carbide ≳ 33%) or the amount, shape and distribution of the carbon separated out as graphite.

The processes known for low-carbon iron-based alloys for improving the strength or ductility properties of the workpieces to be produced have not hitherto been applied to high-carbon iron-based alloys. The reason for this is probably in particular that the different structural parameters and phase compositions in the case of high-carbon-like iron-based alloys require completely different metal-chemical processes than in the case of low-carbon iron-based alloys.

For cast iron alloys, therefore, a different approach has been taken and an attempt has been made to influence the disruptive graphite formation in such a way that the crystallization of the graphite is controlled in a certain way. While the graphite crystallizes out in the form of lamellae in the normal course of the process, a material in which the majority of the carbon in the cast state is precipitated in the form of spheroidal graphite has the particular advantage that it has a higher tensile strength and a better ductility. The formation of spheroidal graphite is only possible in almost sulfur-free melts. In addition, workpieces manufactured in this way do not achieve the strength and ductility values of bodies made of low-carbon iron alloys.

zur Verwendung, das zwingend 0,1 bis 1,5% Bor enthält. Bor verbessert die Unterkühlung und fördert das Entstehen eines homogenen und metastabil kristallisierten Gefüges beim Zerstäuben der Schmelze mit hoher Abkühlungsgeschwindigkeit. Hingegen enthält die Schmelze bzw. Legierung prinzipiell weder Mangan noch Silizium und Nickel, die allenfalls als Verunreinigungen in Frage kommen.A two-stage powder metallurgical process is also known from British Patent Application 2 116 207, in which a melt is atomized at a high cooling rate and the powder is then thermomechanically treated, that is to say hot consolidated. A melt or powder comes with it

for use, which contains 0.1 to 1.5% boron. Boron improves supercooling and promotes the formation of a homogeneous and metastable crystallized structure when the melt is atomized with a high cooling rate. On the other hand, the melt or alloy contains in principle neither manganese, nor silicon and nickel, which can at best be considered impurities.

The invention has for its object to apply the aforementioned method to another alloy powder and to show a way of producing workpieces from carbon-rich iron-based alloys which have both a particularly high strength and particularly advantageous ductile properties.

This object is achieved by a method of the type mentioned in the introduction, in which, according to the invention, an alloy with over 1.7% carbon, over 3.0% nickel and / or manganese, up to 15% chromium and / or cobalt and individually or side by side to 1 % Boron, tellurium, bismuth, selenium, antimony, titanium, niobium, magnesium and phosphorus as well as optionally 2 to 4% silicon, the rest iron is atomized to an average particle diameter of less than 30 µm and then thermomechanically compressed.

Preferred solutions are found in dependent claims 2-17. Depending on the composition of the alloy, in the second process stage (thermomechanical compression) either temperatures below 720 ° C., preferably 650 ° C., are to be regarded as particularly advantageous in the sense of the invention, or the thermomechanical treatment can also be carried out at normal annealing temperatures of 850 to 1000 ° C.

The first stage of the process, the quenching and atomization of the molten metal in such a way that powder particles with a diameter of less than 30 μm are formed, has the effect that the structure structures obtained by normal solidification conditions, such as coarse dendrites and / or acicular carbides, are changed in favor of a fine crystalline structure. This process section is preferably carried out according to the so-called "rapid solidification technology", a temperature gradient of, for example, 10⁴ - 10⁴ K / s being selected. With such a quenching rate, extremely high germination rates can be achieved, however, to keep the germ growth very low due to the short crystallization time until the solid phase is reached. The quenching rate should be chosen depending on the alloy and the particular process so that particles with an average diameter that is smaller than 30 microns are available for the second process stage and the phases of the structure that forms in the particles unite Have a diameter that is less than 0.1 microns.

With regard to a fine final grain size, the quenching according to an exemplary embodiment of the invention is particularly advantageous if additives such as tellurium, bismuth, selenium or antimony, in amounts of up to 1% by weight, achieve a higher supercooling of the melt.

The rapid cooling from the homogeneous melting phase has the further consequence that the crystals formed do not precipitate out in the total weight composition, since the short diffusion times available are not sufficient to bring about complete segregation.

A preferred method for carrying out the first method step according to the teaching of the invention is the so-called “melt spinning” method known for low-carbon steels. The melt, which is saturated with carbon due to its high solubility at high temperatures, is atomized and at the same time extremely quenched, which causes the small particles formed to freeze due to the short diffusion times. In this way, the carbon dissolved in the melt cannot separate out in the form of graphite, but on the other hand, precipitation in carbide form is only fine-grained possible or even completely excluded if suitable additional alloying elements are added.

The production of a powdery material according to process stage 1 then enables powder metallurgical techniques to be used in the second process stage in order to further compact and compact the metal structure, the various workpieces being able to be produced directly or as semi-finished products

A preferred embodiment of the invention provides that after the completion of the first process stage and before the start of the thermomechanical treatment, the powders are pre-compacted in an intermediate stage to form a blank and / or are encased in a metal container. It can also be provided that the powders are sifted to a grain size of less than 30 microns after atomization. Furthermore, it can be provided that the powders are subjected to a reducing annealing before they are compacted, optionally with deoxidizing additives being added.

In order to achieve optimum strength and ductility values of the workpieces made of high-carbon iron alloys, the invention teaches in a first embodiment to work in a temperature range below the A 1 temperature. In this way, depending on the workpiece to be manufactured, the metastable γ phase and the martensite phase in finely disperse cementite with a grain size can be obtained by hot isostatic pressing, forging or extrusion at temperatures between 600 ° C and 720 ° C, preferably in the range around 650 ° C below 0.5 µm and fine-grained ferrite with a particle size below 2 µm can be converted. In addition, the dendritic microstructure is simultaneously molded into a finely crystalline, equiaxial structure made of spherodized, dispersed carbides in the ferrite. The volume fraction of the Carbide particles, for example, is over 50% and thus forms the matrix of these high-carbon iron-based alloys.

When the second process stage is carried out in the temperature interval proposed according to the invention, it is achieved that the carbon previously dissolved in the iron precipitates out as iron carbide, the carbide precipitates having a diameter of approximately 0.1 to 0.01 μm. These fine, but high-strength particles are then embedded in the ferrite matrix due to the process according to the invention and form the cause of the unusually high strength and ductility of the workpieces produced in this way. In contrast to the usual mechanisms for increasing strength in iron, this is essentially a case of dispersion hardening of the ferrite using cementite particles.

According to a second exemplary embodiment of the invention, however, provision is also made for this iron-based alloy to be adjusted by adding up to 1% by weight of boron to the iron-based alloy in such a way that the powders produced therefrom by the first process step according to the invention also at temperatures between 850 ° C. - 1000 ° C, ie at "normal" processing temperatures, can be treated thermomechanically, since the addition of boron reduces the carbon solubility of the austenite. Materials of ferrite and carbide are then produced in such a process.

Instead of adding boron, the alloy can also be adjusted by adding nickel and / or manganese, in an order of magnitude greater than 3% by weight, so that iron-based materials with a purely austenitic structure are formed. Also leave these iron base materials treat themselves thermomechanically at normal processing temperatures.

Another example of such an adjustment of the iron-based alloy is that silicon is added in an order of magnitude of 2-4% by weight to the melt, so that a material with a bainitic matrix and carbides is produced, which is also found in the previous examples mentioned temperatures can be treated.

These three examples of the setting of the iron-based alloy with regard to the high processing temperatures of the powders produced according to the invention each stand on their own and should not be combined with one another.

Through the teaching according to the invention, a method is proposed with which cast iron alloys containing high carbon and having favorable ductility properties can also be produced. The prevailing opinion in the professional world that carbon-rich alloys must be brittle cannot be maintained in this respect. Rather, it is possible with the teaching according to the invention to obtain, through the fine distribution of the carbide phase, high-strength, very ductile materials which, with low alloy contents of metals, have properties which correspond to the high-alloy iron-based alloys.

According to the invention, superplasticity can be achieved in the temperature range between 600 ° C and 720 ° C with deformation values of up to 1,300% with high strength at the same time.

The invention, including advantageous refinements and developments, results from the features of the claims, which follow this description.

Fig. 1

eine Gegenüberstellung einer unverformten und zweier superplastisch bis zum Bruch gedehnten Proben, die nach dem erfindungsgemäßen Verfahren hergestellt worden sind,

Fig. 2

eine Raster-Elektronen-Mikroskop-Gefügeaufnahme einer nach dem erfindungsgemäßen Verfahren hergestellten Eisenlegierung.

The invention is explained in more detail with reference to the drawing and the following exemplary embodiments. Show it:

Fig. 1

a comparison of an undeformed and two superplastically stretched samples that have been produced by the method according to the invention,

Fig. 2

a scanning electron microscope micrograph of an iron alloy produced by the inventive method.

Iron-based alloys of the Fe-CX type (X = Cr., Mn, Co, Ni) were investigated, the carbon content varying between 2 and 4% by weight and the proportion of metallic additives varying between 0 and 15% by weight . The structure, structure, hardness and ductility were tested in the strips obtained.

The conversion behavior was examined using calorimetric and dilatometric methods.

In addition, the creep behavior of the alloys was studied using a thermomechanical test system.

The samples were produced according to the so-called "melt spinning" process.

In the rapidly quenched structure, there are clear differences depending on the alloy content. In Fe, Cr, C alloys with low chromium contents, dendrites form from a quench layer. At higher chromium contents, the structure changes into equiaxial crystallites. With increasing carbon content, the former dendrites are replaced by larger carbide grains replaced. The addition of nickel, silicon or manganese promotes the formation of equiaxial particles, with segregations of carbide being detectable at the grain boundaries.

By briefly annealing the samples, it is possible to achieve fine carbide deposits in an austenitic or ferritic matrix - depending on the composition of the sample. The grain size is then in the range of 0.1 µm and below.

The fracture appearance of tempered samples is different from that of the as-quenched sample. The samples with a content of 6% by weight of chromium and 3% by weight of carbon have different properties after an annealing treatment in that the fracture no longer runs along the former dendrite grain boundaries.

The samples were produced by the powder atomization method, which allows large quantities of rapidly quenched material to be produced, so that further processing by means of powder metallurgical techniques is possible.

Creep properties in the temperature range between 500 and 720 ° C were investigated on rapidly quenched strips of Fe-Cr-C alloys. This leads to changes during heating in the form of changes in length, which are due to the residual austenite transformation, precipitations, etc. (1st-3rd tempering stage). Such falsifying effects can be eliminated by heating once at 10 K / min. The change in length depending on the temperature in the temperature range of 500 - 600 ° C indicates a normal dislocation creep. In the temperature range of 600 to 650 ° C, however, the creep speed drops. This is due on the coagulation of cementite. Above 650 ° C, up to around 720 ° C, effects are obtained which indicate superplasticity.

The martensitic structural components that occur in small volume fractions of the rapidly quenched powder particles are predominantly due to deformation-induced Ms conversions during the particle collisions during the quenching process. It can be assumed that not all powder particles with an average diameter below 30 µm have the Ms temperatures of the alloys investigated calculated from the chemical analysis of austenite: Fe - 3.5% by weight C, T _Ms = 85 K and for Fe 3.5% by weight C + 1.5% by weight Cr, T _Ms = 140 K, in the cooling helium vapor. However, it is evident that the chromium-rich powder particles are favored for the Ms transformation.

The compacting and compacting of the extremely rapidly quenched Fe-C-Cr powder by a combination of powder metallurgical and thermomechanical process techniques, namely hot isostatic pressing and rolling just below the A 1 transformation temperature, causes profound structural changes in the structure. These consist in the transformation of the metastable γ phase and the martensite into finely disperse cementite with a grain size of less than 0.5 µm and fine-grained ferrite with a grain size of less than 2 µm. In addition, the dendritic microstructure is molded into a finely crystalline, equiaxial structure made of spherodized, dispersed carbides in the ferrite. 2 shows a scanning electron microscope micrograph of the equiaxial microstructure of the compacted and thermomechanically treated high-carbide iron alloys. The volume fraction of the carbide particles is approximately 56 vol .-% and thus represents the matrix phase of this high-carbon iron-based alloy.

Texture studies of the rolled state show no preferred crystallographic orientation distribution of these two-phase alloys. This is explained by the texture-inhibiting effect of the carbide particles in large volume fractions.

As can be seen in FIG. 1, the yield stresses and compressive strengths of the two alloys produced according to the invention are different from one another. The higher strength values of the chromium-rich alloy are due to the structurally more stable, fine-crystalline structure after the thermomechanical treatment. The predominant content of chromium is dissolved in the cementite, stabilizes the carbides and prevents undesired carbide growth. In addition, a strength-increasing contribution due to the solid solution hardening of the ferrite by the chromium dissolved in the γ-iron can be assumed.

As the test temperature increases, considerable changes in the deformation and hardening properties of the alloys produced according to the invention occur. At temperatures above about 600 ° C, these fine crystalline, high carbide iron materials become superplastic. According to the invention, the optimal superplastic deformation temperature is about 650 ° C. At this deformation temperature, which is relatively low for iron alloys, the diffusion-controlled accommodation mechanisms of grain boundary sliding are sufficiently thermally activated, and at this temperature the microstructure is stable against a stress or strain-induced grain growth of the cementite and ferrite phase. This applies in particular to the chromium-containing alloy.

Superplastic materials are generally characterized by high amounts of uniform expansion. In the fracture zone, however, local constrictions can often be found, which are caused by the plastomechanical instabilities due to local hardening processes.

These processes obviously do not occur in the present alloys under the production method according to the invention.

According to the teaching of the invention, different consolidation methods are possible as long as they are associated with a sufficiently high deformation so that the pre-pressed powder is formed into a solid body with low porosity and the forming temperature is in the range between 600 and 720 ° C.

Claims (17)

1. Method of making ductile, high-strength bodies of carbon-rich iron-base alloys, wherein an molten iron-base alloy is quenched, pulverised and thermomechanically consolidated, characterised in that an alloy containing more than 1,7% carbon, more than 3,0% nickel and/or manganese, up to 15% chromium and/or cobalt and, singly or in combination, up to 1% of boron, tellurium, bismuth, selenium, antimony, titanium, niobium, magnesium and phosphorus, and optionally 2 to 4% silicon, balance iron, is pulverised to an average particle diameter of less than 30µm and then thermomechanically consolidated.
2. Method according to claim 1, characterised in that the iron-base alloy has an extremely low content of silicon and manganese, preferably less than 0.02% silicon or less than 0.02% manganese or less than 0.02% silicon and manganese.
3. Method according to one of claim 1 and claim 2, characterised in that in the first process step the rate of quenching is from 10⁴ to 10⁷K/s.
4. Method according to any one of claims 1 to 3, characterised in that the quenching in the first process step is performed using a temperature gradient such that the phases of the structure formed in the particles have a diameter less than 0.1µm.
5. Method according to any one of claims 1 to 4, characterised in that the powder particles in the first process step are formed by the powder atomisation process.
6. Method according to any one of claims 1 to 5, characterized in that carbide stabilizing additives are added to the iron-base alloy in order to inhibit grain growth of the carbide and/or of the matrix structure.
7. Method according to any one of claims 1 to 6, characterised in that elements that bind the residual carbon in the ferrite are added to the iron-base alloy.
8. Method according to any one of claims 1 to 7, characterised in that a two-phase structure consisting exclusively of carbide and ferrite is produced.
9. Method according to any one of claims 1 to 8, characterised in that the thermomechanical consolidation treatment in the second process step is performed between 600 and 720°C.
10. Method according to claim 9, characterised in that the thermomechanical consolidation treatment is performed at about 650°C.
11. Method according to claim 9 or claim 10, characterised in that the thermomechanical consolidation treatment is performed under a mechanical load of 1500 to 2000 MPa.
12. Method according to any one of claims 9 to 11, characterised in that the thermomechanical consolidation treatment is performed by hot isostatic pressing.
13. Method according to any one of claims 9 to 11, characterised in that the thermomechanical consolidation treatment is performed by extrusion.
14. Method according to any one of claims 9 to 11, characterised in that the thermomechanical consolidation treatment is performed by forging.
15. Method according to any one of claims 1 to 15, characterised in that the powder produced in the first process step is preconsolidated to a green compact and/or clad in a metal container before the second process step.
16. Method according to claim 15, characterised in that the powder is sieved to a particle size of less than 30µm after atomising.
17. Method according to claim 15 or claim 16, characterised in that before consolidation the powder is subjected to a reducing anneal and/or deoxidising additives are optionally added to it.

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