EP2196553A1 - Steel alloy for machine components - Google Patents

Abstract

The machine component or component consists of thermally hardened steel alloy having carbon (0.48-0.55 wt.%), silicon (0.18-0.25 wt.%), manganese (0.35-0.45 wt.%), chromium (4.40-4.70 wt.%), molybdenum (2.90-3.10 wt.%), vanadium (0.72-0.77 wt.%), iron and accompanying elements necessitated by steel production and impurities as remainder. The accompanying elements or impurity elements consist of phosphorus (0.005 wt.%), sulfur (0.001 wt.%), nickel (0.1 wt.%), copper (0.1 wt.%), cobalt (0.1 wt.%), titanium (0.005 wt.%), aluminum (0.01 wt.%), nitrogen (0.003 wt.%) and oxygen (0.002 wt.%). The machine component or component consists of thermally hardened steel alloy having carbon (0.48-0.55 wt.%), silicon (0.18-0.25 wt.%), manganese (0.35-0.45 wt.%), chromium (4.40-4.70 wt.%), molybdenum (2.90-3.10 wt.%), vanadium (0.72-0.77 wt.%), iron and accompanying elements necessitated by steel production and impurities as remainder. The accompanying elements or impurity elements consist of phosphorus (0.005 wt.%), sulfur (0.001 wt.%), nickel (0.1 wt.%), copper (0.1 wt.%), cobalt (0.1 wt.%), titanium (0.005 wt.%), aluminum (0.01 wt.%), nitrogen (0.003 wt.%), oxygen (0.002 wt.%), calcium (0.001 wt.%), magnesium (0.001 wt.%) and tin (0.005 wt.%). The component has a purity degree of steel alloy. The elasticity module of the material is 205000 MPa. The machine component has a tensile strength of greater than 2000 MPa for changing mechanical loads to a temperature of greater than 160[deg] C, and a hardness of greater than 55 [HRC] produced by thermal tempering.

Description

The invention relates to machine components or components with a tensile strength greater than 2000 [MPa] for varying mechanical loads up to a temperature of 160 ° C, formed from a thermally tempered steel alloy. In particular, the invention relates to engine and / or drive components of vehicles.

Machine components with varying mechanical stress are increasingly loaded in modern technology to the limits of the respective material resistance. In particular, this is true for components in vehicles because the weight reductions achieved thereby are also useful for fuel savings and the like.

Of the materials from which the components are formed, high values for the toughness, strength, and ductility properties are required in the thermally tempered state because these property values are critical for dimensional design of the parts.

As evidenced by failure of parts in long-term operation, the properties of the material fatigue have to be taken into consideration in order to achieve a high level of operational safety.

For parts with significant, mechanical alternating load in the railway, automotive and aerospace sectors are currently used in the state of the art mostly alloyed, possibly low-alloy tempered steels. A preferred representative of these steels is the alloy according to DIN material no. 1.6928. This rather low-alloy material contains 1.40 to 1.90 wt .-% silicon in order to ensure a high creep strength largely. It has also been advantageously attempted to increase the silicon content of this alloy up to 3.0% by weight in order to achieve the best fatigue properties of the material when the parts are stressed.

A use of steel alloys with a composition according to that of tempering steels of the aforementioned type has been well proven for the production of highly stressed machine components according to the prior art, but their fatigue properties are often not sufficient for a mechanical alternating load in the limit value range of material used.

It is now an object of the invention to provide machine components or components with a tensile strength of greater than 2000 [MPa], which are exposed in the thermally tempered state, mechanical stresses are exposed to a temperature of 160 ° C and significantly improved long-term properties and a high modulus of elasticity exhibit.

This goal is achieved with a thermally tempered steel alloy for machine components and / or components of the type mentioned, which has a chemical composition in wt .-% of Carbon (C) 12:48 to 12:55 Silicon (Si) 12:18 to 12:25 Manganese (Mn) 12:35 to 12:45 Chrome (Cr) 4:40 to 4.70 Molybdenum (Mo) 2.90 to 3.10 Vanadin (V) 0.72 to 0.77 Iron (Fe) and accompanying elements caused by melting and impurities as the remainder
has.

The advantages achieved with the use of a material according to the invention are essentially to be seen in the fact that the machine components of the type mentioned have the same or improved, mechanical strength properties a much higher fatigue resistance at high loads. Furthermore, the material or the component according to the invention has a much higher modulus of elasticity, which leads to lower strain values in the elastic range and thus to a longer service life of the parts given the same specific mechanical stress.

Accompanying and impurity elements may be the cause of reduced long-term properties, because these elements are enriched at the grain boundaries of the structure or can form compounds. It has been found that the material properties are only slightly impaired in the case of long-term alternating load if the highest contents of one or more of the following accompanying or impurity elements are present in% by weight. Phosphorus (P) 0005 Sulfur (S) 0001 Nickel (Ni) 0.1 Copper (Cu) 0.1 Cobalt (Co) 0.1 Titanium (Ti) 0005 Aluminum (Al) 12:01 Nitrogen (N) 0003 Oxygen (O) 0002 Calcium (Ca) 0001 Magnesium (Mg) 0001 Tin (Sn) 0005 be.

Advantageously, in the case of an aforementioned chemical composition by thermal tempering, a homogeneous distribution and a hardness of greater than 54 HRC, in particular greater than 55 HRC, formed free of peak values can be set, which increases the fatigue resistance.

Of particular importance is the degree of purity of the steel alloy with regard to crack initiation. It has been found that in a material thermally tempered to high strength values, even small, non-metallic inclusions, even with somewhat rounded edge shapes, have the greatest adverse effect on fatigue resistance with varying mechanical stress. This situation must also be taken into account melting technology, after a reaction-kinetic liquid steel treatment usually a two-fold vacuum arc remelting of the steel alloy is provided to a degree of purity of the steel alloy according to the invention of less than / D / 0.5 / THIN 1 (A, B, C type inclusions not available) according to ASTM E 45 (measuring surface 160mm ² ).

If the machine components or the component have (has) a modulus of elasticity of the material greater than 200,000 MPa, these have or have this in the elastic region of the mechanical stresses under alternating load lower strain and compression values, whereby a longer life is achieved or better fatigue values are given.

Particularly well-proven, in terms of the overall property profile, the quenched steel alloy or the material has as a machine component in vehicle construction and especially as a motor and / or drive and / or spring part.

• Fig. 1 Zugfestigkeit
• Fig. 2 Fließgrenze
• Fig. 3 Bruchdehnung und Einschnürung
• Fig. 4 E-Modul
• Fig. 5 Bruchlastspielzahl
• Fig. 6 Erprobungsanordnung

von Vergleichswerkstoffen und erfindungsgemäßen Maschinenkomponenten.In the following, the invention is explained in more detail on the basis of examination results and comparative diagrams.
Show it:

• Fig. 1 tensile strenght
• Fig. 2 yield
• Fig. 3 Elongation at break and constriction
• Fig. 4 Modulus
• Fig. 5 Fatigue life
• Fig. 6 testing arrangement

of comparative materials and machine components according to the invention.

Steel alloys containing essentially the elements in wt% carbon 0.49 to 0.53, silicon 0.20 to 0.23, manganese 0.36 to 0.42, chromium 4.50 to 4.60, molybdenum 2.80 to 3.00, vanadium 0.70 to 0.85, balance iron and impurities, were due to Results from preliminary tests as materials with a property potential according to the invention determined and produced with the highest possible degree of purity.

Such chemically combined materials are, as is familiar to those skilled in the art, hot working steels for use temperatures up to 500 ° C. It was surprising It has been found that these alloys in the thermally tempered state are advantageously applicable to machine components or components with alternating, mechanical stress at low temperatures, if a chemical composition is present within narrow limits of the alloying elements according to the invention.

High purity steel alloys according to the invention marked W366 had been made into deformed and thermally tempered samples which were tested in investigations to determine the material characteristics.

Comparing with the material according to the invention, a determination was made of the material characteristics of materials treated in the same way, which according to the prior art have hitherto been used for machine components of the type mentioned and according to a US standard with a designation 300 M, corresponding to DIN material No. 1.6928, as well as 300 M "improved" with higher Si content in the comparisons are characterized.

In Fig. 1 the tensile strength with the highest values for the material proposed according to the invention is reproduced comparatively.

Fig. 2 shows in one, as in Fig. 1 4, the bar graph shows the 0.2% proof strength of the materials, with the values of the samples of components having a composition W366 being of the highest level.

Fig. 3 teaches that both the elongation at break and the W366 fracture throat values are significantly higher as compared to 300 and 300 M improved, revealing significant advantages in its use for machine components with varying mechanical stress.

Also, the modulus of elasticity of material W366 is, as out Fig. 4 can be seen in comparison with the materials according to the prior art higher, so that in heavy use lower, elastic deformations in a mechanical Load of the material, whereby a fatigue failure of a part of W366 is substantially reduced.

Fig. 5 shows the fatigue behavior of the thermally tempered samples from the investigated alloys in comparison.

Fatigue behavior:

Cyclically repeated stress results in subcritical crack growth in one material. The cause is microplastic deformations, which add up to a relatively high overall deformation in the course of the alternating stress. This form of material damage is called fatigue. Even cyclic, mechanical stresses that are far below the yield strength, can lead to cracking and crack growth or even breakage of the material. The fatigue strength (fatigue strength) is the limit of the stress at which no break occurs even after an infinite number of cycles (load changes). To determine the fatigue strength, the Wöhler test must be carried out until a limit number of cycles N _G is reached.

For tool steels, fractures up to 10 ⁷ load changes can occur. In this study, however, a limit of 2 x 10 ⁶ load changes was chosen.

The fatigue tests were carried out on a TESTRONIC type resonance testing machine by means of a four-point bending arrangement. This so-called Dauerschwingprüfmaschine is a dynamic testing machine that works at full resonance.

In Fig. 6 the four-point bending arrangement is shown schematically. The loading of the samples was carried out on rolls with a diameter of 5mm. The distance between the rollers was 15mm at the top and 30mm at the bottom. For this test, square samples with the following dimensions were used: height h = 5mm; Width b = 7mm; Length I = 55mm.

Dabei ist M_b = x' x F/2 das Biegemoment und W_b = b x h²/6 das Widerstandsmoment der Probe. F ist die Kraft, die auf die Rollen wirkt und x' (= 7.5mm) ist der Hebelarm, der zusammen mit der zeitabhängigen Belastung F das Biegemoment bildet.The marginal fiber stress σ _b was assumed to be linearly elastic Voltage distribution determined according to the equation. $${\mathrm{\sigma }}_{\mathrm{b}}\mathrm{=}\frac{{\mathrm{M}}_{\mathrm{b}}}{{\mathrm{W}}_{\mathrm{b}}}\mathrm{=}\frac{\mathrm{3}\mathrm{\times }\mathrm{F}\mathrm{\times }\mathrm{x\; \text{'}}}{\mathrm{b}\mathrm{\times }{\mathrm{<figure-callout\; id="2"\; label="H"\; filenames="imgf0003.png"\; state="\{\{state\}\}">H</figure-callout>}}^{\mathrm{2}}}$$

Here, M _b = x 'x F / 2, the bending moment and W _b = wxh ^2/6, the section modulus of the sample. F is the force acting on the rollers and x '(= 7.5mm) is the lever arm which together with the time dependent load F forms the bending moment.

Out Fig. 5 clearly show the advantages with respect to an improved fatigue behavior of machine components or components according to the invention, wherein the value range "pass level" indicates the voltage amplitude up to which no break of the sample occurs at infinite load cycle.

In order to determine the effect of accompanying and impurity elements on the property profile, the steel alloy according to the invention was doped with these elements with different concentrations and quenched samples were investigated therefrom. The results of the tests and the resulting limit values are given below.

The impurity elements phosphorus and sulfur lead to brittle precipitates at strength values of the material of more than 53 HRC, whereby a significant increase of the embrittlement from 0.005 wt.% P and 0.001 wt.% S was observed.

Calcium, magnesium, aluminum are deoxidizing elements and form oxygen inclusions with oxygen, which due to the sharp-edged shape and deformed materials of the linear arrangement cause disadvantages in terms of fatigue resistance of the material, which also depends on the direction of deformation, if necessary. Despite repeated vacuum arc remelting, the material investigations revealed upper limit values which should not be exceeded in the materials according to the invention. These limits are 0.01 wt% Al, 0.001 wt% Ca, 0.001 wt% Mg and 0.002 wt% O.

Nitrogen, in particular with alloying elements as well as titanium and oxygen, can form sharp-edged nitrides, which by means of increased strength cause voltage peaks in the micro range and thus crack initiation. The upper limit values found are 0.003 wt% N and 0.005 wt% Ti.

Nickel, copper and cobalt in low concentrations are storage elements in the crystal formation of the alloy, but should not exceed levels of 0.1 wt .-% due to an adverse effect of lattice defects on the long-term properties of the material.

Tin is due to the extremely low solubility in iron-based materials as a grain boundary occupying element to see and acts from a concentration of 0.005 wt .-% extremely negative on the fatigue and especially toughness properties of a component with alternating mechanical stress.

Claims (6)

Machine components or components with a tensile strength of more than 2000 [MPa] for alternating mechanical loads up to a maximum temperature of 160 ° C, formed from a thermally-quenched steel alloy containing, in% by weight Carbon (C) 12:48 to 12:55 Silicon (Si) 12:18 to 12:25 Manganese (Ma) 12:35 to 12:45 Chrome (Cr) 4:40 to 4.70 Molybdenum (Mo) 2.90 to 3.10 Vanadin (V) 0.72 to 0.77 Iron (Fe) and accompanying elements caused by melting and impurities as the remainder. Machine components according to claim 1, having the highest contents of one or more of the following concomitant or impurity elements in% by weight. Phosphorus (P) 0005 Sulfur (S) 0001 Nickel (Ni) 0.1 Copper (Cu) 0.1 Cobalt (Co) 0.1 Titanium (Ti) 0005 Aluminum (AI) 12:01 Nitrogen (N) 0003 Oxygen (O) 0002 Calcium (Ca) 0001 Magnesium (Mg) 0001 Tin (Sn) 0005 Machine components according to claim 1 or 2 with a hardness created by thermal treatment of greater than 54 [HRC], in particular greater than 55 [HRC]. Machine components according to one of claims 1 to 3 with a Purity of the steel alloy. Machine components according to one of claims 1 to 4 with a modulus of elasticity of the material of E = greater than 200,000 MPa, in particular greater than 205,000 MPa. Machine components according to one of claims 1 to 5, used in vehicle construction, in particular as a motor and / or drive and / or spring part.

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