Patent Application
20180274066
This application claims the benefit and priority of European Patent Application No. 17 162 715.1 filed Mar. 24, 2017. The entire disclosure of the above application is incorporated herein by reference.
The invention relates to a nodular cast alloy having a perlitic-ferritic microstructure for cast iron products having, even in the cast state without subsequent heat treatment, a high static strength of a 0.2% offset yield strength of ≥600 MPa and a tensile strength of ≥750 MPa combined with good ductility of an elongation at break of from 2% to 10%, comprising the nonferrous constituents C, Si, P, Mg, S, Mn and Ni and also normal impurities. Possible uses for motor vehicle construction are, for example, chassis components such as wheel carriers, structural vehicle components and also crankshafts.
This section provides background information related to the present disclosure which is not necessarily prior art.
Higher-strength cast iron alloys which display higher strengths for potential exploitation of weight reduction are increasingly being used in motor vehicle construction. For cost reasons, the focus is on dispensing with any heat treatment processes where possible and also achievement of the required mechanical properties at only moderate amounts of alloying constituents.
EP 1 225 239 A1 discloses a higher-strength bainitic nodular cast alloy which comprises, as nonferrous constituents, from 2 to 4% by weight of Ni and from 0.05 to 0.45% by weight of Mn, with the Ni—Mn range serving to adjust the variable ratio of strength to elongation. To implement the invention, preference is given to the nonferrous constituents being from 3.1 to 4% by weight of C and from 1.8 to 3% by weight of Si. A material having this composition at this microstructure displays a high tensile strength of from 650 to 850 MPa and a 0.2% offset yield strength of ≥500 MPa combined with an elongation at break of from 14.5 to 7%. Although these properties are achieved without heat treatment, the achievable strengths are limited by the alloy composition.
DE 10 2004 040 056 A1 discloses a further cast iron alloy which is described as high-strength and wear resistant and corrosion resistant. It is composed of from 3 to 4.2% by weight of C, from 1 to 3.5% by weight of Si, from 1 to 6% by weight of Ni, ≤5% by weight of Cr, ≤3% by weight of Cu, ≤3% by weight of Mo, ≤1% by weight of Mn, ≤1% by weight of V, ≤0.4% by weight of P, ≤0.1% by weight of S, ≤0.08% by weight of Mg, ≤0.3% by weight of Sn and production-related impurities. These wide alloy ranges result in a variety of matrix compositions of >50% of acicular ferrite with different proportions of austenite (<20%), martensite (<30%), perlite (<50%) and carbides (<15%); graphite formation is not restricted to spheroidal graphite but can also be of a vermicular and lamellar type. Flexural fracture strengths which can be achieved for the use example of a piston ring are >1100 MPa and the hardness is 320 HB2.5; a high toughness/ductility which is not specified in more detail is emphasized. However, the elongation at break could, in particular, be reduced significantly in the case of alloy variants having 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 of <700° C. can also be necessary.
A higher-strength nodular cast alloy is known from CA 122 40 66 A1/US 448 49 53 A, with the nodular cast alloy containing, as nonferrous constituents, from 3 to 3.6% by weight of C, from 3.5 to 5% by weight of Si, from 0.7 to 5% by weight of Ni, from 0 to 0.3% by weight of Mo, from 0.2 to 0.4% by weight of Mn, ≤0.06% by weight of P and ≤0.015% by weight of S. A disadvantage here is that a ferritic-bainitic microstructure, for which a ferritizing heat treatment is absolutely necessary, is required to achieve the indicated tensile strength of ≥950 MPa, 0.2% offset yield strength of ≥550 MPa and elongation at break of from 6 to 10%.
US 370 22 69 A discloses a high-strength relatively highly alloyed nodular cast alloy whose nonferrous constituents comprise from 2.6 to 4% by weight of C, from 1.5 to 4% by weight of Si, from 6 to 11% by weight of Ni, ≤7% by weight of Co, ≤0.4% by weight of Mo, ≤1% by weight of Mn and ≤0.2% by weight of Cr. The high tensile strength of ≥1000 MPa is due to a fine-grained bainitic microstructure, with the targeted microstructure having to be set by means of a necessary heat treatment in the form of tempering, which in turn requires an additional outlay.
US 585 35 04 A describes an iron-based relatively highly alloyed cast material whose nonferrous constituents comprise from 0.8 to 3.5% by weight of C, from 1 to 7% by weight of Si, from 5 to 15% by weight of Ni, ≤1% by weight of Mn, ≤2% by weight of Cr, ≤0.1% by weight of at least one element from the group consisting of Mg, Ca and Ce and ≤2% by weight of at least one element from the group consisting of Mo, Nb, Ti and V. The material has a hardness of at least 250 HV at a proportion of at least 30% of martensite in the microstructure; the graphite formation is predominantly spheroidal. As target product, mention is made of a lapping disc, preferably for use in semiconductor manufacture. Despite an optional heat treatment, only a low elongation at break can be assumed because of the from 5 to 10% of carbides present in the alloy and the largely martensitic matrix. For safety reasons, this rules out use for dynamically stressed motor vehicle cast products such as structural/chassis components.
A higher-strength bainitic nodular cast alloy is known from US 354 94 30 A, where the nodular cast alloy contains, as nonferrous constituents, from 2.9 to 3.9% by weight of C, from 1.7 to 2.6% by weight of Si, from 3.2 to 7% by weight of Ni, from 0.15 to 0.4% by weight of Mo, ≤0.2% by weight of Cr and ≤1% by weight of Mn. The alloy displays a high tensile strength of 2 820 MPa, a 0.2% offset yield strength of ≥520 MPa combined with an elongation at break of at least 2%. In order to achieve these properties, a heat treatment is necessary, and locally used chill moulds can additionally be necessary in the case of relatively large wall thicknesses.
Furthermore, DE 180 85 15 A1 describes a high-strength nodular cast alloy whose nonferrous constituents comprise from 2.9 to 3.9% by weight of C, from 1.7 to 2.6% by weight of Si, from 3.2 to 7% by weight of Ni, from 0.15 to 0.4% by weight of Mo, ≤0.1% by weight of Mg, from 0 to 1% by weight of Mn and from 0 to 0.25% by weight of Cr with a total content of Mo and Cr of not more than 0.5% by weight. This material has a tensile strength of ≥1000 MPa and a 0.2% offset yield strength of ≥750 MPa combined with an elongation at break of at least 4%. However, the central feature of this material is a heat treatment in the form of tempering for a number of hours at temperatures of from 200 to 315° C., since the properties indicated cannot be achieved without tempering of the matrix microstructure.
A higher-strength, predominantly perlitic nodular cast alloy for applications in motor vehicle construction is known from EP 1 834 005 B1. This contains the nonferrous constituents of from 3.0 to 3.7% by weight of C, from 2.6 to 3.4% by weight of Si, from 0.02 to 0.05% by weight of P, from 0.025 to 0.045% by weight of Mg, from 0.01 to 0.03% by weight of Cr, from 0.003 to 0.017% by weight of Al, from 0.0005 to 0.012% by weight of S and from 0.0004 to 0.002% by weight of B, from 0.1 to 1.5% by weight of Cu, from 0.1 to 1.0% by weight of Mn and unavoidable impurities. The chassis components produced with this composition have, even in the cast state without an additional heat treatment, a tensile strength of from 600 to 900 MPa, a 0.2% offset yield strength of from 400 to 600 combined with an elongation at break of from 14 to 5%.
Proceeding from this prior art, it is a central object of the invention to provide a high-strength nodular cast alloy whose requirements in terms of the 0.2% offset yield strength, tensile strength and elongation at break can readily be achieved even in the cast state, i.e. which advantageously does not require a separate heat treatment, in contrast to the known high-strength cast iron alloys such as ADI materials (=Austempered Ductile Iron).
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
Example embodiments will now be described more fully with reference to the accompanying drawings.
The above object is achieved by the nodular cast alloy of the invention comprising from 2.8 to 3.7% by weight of C, from 1.5 to 4% by weight of Si, from 1 to 6.2% by weight of Ni, from 0.02 to 0.05% by weight of P, from 0.025 to 0.06% by weight of Mg, from 0.01 to 0.03% by weight of Cr, from 0.003 to 0.3% by weight of Al, from 0.0005 to 0.012% by weight of S, from 0.03 to 1.5% by weight of Cu and from 0.1 to 2% by weight of Mn, balance Fe and unavoidable impurities, where the nodular cast alloy in the cast state without subsequent heat treatment achieves a high static strength of a 0.2% offset yield strength of ≥600 MPa and a tensile strength of ≥750 MPa combined with good ductility of an elongation at break A5 of from 2 to 10%.
The matric microstructure surrounding the spheroidal graphite precipitates has a perlitic-ferritic structure comprising >50% of perlite; the perlite is preferably present as fine streaks and the ferrite is preferably present in globular form. This is another significant difference between the nodular cast alloy of the invention and the alloy known from US 585 35 04 A having a partly overlapping Ni alloying range in addition to the mechanical properties and the omission of the carbide formers Mo, Nb, Ti and V. Likewise, the alloy differs from the cast iron alloy known from DE 10 2004 040 056 A1 since the mechanical properties of an acicular ferrite differ significantly from those of a globular ferrite.
The nodular cast alloy is preferably in the form of a nodular cast alloy cast in sand.
The key concept of the invention is to provide a nodular cast alloy which, owing to appropriately matched compositions of the nodular cast alloy of the invention and the resulting combinations of mechanical properties, can be used in motor vehicle construction, for example for axle and chassis components, which in the case of collision of the motor vehicle have to deform plastically and must not break, but also for structural components and crankshafts which are subjected to high dynamic stresses.
It is worth mentioning that, given its mechanical properties and possible uses, the nodular cast alloy of the invention requires only moderate alloying additions compared to austenitic nodular cast alloys.
It is known that Ni and Si increase the 0.2% offset yield strength. This is attributed firstly to mixed crystal strengthening (Si and Ni) and secondly to perlite refining by shifting the austenite-ferrite transformation temperature to low temperatures (Ni). It is advantageous that the alloy has a very high 0.2% offset yield strength at elongation at break values which are not too low (high lightweight construction potential). This is achieved first and foremost by the nodular cast alloy comprising from 1 to 6.2% by weight of Ni, preferably from 2.5 to 5.2% by weight of Ni and particularly preferably from 4 to 5.2% by weight of Ni.
Good strength properties combined with elongation at break values which are not too low are achieved particularly in combination with from 1.5 to 4% by weight of Si, preferably from 2 to 3.5% by weight of Si and particularly preferably from 2.2 to 3.3% by weight of Si. For example, compared to the bainitic alloy known from EP 1 225 239 A1, which likewise does not require any heat treatment, the 0.2% offset yield strength of the perlitic-ferritic nodular cast alloy of the invention is significantly higher at ≥600 MPa compared to ≥500 MPa (tensile strength likewise somewhat higher). Thus, the working examples given in EP 1 225 239 A1 do not contain any values of the 0.2% offset yield strength above 550 MPa.
Adherence to the indicated upper and lower limits for the nonferrous constituents Si and Ni are critical for the perlitic-ferritic target microstructure and thus for achievement of the mechanical properties of the nodular cast alloy of the invention.
At Ni contents of ≤1% by weight, no significant offset yield strength increase is observed; contents >6.2% by weight are to be avoided because of an increased risk of martensite formation. In respect of this risk of martensite formation, the nodular cast alloy of the invention has a significant advantage over the alloy of DE 10 2004 040 056 A1 having similar Ni content limits: thus, even at low wall thicknesses of about 8 mm a reliably martensite-free microstructure is achieved without the need for a subsequent tempering step. In a preferred embodiment of the nodular cast alloy of the invention, this can be achieved by adherence to particular composition ratios of Ni, Si and Mn contents. Preference is therefore given to the sum of the contents of Ni and Si being ≤9% by weight in order to achieve a martensite-free perlitic-ferritic microstructure in the cast state, while at the same time the ratio (Ni+0.5*Mn)/(1.5*Si) should not exceed a value of 1.5.
Contents of Si of <1.5% by weight increase the risk of carbide formation; in the worst case, solidification as white cast iron can be the result. Contents of Si of >4% by weight lead to a significant decrease in the elongation at break and, owing to the reduced solubility of carbon in austenite, likewise increase the risk of martensite formation. In addition, the Si content should also be limited because silicon shifts the austenite-ferrite transformation temperature to higher temperatures and thus acts counter to the perlite refinement sought by means of additions of nickel.
The addition of from 0.03 to 1.5% by weight of Cu to the alloy is carried out, particularly with regard to the low limiting Ni contents indicated for the nodular cast alloy of the invention at simultaneously high Si contents, to ensure the predominantly perlitic microstructure comprising >50% of perlite, balance ferrite, necessary for achieving the mechanical properties; ferrite is preferably present in globular form.
Mn is, in increasing proportions, a scrap accompaniment. Up to a moderate content, Mn is advantageous for increasing the offset yield strength. In addition, Mn reduces the martensite start temperature and can thus contribute to reducing the risk of martensite formation in thin-walled component parts which cool more rapidly. The upper limit of 2% by weight of Mn for the nodular cast alloy of the invention is determined by great embrittlement due to carbide formation; however, an increase in segregating grain boundary carbides is found even at lower Mn contents, especially at simultaneously relatively high Si contents.
The addition of from 0.003 to 0.3% by weight of Al to the alloy can be carried out in order to achieve a further increase in strength due to mixed crystal strengthening. However, the content of Al should be limited to <0.3% by weight since Al simultaneously acts as ferrite stabilizer and thus contrary to the predominantly perlitic microstructure comprising >50% of perlite which is necessary for the mechanical properties.
Adherence to the indicated upper limits for the nonferrous constituents Mn, Cu, Mg, Cr, Al, P, S are critical for achieving the mechanical properties and also for the processability of cast parts composed of the nodular cast alloy of the invention. Excessive contents of Cu, Mg, Al and S can have an adverse effect on graphite formation, and deviations of the graphite shape from the desired spheroidal shape lead to significant worsening of elongation at break and achievable strength. Cr likewise has an embrittling effect, in this case by promotion of carbide formation.
P has to be limited because of the well-known embrittling effect of low-melting P-rich phases which can be formed at grain boundaries (former, P-enriched residual melt regions).
Preference is given to more than 90% of the graphite present in the cast state immediately after the casting process, i.e. after casting and cooling in the mould, being spherical.
It is advantageous for the matrix microstructure of the cast part in the cast state immediately after the casting process, i.e. after casting and cooling of the mould, to be made up to an extent of from 50 to 90% of perlite.
In an advantageous embodiment, the microstructure of the cast part in the cast state immediately after the casting process, i.e. after casting and cooling in the mould, has from 200 to 1200 spheroids per mm2.
The graphite particles preferably have a size distribution of at least 5% of the size 8, from 40% to 70% of the size 7 and not more than 35% of the size 6, in accordance with DIN EN ISO 945.
It is advantageous for the cast part to have a Brinell hardness of from 260 to 320 HBW.
A working example of the invention will be described below, but the invention is not restricted only to or by the following working example.
An Y2 specimen was cast in sand from the nodular cast alloy of the invention. The chemical composition is 2.87% by weight of C, 5.12% by weight of Ni, 3.25% by weight of Si, 0.03% by weight of Cu, 0.22% by weight of Mn, 0.046% by weight of Mg, 0.037% by weight of P, 0.022% by weight of Cr, 0.013% by weight of Al and 0.003% by weight of S, balance Fe and usual impurities. The sum of the Ni+Si contents is thus ≈8.4% by weight (≤9% by weight is preferred), and the ratio (Ni+0.5*Mn)/(1.5*Si) is =1.1 (≤1.5 is preferred). The casting was examined in the cast state for spheroid count, graphite content, graphite shape and graphite size, perlite content and also in respect of properties from the tensile test and in respect of Brinell hardness and impact work. The spheroid count is 218 spheroids per mm2, and the graphite content is 10.6%. The graphite shape in accordance with DIN EN ISO 945 is 94% of the shape VI. The size distribution in accordance with DIN EN ISO 945 is 8% of the size 8, 57% of the size 7 and 33% of the size 6. The perlite content of the matrix is 79% (for image of the microstructure, see
Tensile specimen blanks whose cast wall thickness in the test region was about 8 mm were also cast from the same melt of the above-described example of the nodular cast alloy of the invention. 6 mm tensile specimens taken therefrom confirmed the Y2 specimen results: a 0.2% offset yield strength of 652 MPa and a tensile strength of 872 MPa combined with an elongation at break of 6.9% could be achieved.
The specimens of this illustrated variant of the nodular cast alloy of the invention are, in respect of the tensile test property values, even in the cast state in the order of magnitude of ADI (=Austempered Ductile Iron), a nodular cast material which is standardized in Europe under EN 1564 and is produced by means of a very complicated heat treatment, can be obtained in relatively large wall thicknesses only by addition of the elements Ni and/or Mo to the alloy and is thus, as expected, expensive.
For illustration, the offset yield strength Rp0.2 is shown as a function of the elongation at break A5 in
The grey lines in
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17162715.1 | Mar 2017 | EP | regional |
