EP3243920A1 - Spheroidal cast alloy - Google Patents

Abstract

Nodular cast iron alloy as well as castings and their manufacturing processes with a pearlitic-ferritic structure for cast iron products with a high strength combined with good ductility and toughness already in the cast state, comprising as non-iron components C, Si, Ni, Mn, Cu, Mg, Cr, Al, P, S and the usual impurities, characterized in that the nodular cast iron alloy 2.8 to 3.7% by weight of C, 1.5 to 4% by weight Si, 1 to 6.2% by weight Ni, 0.02 to 0.05% by weight of P, 0.025 to 0.06% by weight Mg, 0.01 to 0.03 wt.% Cr, 0.003 to 0.3% by weight Al, 0.0005 to 0.012% by weight S, 0.03 to 1.5 wt.% Cu and 0.1 to 2% by weight of Mn, the balance Fe and unavoidable impurities, whereby the nodular cast iron alloy in the cast state without subsequent heat treatment has a high static strength of a 0.2% yield strength of ¥600 MPa and a tensile strength of ¥750 MPa with at the same time good ductility of one Elongation at break A5 of 2 to 10% achieved.

Description

The invention relates to a nodular cast iron alloy with pearlitic-ferritic structure for cast iron products with a high static strength of 0.2% proof stress ≥600 MPa and a tensile strength ≥750 MPa with a good ductility of at least 2% until a ductile state without subsequent heat treatment 10%, comprising the non-iron components C, Si, P, Mg, S, Mn and Ni and the usual impurities. Possible applications for motor vehicle construction are, for example, chassis components such as wheel carriers, vehicle structural parts and crankshafts.

In the automotive industry increasingly high-strength cast iron alloys are used, which are characterized for potential exploitation of a weight reduction by higher strengths. For reasons of cost, the focus is as far as possible on the renunciation of any heat treatment processes and achieving the required mechanical properties with only moderate alloying additions.

From the EP 1 225 239 A1 For example, a higher strength bainitic nodular cast iron alloy having 2 to 4% by weight of Ni and 0.05 to 0.45 by weight of Mn is known as non-iron components, and the Ni-Mn margin is to adjust the variable ratio of strength to elongation. For the implementation of the invention, the non-iron components are preferably 3.1 to 4% wt.% C and 1.8 to 3 wt.% Si. A material of this composition with this microstructure is characterized by a tensile strength of 650 to 850 MPa and a 0.2% proof stress of ≥ 500 MPa with an elongation at break of 14.5 to 7%. Although these properties are achieved without heat treatment, the achievable strengths are limited due to the alloy composition.

From the DE 10 2004 040 056 A1 Another cast iron alloy is known, which is described as high and wear resistant and corrosion resistant. It is composed of 3 to 4.2 wt.% C, 1 to 3.5 wt.% Si, 1 to 6 wt.% Ni, ≤5 wt.% Cr, ≤3 wt.% Cu, ≤3 wt.% Mo, ≤1% by weight Mn, ≤1% by weight V, ≤0.4 % P, ≤0.1% by weight S, ≤0.08% by weight Mg, ≤0.3% by weight Sn and manufacturing impurities. These wide alloy ranges result in a variety of matrix compositions of> 50% needlelike ferrite with different proportions of austenite (<20%), martensite (<30%), perlite (<50%) and carbides (<15%), graphite formation is not evident Nodular graphite limited but may also be vermicular and lamellar type. Achievable bending fracture strengths in the application example of a piston ring are> 1100 MPa, the hardness is 320 HB2.5, emphasizing unspecified high toughness / ductility. The elongation at break, however, should be significantly reduced, especially in the case of alloy variants with carbide contents of up to 15% in the microstructure. In the case of small wall thicknesses (modulus ≤ 1.5 cm), an additional process step in the form of tempering at temperatures <700 ° C may be necessary.

From CA 122 40 66 A1 / US 448 49 53 A a higher-strength nodular cast iron alloy is known, wherein the nodular cast iron alloy as non-iron constituents 3 to 3.6 wt.% C, 3.5 to 5 wt.% Si, 0.7 to 5 wt.% Ni, 0 to 0.3 wt.% Mo, 0.2 to 0.4 wt. % Mn, ≤0.06 wt% P and ≤0.015 wt% S. The disadvantage here is that in order to achieve the stated tensile strength ≥ 950 MPa, 0.2% proof stress ≥ 550 MPa and elongation at break of 6 to 10%, a ferritic-bainitic structure is required, which necessarily requires a ausferritisierende heat treatment.

From the US 370 22 69 A is a high strength high alloyed nodular cast iron alloy whose non-iron constituents comprise 2.6 to 4 wt% C, 1.5 to 4 wt% Si, 6 to 11 wt% Ni, ≤ 7 wt% Co, ≤ 0.4 wt% Mo, ≤1 wt% Mn and ≤0.2 wt% Cr. The high tensile strength of ≥ 1000 MPa is due to a fine-grained bainitic structure, the target structure must be adjusted by means of a required heat treatment in the form of tempering, which in turn requires more effort.

In the US 585 35 04 A describes an iron-based higher alloy cast material whose non-iron constituents comprise 0.8 to 3.5 wt% C, 1 to 7 wt% Si, 5 to 15 wt% Ni, ≤1 wt% Mn, ≤ 2 wt% Cr, ≤0.1% by weight of at least one element of the group Mg, Ca and Ce and ≤2 wt.% Of at least one element of the group Mo, Nb, Ti and V. The material has a hardness of at least 250 HV at a microstructural content of at least 30% martensite, the Graphite formation is predominantly spherulitic. The target product is a lapping disk, preferably for use in semiconductor production. In spite of an optional heat treatment, only 5 to 10% carbides and a large part of the martensitic matrix can be expected to have a low elongation at break due to the alloy contained in the alloy. For safety reasons, this excludes use for dynamically stressed motor vehicle casting products, such as structural / chassis parts.

From the US 354 94 30 A is a high-strength bainitic ductile iron alloy known, wherein the nodular cast iron alloy as non-iron constituents 2.9 to 3.9 wt.% C, 1.7 to 2.6 wt.% Si, 3.2 to 7 wt.% Ni, 0.15 to 0.4 wt.% Mo, ≤0.2 wt. % Cr and ≤1 wt.% Mn. The alloy is characterized by a high tensile strength ≥ 820 MPa, a 0.2% proof stress of ≥ 520 MPa with an elongation at break of at least 2%. In order to achieve these properties, a heat treatment is required; in addition, locally used cooling molds may be required for larger wall thicknesses.

Further describes DE 180 85 15 A1 a high strength nodular cast iron alloy whose non-iron components comprise 2.9 to 3.9 wt% C, 1.7 to 2.6 wt% Si, 3.2 to 7 wt% Ni, 0.15 to 0.4 wt% Mo, ≤0.1 wt% Mg, 0 to 1 wt.% Mn and 0 to 0.25 wt.% Cr at a total content of Mo and Cr of at most 0.5 wt.%. This material has a tensile strength of ≥ 1000 MPa and a 0.2% proof stress of ≥ 750 MPa with an elongation at break of at least 4%. The central feature of the material, however, is a heat treatment in the form of a tempering of several hours at temperatures of 200 to 315 ° C, since the specified parameters can not be achieved without starting the matrix structure.

Out EP 1 834 005 B1 is a high-strength, mainly pearlitic ductile iron alloy known for automotive applications. This contains the non-iron components 3.0 to 3.7 wt.% C, 2.6 to 3.4 wt.% Si, 0.02 to 0.05 % P, 0.025 to 0.045 wt% Mg, 0.01 to 0.03 wt% Cr, 0.003 to 0.017 wt% Al, 0.0005 to 0.012 wt% S and 0.0004 to 0.002 wt% B, 0.1 to 1.5 wt. % Cu, 0.1 to 1.0 wt% Mn and unavoidable impurities. The chassis components produced in this composition already have a tensile strength of 600 to 900 MPa, a 0.2% proof stress of 400 to 600 at an elongation at break of 14 to 5% already in the cast state without an additional heat treatment.

Starting from this prior art, it is a central object of the invention to provide a high-strength ductile iron alloy, the requirements of the 0.2% proof strength, tensile strength and elongation at break already in the cast state without further action are achieved, which advantageously in contrast to the known high-strength cast iron alloys such. ADI materials (= Austempered Ductile Iron) so no separate heat treatment required.

This object is achieved by the nodular cast iron alloy according to the invention comprising 2.8 to 3.7% by weight C, 1.5 to 4% by weight Si, 1 to 6.2% by weight Ni, 0.02 to 0.05% by weight P, 0.025 to 0.06% by weight Mg, 0.01 to 0.03 wt.% Cr, 0.003 to 0.3 wt.% Al, 0.0005 to 0.012 wt.% S, 0.03 to 1.5 wt.% Cu and 0.1 to 2 wt.% Mn, remainder Fe and unavoidable impurities achieved, the nodular cast iron alloy in the cast state without subsequent heat treatment achieves a high static strength of a 0.2% proof stress ≥600 MPa and a tensile strength ≥ 750 MPa with simultaneously good ductility of an elongation at break A5 of 2 to 10%.

The matrix structure surrounding the spherulitic graphite precipitates is perlitic-ferritic with> 50% perlite, preferably the perlite is finely striated and the ferrite is globular. As a result, in addition to the mechanical properties and the absence of the carbide formers Mo, Nb, Ti and V, the nodular cast iron alloy according to the invention differs markedly from that of the present invention US 585 35 04 A known alloy with a partially overlapping Ni alloy region. Likewise, there is a difference to this DE 10 2004 040 056 A1 well-known cast iron alloy, since mechanical properties of a needle-like ferrite differ significantly from those of a globular formed ferrite.

Preferably, the nodular cast iron alloy is formed as a sand nodular cast iron alloy.

The core idea of the invention is to specify a nodular cast iron alloy which can be used on the basis of suitably coordinated compositions of the nodular cast iron alloy according to the invention and the resulting combinations of mechanical properties in motor vehicle construction, for example for axle and chassis parts which must plastically deform in the event of a collision of the motor vehicle and not allowed to break, but also for structural parts and crankshafts, which are exposed to high dynamic loads.

It is worth noting that, in view of their mechanical properties and possible applications, the nodular cast iron alloy according to the invention already satisfies moderate alloy additions compared with austenitic nodular cast iron alloys.

Ni and Si are known to increase the 0.2% proof stress. This is attributed on the one hand to the solid solution hardening (Si and Ni), on the other hand to a pearlite refining by lowering the austenite-ferrite transformation temperature to lower temperatures (Ni). It is advantageous that the alloy has the highest possible 0.2% proof strength at not inconsiderable elongation at break values (high lightweight potential). This is achieved primarily by the nodular cast iron alloy having from 1 to 6.2% by weight of Ni, preferably 2.5 to 5.2% by weight of Ni and particularly preferably 4 to 5.2% by weight of Ni.
In particular in connection with 1.5 to 4 wt.% Si, preferably 2 to 3.5 wt.% Si and particularly preferably 2.2 to 3.3 wt.% Si good strength properties are achieved at not too low elongation at break values. For example, compared to the EP 1 225 239 A1 known bainitic alloy, which also requires no heat treatment, the 0.2% proof strength of the inventive pearlitic-ferritic ductile iron alloy with ≥600 MPa compared to ≥ 500 MPa significantly higher (tensile strength also slightly higher). So included in EP 1 225 239 A1 mentioned embodiments, no values of 0.2% proof strength above 550 MPa.

Compliance with the specified upper and lower limits for the non-iron constituents Si and Ni are decisive for the pearlitic-ferritic target structure and thus for the achievement of the mechanical properties of the ductile iron alloy according to the invention.
At Ni contents <1% by weight, there is no appreciable increase in yield strength. Contents> 6.2% by weight are to be avoided owing to an increased risk of martensite formation. With regard to this risk of martensite formation, the nodular cast iron alloy according to the invention has a distinct advantage over the alloy DE 10 2004 040 056 A1 with similar Ni content limits, so even with small wall thicknesses of about 8 mm, a sure martensitfreies structure is achieved without the need for a subsequent annealing. In a preferred embodiment of the nodular cast iron alloy according to the invention, this is possible by observing certain composition ratios of Ni, Si and Mn contents. Thus, it is preferable that for a martensite-free pearlitic-ferritic microstructure of the ductile iron alloy according to the invention, the sum of the contents of Ni and Si is ≦ 9 wt.%, At the same time the ratio should be (Ni + 0.5 * Mn) / (1.5 * Si) do not exceed 1.5.
Levels of Si <1.5 wt% increase the risk of carbide formation, in the worst case, white whitening may result. Contents of Si> 4 wt.% Lead to a significant decrease in the elongation at break and also increase the risk of martensite formation due to the reduced carbon solubility in austenite. In addition, the Si content must also be limited for the reason that silicon shifts the austenite-ferrite transformation temperature toward higher temperatures and thus counteracts pearlite refinement aimed at via nickel additions.

The alloying of 0.03 to 1.5% by weight of Cu takes place - in particular with respect to the limits specified for the ductile iron alloy according to the invention, low Ni contents and simultaneously high Si contents - to secure> 50% of the predominantly pearlitic microstructure for achieving the mechanical properties. Perlite, remainder ferrite, while ferrite is preferably globular.

Mn is in increasing proportions a scrap companion. For an increase in yield strength, Mn is advantageous up to a moderate level. Mn also lowers the martensite start temperature and can thus help to reduce the risk of martensite formation in faster cooling thin-walled component parts. The upper limit for the nodular cast iron alloy according to the invention of 2% by weight Mn is due to a strong embrittlement due to carbide formation, but an increase of segregating grain boundary carbides, especially with simultaneously higher Si contents, can be observed even at lower Mn contents.

The alloying of 0.003 to 0.3 wt.% Al can be carried out in order to achieve a further increase in strength through solid solution hardening. However, the Al content must be limited to <0.3% by weight, since Al simultaneously acts as a ferrite stabilizer and thus, contrary to the predominantly pearlitic microstructure with> 50% perlite necessary for the mechanical properties.

Compliance with the upper limits specified for the non-iron components Mn, Cu, Mg, Cr, Al, P, S are crucial for achieving the mechanical properties and the machinability of castings from the ductile iron alloy according to the invention. Excessive contents of Cu, Mg, Al and S can adversely affect the graphite formation, and corresponding deviations of the graphite shape from the desired spherulitic form lead to significant deterioration of elongation at break and attainable strength. Also embrittling acts Cr, in turn, by promoting carbide formation.
P is limited due to the well-known embrittling effect of low-melting P-rich phases that can form at grain boundaries (former, P-enriched residual melt areas).

Preferably, the graphite content is in the casting condition immediately after the casting process, i. after casting and cooling in the mold, formed spherical to more than 90% of the graphite present.

It is advantageous if the matrix structure of the casting immediately after the casting process in the cast state, ie after casting and cooling in the mold, is formed to 50 to 90% pearlitic.

In an advantageous embodiment, the structure of the casting immediately after the casting process in the cast state, i. after casting and cooling in the mold, from 200 to 1200 spherulites per mm 2.

The graphite particles preferably have a size distribution of at least 5% of the size 8, 40% to 70% of the size 7 and at most 35% of the size 6 according to DIN EN ISO 945.

It is advantageous if the casting has a Brinell hardness of 260 to 320 HBW.

An embodiment of the invention will be described as follows, the invention not being limited only to or by the following embodiment.

• 0.2%-Dehngrenze: 658 bis 663 MPa,
• Zugfestigkeit: 884 bis 889 MPa,
• Bruchdehnung: 6.2 bis 7.9 %,
• Elastizitätsmodul (ermittelt über Regression im Bereich 100 - 300 MPa): 175 bis 186 GPa.

Aus derselben Schmelze des beschriebenen Beispiels der erfindungsgemässen Sphärogusslegierung wurden auch Zugprobenrohlinge abgegossen, deren Gusswandstärke im Prüfbereich ca. 8 mm betrug. Daraus entnommene 6mm-Zugproben bestätigen die Y2-Proben-Resultate, erreicht werden konnten eine 0.2%-Dehngrenze von 652 MPa und eine Zugfestigkeit von 872 MPa bei einer Bruchdehnung von 6.9 %.
Damit liegen die Proben dieser Beispielvariante der erfindungsgemässen Sphärogusslegierung hinsichtlich der Zugprüfkennwerte bereits im Gusszustand in der Grössenordnung von ADI (=Austempered Ductile Iron), einem durch eine sehr aufwendige Wärmebehandlung erzeugten, in grösseren Wanddicken nur durch Zulegieren der Elemente Ni und/oder Mo realisierbaren und damit erwartungsgemäss teureren Sphärogusswerkstoff, der in Europa unter EN 1564 genormt ist.A Y2 sample was poured from the nodular cast iron alloy according to the invention into sand. The chemical composition is 2.87% by weight C, 5.12% by weight Ni, 3.25% by weight Si, 0.03% by weight Cu, 0.22% by weight Mn, 0.046% by weight Mg, 0.037% by weight P, 0.022% by weight Cr, 0.013 wt.% Al and 0.003 wt.% S, balance Fe and the usual impurities. The sum of the contents Ni + Si is thus ≈ 8.4 wt.% (≤ 9 wt.% Preferred), the ratio (Ni + 0.5 * Mn) / (1.5 * Si) ≈ 1.1 (≤ 1.5 preferred).
The casting was examined as cast for spherulite number, graphite content, graphite form and graphite size, perlite content, as well as for tensile strength, Brinell hardness and impact strength. The number of spherulites is 218 spherulites per mm 2, the graphite content is 10.6%. The graphite mold according to DIN EN ISO 945 is 94% of the form VI. The size distribution according to DIN EN ISO 945
is 8% of the size 8, 57% of the size 7 and 33% of the size 6. The pearlite content of the matrix is 79% (micrographs see illustration 1 , Remainder: ferrite, globular). The Brinell hardness is 310 +/- 2 HBW5 / 750. The impact energy of individual samples at room temperature was 30.1 J, respectively 12.5 J at -30 ° C.
The room temperature tensile tests according to DIN EN ISO 6892-1 gave the following characteristics:

• 0.2% proof stress: 658 to 663 MPa,
• Tensile strength: 884 to 889 MPa,
• Elongation at break: 6.2 to 7.9%,
• Modulus of elasticity (determined by regression in the range 100-300 MPa): 175 to 186 GPa.

From the same melt of the described example of the spheroidal cast iron alloy according to the invention, train blanks were also cast off whose cast wall thickness in the test area was about 8 mm. The resulting 6mm tensile specimens confirm the Y2 sample results, yielding a 0.2% yield strength of 652 MPa and a tensile strength of 872 MPa at an ultimate elongation of 6.9%.
Thus, the samples of this example variant of the nodular cast iron alloy according to the invention are already cast in the cast state in the order of magnitude of ADI (= austempered ductile iron), produced by a very complex heat treatment, can be realized in larger wall thicknesses only by alloying the elements Ni and / or Mo. Expectantly more expensive nodular cast iron, which is standardized in Europe under EN 1564.

By way of illustration is in Figure 2 the yield strength Rp0.2 is shown as a function of the breaking elongation A5. Registered is the described embodiment of the inventive nodular cast iron alloy as well as representatives of the standardized in DIN EN 1563 and DIN EN 1564 ductile iron alloys. The gray lines in Figure 2 combine the minimum values according to the standard DIN EN 1563 for spheroidal graphite cast irons of grades produced in the cast state. The solid black line in Figure 2 combines the minimum values according to DIN EN 1564 for spheroidal graphite cast iron of heat-treated ADI grades. Black dashed lines show patented nodular cast iron alloys from Georg Fischer ( EP 1 834 005 B1 and EP 1 270 747 B1 ).

Claims (13)

Spheroidal cast iron alloy with pearlitic-ferritic structure for cast iron products with a high strength at the same time good ductility and toughness already in the casting state, comprising as non-iron components C, Si, Ni, Mn, Cu, Mg, Cr, Al, P, S and the usual impurities , characterized in that the spheroidal cast iron alloy 2.8 to 3.7% by weight of C, 1.5 to 4% by weight of Si, 1 to 6.2% by weight of Ni, 0.02 to 0.05% by weight P, 0.025 to 0.06 wt.% Mg, 0.01 to 0.03 wt.% Cr, 0.003 to 0.3% by weight Al, 0.0005 to 0.012% by weight of S, 0.03 to 1.5 wt.% Cu and 0.1 to 2% by weight of Mn, Residual Fe and unavoidable impurities containing the nodular cast iron alloy in the cast state without subsequent heat treatment, a high static strength of 0.2% proof stress ≥600 MPa and a tensile strength ≥750 MPa with good ductility of an elongation at break A5 of 2 to 10% achieved. Ductile cast iron alloy according to Claim 1, characterized in that the alloy contains 2 to 3.5% by weight of Si, particularly preferably 2.2 to 3.3% by weight of Si, the sum of the alloy contents of Ni and Si being ≦ 9% by weight and at the same time the ratio ( Ni + 0.5 * Mn) / (1.5 * Si) ≤ 1.5, whereby on cooling from the casting heat to room temperature a purely pearlitic-ferritic microstructure> 50% perlite, the remainder ferrite sets. Ductile cast iron alloy according to one of claims 1 or 2, characterized in that the alloy contains 2.5 to 5.2 wt.% Ni, particularly preferably 4.0 to 5.2 wt.% Ni, wherein the sum of the alloy contents of Ni and Si ≤ 9 wt.% Is and at the same time the ratio (Ni + 0.5 * Mn) / (1.5 * Si) ≤ 1.5, whereby on cooling from the casting heat up to At room temperature a purely pearlitic-ferritic microstructure> 50% perlite, remainder sets ferrite. Ductile cast iron alloy according to one of Claims 1 to 3, characterized in that the alloy contains 0.2 to 0.5% by weight of Mn, particularly preferably 0.15 to 0.4% by weight of Mn, the sum of the alloy contents of Ni and Si being ≦ 9% by weight, and at the same time the ratio (Ni + 0.5 * Mn) / (1.5 * Si) ≤ 1.5, whereby on cooling from the casting heat up to room temperature a purely pearlitic-ferritic microstructure> 50% perlite, remainder ferrite sets. Ductile cast iron alloy according to claim 1, characterized in that the alloy contains 0.03 to 0.5% by weight of Cu, particularly preferably 0.03 to 0.1% by weight of Cu, the sum of the alloy contents of Ni and Si being ≦ 9% by weight and at the same time the ratio ( Ni + 0.5 * Mn) / (1.5 * Si) ≤ 1.5, whereby on cooling from the casting heat to room temperature a purely pearlitic-ferritic microstructure> 50% perlite, the remainder ferrite sets. Spheroidal cast iron alloy according to claim 1, characterized in that the alloy contains 0.003 to 0.25% by weight Al, more preferably 0.003 to 0.02% by weight Al, the sum of the alloy contents of Ni and Si being ≦ 9% by weight and at the same time the ratio ( Ni + 0.5 * Mn) / (1.5 * Si) ≤ 1.5, whereby on cooling from the casting heat to room temperature a purely pearlitic-ferritic microstructure> 50% perlite, the remainder ferrite sets. Ductile cast iron alloy according to one of claims 1 to 6, characterized in that the graphite part is formed spherical immediately after the casting and cooling to more than 90% of the graphite present. Ductile cast iron alloy according to one of claims 1 to 7, characterized in that the pearlitic-ferritic matrix structure of the casting immediately after the casting and cooling to 55 to 90% perlitic and the remaining ferrite is preferably globular. Spheroidal cast iron alloy according to one of claims 1 to 8, characterized in that the structure of the casting immediately after casting and cooling 200 to 1200 spherulites per mm ² . Ductile cast iron alloy according to one of claims 1 to 9, characterized in that the graphite particles have a size distribution of at least 5% of the size 8, 40% to 70% of the size 7 and at most 35% of the size 6 according to DIN EN ISO 945. Ductile cast iron alloy according to one of claims 1 to 10, characterized in that the casting has a Brinell hardness of 260 to 320 HBW. Use of a nodular cast iron alloy according to claim 1 for the production of chassis parts in motor vehicles with a high static strength of a 0.2% proof stress ≥600 MPa and a tensile strength ≥750 MPa with simultaneously good ductility of an elongation at break A5 of 2 to 10%, preferably of wheel carriers, Swing bearings, axle links, crankshafts and / or rear axle housings in motor vehicles. A method for producing a cast iron from the ductile iron alloy according to claim 1, characterized in that after casting and cooling of the casting no heat treatment of the casting takes place and the casting has a high static strength of 0.2% proof stress ≥600 MPa and a tensile strength ≥750 MPa has at the same time good ductility of an elongation at break A5 of 2 to 10%.

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