Casting material made of a copper-zinc alloy, method for producing a cast product and cast part

Abstract

The invention relates to a casting material made of a copper-zinc alloy with the following composition in wt.%: Cu: 58.0 to 66.0 %, Si: 0.15 bis 1,2 %, P: 0.20 to 0.38 %, Sn: optionally up to 0.5 %, Al: optionally up to 0.05 %, Fe: optionally up to 0.3 %, Ni: optionally up to 0.3 %, Pb: optionally up to 0.25 %, Bi: optionally up to 0.1 %, Te, Se, In, each optionally up to 0.1 %, B: optionally up to 0.01 %, with the rest being Zn and unavoidable impurities, wherein the proportion of unavoidable impurities is less than 0.2 wt.%. The alloy has a structure of α-phase, β-phase and phosphide particles. The proportion of β-phase in the sum of the α-phase and β-phase is at least 20 vol.% and max. 70 vol.%. Silicon is present in both the α-phase and β-phase. In an area of 21000 μm2, there are 20 to 300 phosphide particles with an equivalent diameter of 0.5 to 1 μm, 30 to 120 phosphide particles with an equivalent diameter of 1 to 2 μm, and 20 to 100 phosphide particles with an equivalent diameter of 2 to 5 μm. The proportion of β-phase and the proportions of Si and P are selected such that the condition 92, 7249-0, 473254 [Beta]-80,6378 [Si]-142,65 [P]+279,309 [Si] [P] < 40 is fulfilled, wherein [Beta] represents the proportion of β-phase in vol.%, [Si] represents the proportion of silicon in wt.% and [P] represents the proportion of phosphorus in wt.%.

Description

[0001] Description

[0002] Casting material made of a copper-zinc alloy, method for producing a cast product and casting

[0003] The invention relates to a casting material made of a copper-zinc alloy, a method for producing a cast product and a cast part.

[0004] Copper-zinc casting materials (also called "casting alloys") consisting of the α- and β-phases with approximately 1 wt.% lead, such as CuZn39Pb1Al-B (CB757S), are excellently castable, very easy to machine, and also easy to polish. Lead-containing copper-zinc casting alloys are used in a wide variety of applications in the plumbing sector, especially for faucets and fittings in the drinking water sector.

[0005] The positive effect of lead in copper-zinc alloys is based on the fact that lead is present in elemental form as particles in the structure, and these particles act as chip breakers. During machining, lead is present as a liquid phase due to the strong local deformation in the workpiece and the resulting local temperature increase. Since lead cannot absorb stresses in its liquid state, this leads to a stress concentration in the load-bearing, weakened matrix and thus to easier chip breakage. In addition, lead is incorporated into the tribological layer between the material and the tool during machining, thus leading to effective lubrication and thus a reduction in friction and wear. In addition, it is known that

[0006] CONFIRMATION COPY Lead causes significant grain refinement in copper-zinc alloys. This is particularly beneficial for polishability in cast materials. Furthermore, lead is inexpensive.

[0007] However, lead is harmful to the environment. Even the smallest amounts of lead accumulate in the human body and can cause health problems. Therefore, the EU, the USA, China, and other countries have continually reduced the limits for copper alloys and are striving to replace lead-containing brass with lead-reduced or lead-free machinable copper alloys. Limits are set by EU directives, such as Directive 2011 / 65 / EU (RoHS), which stipulates a maximum of 1000 ppm (0.1%) lead. To ensure good machinability of the material even at such low lead levels, various alloying elements are proposed as alternatives to lead.

[0008] Numerous publications disclose the use of bismuth (Bi) as an alternative to lead to improve machinability. To mitigate the problem of Bi film formation along grain boundaries and the associated susceptibility to stress cracking and hot cracking, the addition of additional elements is proposed. Reference is made in particular to publications KR 10 0 555 854 B1, KR 10 2006 096 877 A, JP 2005 290 475 A, JP 2014 122 427 A, and JP 2006 083443 A. Nevertheless, Bi is undesirable because, on the one hand, it is a scarce, limited-abundance metal and, on the other hand, it leads to hot brittleness within the material cycles of copper materials.

[0009] Furthermore, copper-zinc alloys are known from the document EP 2 194 150 B1 which contain 0.1 to 1.5 wt.% Si, 0.03 to 0.4 wt.% Al, 0.01 to 0.36 wt.% P, 0.05 to 0.5 wt.% Sn and 0.001 to 0.05 wt.% rare earths. The alloys are easy to machine due to the formation of an α, β and possibly γ microstructure. The Al content leads to the formation of undesirable Al phosphides. Although the γ-phase and Al phosphides improve chip formation, they reduce the service life of the tool. In addition, the proportion of rare earths probably leads to embrittlement of the microstructure. The alloys are used for castings and hot-pressed parts.

[0010] The replacement of lead with phosphorus, which forms brittle phosphides in the alloy, is further described in EP 3 992 321 A1 for a cast alloy containing Cu from 58.5 to 65.0 wt.%, Si from 0.40 to 1.40 wt.%, P from 0.003 to 0.19 wt.%, Pb from 0.002 to 0.25 wt.%, the remainder zinc, and other optional elements. The addition of 0.003 to 0.19 wt.% P to form phosphides and 0.4 to 1.4 wt.% Si to harden the α- and β-crystallites results in a readily machinable cast material.

[0011] The invention is based on the object of providing a cast material made of a copper-zinc alloy that exhibits excellent machinability, good mechanical properties, a small grain size and thus good polishability, a low tendency to shrink holes, and the lowest possible content of environmentally harmful alloy components. Furthermore, the invention is based on the object of specifying a method for producing a cast product and a cast part.

[0012] The invention is described with respect to a cast material made of a copper-zinc alloy by the features of claim 1 and with respect to a manufacturing method by the features of claim 7. The further dependent claims relate to advantageous embodiments and developments of the invention. The invention relates to a cast material made of a copper-zinc alloy with the following composition in weight percent:

[0013] Cu: 58.0 to 66.0%,

[0014] Si: 0.15 to 1.2%,

[0015] P: 0.20 to 0.38%,

[0016] Sn: optional up to 0.5%,

[0017] AI: optional up to 0.05%,

[0018] Fe: optional up to 0.3%,

[0019] Ni: optional up to 0.3%,

[0020] Pb: optionally up to 0.25%, preferably up to 0.10%

[0021] Bi: optional up to 0.1%,

[0022] Te, Se, In optionally up to 0.1% each, B: optionally up to 0.01%,

[0023] The remainder is Zn and unavoidable impurities, with the proportion of unavoidable impurities being less than 0.2 wt.%. The alloy has a microstructure of α-phase, β-phase, and phosphide particles. The phosphide particles preferably contain or are copper- and / or zinc-containing phosphides. The proportion of β-phase in the sum of α-phase and β-phase is at least 20 vol.%, preferably at least 40 vol.%, and at most 70 vol.%, preferably at most 60 vol.%. Silicon is present in both the α-phase and the β-phase. In an area of ​​21,000 pm 2 20 to 300 phosphide particles with an equivalent diameter of 0.5 to 1 pm, 30 to 120 phosphide particles with an equivalent diameter of 1 to 2 pm and 20 to 100 phosphide particles with an equivalent diameter of 2 to 5 pm are present. The volume fraction of the ß-phase as well as the proportions of Si and P are selected so that the alloy meets the condition

[0024] 92, 7249-0, 473254 [Beta]-80.6378 [Si]-142.65 [P]+279.309 [Si] [P] < 40, where [Beta] denotes the proportion of the ß-phase in vol%, [Si] the proportion of silicon in weight% and [P] the proportion of phosphorus in weight%.

[0025] The invention is based on the idea of ​​reducing the proportion of lead in the copper-zinc alloy as much as possible without impairing the material's machinability. To this end, Si and P are selectively added to the alloy, and the volume fraction of the ß-phase is adjusted to achieve favorable machining properties while refining the cast structure of the original ß-phase matrix. Furthermore, the casting process is selected to achieve the desired properties.

[0026] A small ß-grain size is advantageous for good polishability. Therefore, the ß-phase must be fine-grained in the as-cast state. Surprisingly, it has been shown that with increasing P content, a significant grain refinement of the cast structure of the original basic matrix of ß-phase occurs. To achieve sufficient grain refinement of the cast structure, the addition of at least 0.20 wt.% P is necessary. This is similar to the effect of 1 to 3 wt.% Pb on the grain refinement of α-ß-brass. During the primary crystallization of the ß-crystallites, the residual melt becomes enriched with P, leading to subdivision and thus to grain refinement of the ß-phase. During solidification, a eutectic of phosphide and ß-phase forms. In addition to the grain refinement of the basic matrix of ß-phase, grain refinement of the α-crystallites is observed.

[0027] With a P content of at least 0.20 wt.%, phosphide particles are present in the cast material in both the α-phase and the β-phase. The alloy preferably contains at least 0.22 wt.% P. With a P content of more than 0.38 wt.%, coarse phosphides form in the as-cast state, with individual phosphides coagulating and forming long network-like structures. These coarse phosphides wet the grain boundaries and reduce ductility. For use as mechanical components, such as drinking water valves or connections, the material requires high strength corresponding to high hardness.

[0028] However, coarse, undesirable phosphides can also occur at P contents below 0.38 wt.% if the cooling rate during casting of the alloy is too low, such as when casting into a stationary mold. The necessary high cooling rates are achieved, for example, in the Tammann casting of small 25 x 55 x 160 mm ingots into a steel mold. In this way, with a P content of 0.20 to 0.38 wt.%, the phosphide particles are already present in the as-cast state in a globular and finely distributed structure. These act as separation points during machining and promote chip breakage. In order to adjust the phosphide particles appropriately, the cooling rate during solidification must be at least 20 °C per minute (°C / min), preferably at least 30 °C per minute, and at most 60 °C per minute, preferably at most 50 °C per minute, in a temperature range of 550 °C to 350 °C.The distribution of phosphides and the grain size of the ß-phase are therefore determined not only by the chemical composition of the alloy, but also by the casting conditions. The characteristics of the phosphides in the as-cast state are thus like a fingerprint left on the product by the specific process. The distribution of the phosphides in the as-cast state can be characterized as follows: In an area of ​​21,000 pm. 2There are 20 to 300 phosphide particles with an equivalent diameter of 0.5 to 1 pm, 30 to 120 phosphide particles with an equivalent diameter of 1 to 2 pm, and 20 to 100 phosphide particles with an equivalent diameter of 2 to 5 pm. The equivalent diameter of a phosphide particle is understood to be the diameter of a circle with the same area as the phosphide particle. The majority of the phosphide particles with an equivalent diameter of at least 0.5 pm have an equivalent diameter of 2 pm at most. The alloy is also suitable for continuous casting and mold casting, for example in the form of sand casting or permanent mold casting, provided the cooling conditions are observed.

[0029] Brittle structural components are advantageous for the machinability of the material, acting as separation points during machining and thus promoting chip breaking. The ß-phase is brittle and promotes machinability. An increase in the proportion of ß-phase can be achieved by increasing the Zn content and / or by alloying with silicon, because silicon stabilizes the ß-phase. For good machinability, it has also proven advantageous to reduce the ductility of the α-phase. This is achieved by alloying and incorporating silicon into the α-phase as well as by finely distributed phosphides in the α-phase. Therefore, the Si content in the alloy must be at least 0.15 wt.%. The phosphide particles described above represent particles that act as separation points during machining and promote chip breaking. A P content of at least 0.20 wt.% not only improves machinability but, in particular, refines the grain.Furthermore, a small optional proportion of Pb has a beneficial effect on machinability.

[0030] The machinability of the material is therefore determined by the combined selection of the parameters ß-phase, Si and P and an optionally present, small proportion of Pb. In the proposed cast material made of a copper-zinc alloy, the proportion of the ß-phase in the sum of the α-phase and ß-phase is at least 20 vol.%, preferably at least 35 vol.%, particularly preferably at least 40 vol.%. A high proportion of the ß-phase has a negative effect on the ductility. Therefore, the proportion of the ß-phase is at most 70 vol.%, preferably at most 60 vol.%. The Si content of the alloy is 0.15 to 1.2 wt.%, the P content 0.20 to 0.38 wt.%. Furthermore, up to 0.25 wt.% Pb, preferably up to a maximum of 0.10 wt.% Pb, can be added. The proportion of the ß-phase as well as the proportions of Si and P are selected so that the alloy meets the condition

[0031] 92, 7249-0, 473254 [Beta]-80, 6378 [Si]-142, 65 [P]+279, 309 [Si] [P] < 40, where [Beta] denotes the proportion of the ß-phase in vol.%, [Si] the proportion of silicon in weight%, and [P] the proportion of phosphorus in weight%. This relationship quantitatively describes the influence of the parameters ß-phase, Si, and P on the machining properties of the alloy, as well as the interaction between these parameters. For example, a low proportion of ß-phase can be compensated by a higher proportion of silicon and / or phosphorus within the scope of the specification of the alloy composition, and vice versa.

[0032] The Cu content of the alloy is 58.0 to 66.0 wt.%. At a Cu content of less than 58.0 wt.%, the ductility of the alloy is too low. At a Cu content of more than 66.0 wt.%, the zinc content in the alloy is too low to achieve good machinability.

[0033] Furthermore, the composition of the alloy can preferably be selected such that the Si / P ratio is at least 0.6, particularly preferably at least 0.9. In a further preferred embodiment, the composition of the alloy can be selected such that the sum of Si and P is at least 0.58 wt.%, particularly preferably at least 0.64 wt.%. Both of the above-mentioned measures, both independently of one another and in combination, contribute to fulfilling the relationship described above and thus to achieving favorable machining properties. The optional elements Sn and Al support the formation of the β phase. With an Sn content of more than 0.5 wt.%, further tin-containing phases can form, which can have a negative effect on the properties of the alloy. The proportion of tin should preferably be at most 0.3 wt.%, particularly preferably at most 0.2 wt.%. Furthermore, aluminum forms aluminum phosphides with phosphorus.However, these are undesirable, which is why the Al content should not exceed 0.05 wt.%.

[0034] Iron leads to grain refinement of the microstructure. Furthermore, iron forms hard phosphides and silicides, which adversely affect tool life and polishability during machining. Therefore, the iron content may not exceed 0.3 wt.%, preferably 0.1 wt.%.

[0035] Nickel supports the formation of the α-phase. Furthermore, nickel forms phosphides, which have no beneficial effect on machinability. Therefore, the nickel content may not exceed 0.3 wt.%, preferably 0.1 wt.%.

[0036] The element Bi is present as an impurity in secondary raw materials, such as scrap. It can improve the machinability of the alloy. In amounts up to 0.1 wt.%, Bi has no negative impact on the alloy. Therefore, up to 0.1 wt.% Bi is tolerated in the alloy. The Bi content is preferably less than 0.015 wt.%.

[0037] The elements Te, Se, and In can have a beneficial effect on the machinability of the alloy. In amounts up to 0.1 wt.% each, they have no negative impact on the alloy. Therefore, up to 0.1 wt.% of Te, Se, and In are tolerated in the alloy. An optional amount of up to 0.01 wt.% boron contributes to grain refinement.

[0038] The remainder of the alloy composition consists of zinc and unavoidable impurities. To avoid uncontrollable influences of these impurities on the alloy's properties, the maximum content of these impurities is 0.2 wt.%. Preferably, the Mn and Mg contents should each be no more than 0.1 wt.%, and particularly preferably no more than 0.05 wt.%, because these elements can form phosphides that can compete with the copper- and / or zinc-containing phosphides.

[0039] In a preferred embodiment of the invention, the Pb content in the alloy can be at least 0.02 wt.%. Even such a low Pb content improves the machining properties.

[0040] Advantageously, the ratio of the weight fractions of P to the sum of Fe and Ni can be greater than 2.0, i.e., P / (Fe+Ni) > 2.0. This ensures that predominantly copper- and / or zinc-containing phosphides, which are favorable for machining properties, are formed. The formation of iron phosphides or nickel phosphides is suppressed.

[0041] It can be particularly advantageous if the total Fe and Ni contents do not exceed 0.1 wt.%. This restriction also inhibits the formation of iron phosphides and nickel phosphides, while the formation of copper- and / or zinc-containing phosphides is inhibited.

[0042] In a particularly advantageous embodiment of the invention, the P content can be at least 0.26 wt.% and at most 0.33 wt.%. If the P content is at least 0.26 wt.%, a sufficient number of phosphide particles are formed to achieve a particularly fine grain and very good machinability.

[0043] In an advantageous embodiment of the invention, the Si content can be at least 0.50 wt.% and at most 1.0 wt.%. A cast material made of a copper-zinc alloy with a Si content in this range is characterized by excellent machining properties.

[0044] A further aspect of the invention relates to a method for producing a cast product, the method comprising the following steps: a) melting a copper-zinc alloy having a composition as described above, b) casting a cast product followed by cooling the cast product, wherein in a temperature range of 550 to 350 °C the cooling rate is at least 20 °C per minute and at most 60 °C per minute.

[0045] The process can be used to produce a cast product from a copper-zinc alloy cast material described above. Cu cathodes, Zn blocks, brass scrap, Cu-P master alloys, and Cu-Si master alloys can be used to melt the alloy. Melting is preferably carried out in an induction furnace. The melt is then poured into a cast product. The cast product is cooled, with the cooling rate being at least 20 °C per minute, preferably at least 30 °C per minute, and at most 60 °C per minute, preferably at most 50 °C per minute, within a temperature range of 550 °C to 350 °C. The defined cooling establishes a favorable ratio of the volume fractions of the α-phase and the β-phase to one another and a favorable particle distribution of copper- and / or zinc-containing phosphides in the cast material of the cast product.With regard to further technical features and advantages of the method according to the invention, reference is hereby explicitly made to the explanations in connection with the casting material according to the invention made of a copper-zinc alloy and to the exemplary embodiments.

[0046] A further aspect of the invention relates to a casting made from a cast material described above. A casting refers to a product whose material is no longer formed after casting and cooling. The production of a casting starts from a cast product that is melted and cast according to the method described above. To shape it, the cast product is machined. Furthermore, at least part of the surface can be polished. Optionally, the casting can be fully or partially coated. The casting is thus produced from a cast material or cast product described above by machining and optional further processing steps. Such a casting can be, for example, a connector, a T-piece, part of a valve, a faucet, or a water meter.

[0047] The invention is explained in more detail using exemplary embodiments and comparative examples.

[0048] Samples No. 1 to No. 12 were melted in an induction furnace and then cast into small ingots in molds. The cooling rate during solidification was 36 °C per minute (°C / min) in the temperature range from 550 °C to 350 °C. The composition of the samples is documented in Table 1. Sample No. 5 represents the lead-containing reference alloy CuZn39Pb1Al-B. Cross slices were taken from the cast ingots, and their microstructure was examined using a light microscope. The stated volume fractions of the α-phase and the β-phase are normalized to the sum of the α-phase and β-phase. The hardness HV was determined. The electrical conductivity was determined using the eddy current method using a probe.

[0049] The grain size of the ß-grains was determined based on EN ISO 2624. Line cuts were made in the width direction of the cross slices (referred to as "vertical"), and the number of cut ß-grains along these line cuts was determined. The mean line cut length in the width direction corresponds to the mean ß-grain diameter in the width direction. Similarly, line cuts were made in the thickness direction of the cross slices (referred to as "horizontal"), and the number of cut ß-grains along these line cuts was determined. The mean line cut length in the thickness direction corresponds to the mean ß-grain diameter in the thickness direction.

[0050] For the quantitative determination of the size distribution of the phosphide particles, the light microscopic images of the unetched samples were used. Image sections measuring 167 pm x 126 pm (corresponding to an area of ​​21,000 pm 2) and analyzed at 1000x magnification using the ImageJ software. This made it possible to identify individual particles and determine their equivalent diameter and area. The phosphide particles were classified based on their equivalent diameter into the categories 0.5 to 1 pm, 1 to 2 pm, 2 to 5 pm, and - if present - larger than 5 pm.

[0051] Machinability was determined by a planing test. An indexable insert with a contour that favors chip breaking was used. The depth of cut was 125 pm and the planing speed was -14.

[0052] 86 m / min. During the planing process, the bending moment acting on the tool was measured, and the average value of the bending moment was determined. The resulting chips were visually assessed and categorized according to chip shape. A chip shape number was assigned to each chip shape according to the following list:

[0053] The chip break number 1 .0 corresponds to the reference alloy CuZn39Pb1AI-B, which

[0054] Contains 1 wt% lead (sample no. 5).

[0055] The machinability of the specimens was assessed based on the bending moment measured during planing and the shape of the chips. An average bending moment of 37 Nm or less and chips corresponding to a chip shape index of 1.0 or 1.25 were considered very favorable.

[0056] The results of the tests are documented in Table 1. Samples 1 to 4 are samples according to the invention. Samples 5 to 12 are comparison samples and are marked with (*).

[0057] Furthermore, an attempt was made to parameterize the measured average bending moment as a function of the volume fraction of the ß-phase as well as the weight fractions of Si and P. The functional relationship thus determined can be represented as follows: f = 92.7249-0.473254 [Beta]-80.6378 [Si]-142.65 [P]+279.309-[Si] [P], where f approximately quantifies the measured bending moment in Nm and where [Beta] denotes the proportion of the ß-phase in vol%, [Si] the proportion of silicon in weight%, and [P] the proportion of phosphorus in weight%. The value of f calculated according to this formula is documented in the last column of Table 1. A comparison of this value f with the measured bending moment shows (with the exception of the two very poorly machinable samples No. 6 and No. 7) a very good agreement between the two values. The inventive samples No. 1 to No.4, all of which have a measured bending moment of less than 37 Nm, are characterized by the value of f being less than 40.

[0058]

[0059] Table 1 : Composition and structural properties of the samples

[0060]

[0061] Table 1 (continued): Composition and technological properties of the samples

[0062] Samples No. 1 to No. 4 are samples according to the invention. The volume fraction of the ß-phase is at least 38% and at most 57%. The ß-grain size is a maximum of 645 pm in the width direction (“vertical”) and a maximum of 781 pm in the thickness direction (“horizontal”). The ratio of ß-grain size in the thickness direction to ß-grain size in the width direction is a maximum of 1.21. The grains therefore have a topology without a preferred direction and are assessed as globular. The hardness is at least 110 HV10. The measured bending moment is a maximum of 36.5 Nm. The shape of the chips corresponds to a chip break number of 1.25 in all samples. The shape of the chips is therefore very favorable.

[0063] Samples No. 5 to No. 12 are comparison samples. Reference sample No. 5 contains 1.1 wt.% lead and is characterized by a very small grain size, a very low bending moment, and good chip shape. Samples No. 6 and No. 7 contain only very small amounts of silicon and phosphorus in addition to copper and zinc. In both samples, the ß-grain size is very large, the bending moment measured during machining is high, and the chip shape is poor.

[0064] Samples No. 8 to No. 11 each contain approximately 0.55 wt.% silicon. Samples No. 8 and No. 10 contain no or very little phosphorus, while samples No. 9 and No. 11 contain phosphorus in amounts of 0.126 wt.% and 0.067 wt.%, respectively. In samples No. 10 and No. 11, the zinc content is slightly more than 1 wt.% higher than the zinc content of samples No. 8 and No. 9. The higher zinc content leads to a larger volume fraction of the ß-phase. A comparison of samples No. 8 and No. 10 with samples No. 6 and No. 7 shows that by alloying with approximately 0.55 wt.% silicon, the bending moment acting during machining is significantly reduced due to the hardening of the α-phase and the ß-phase. The higher volume fraction of the ß-phase in samples No. 6 and No. 7 is more than compensated for by the silicon content in samples No. 8 and No. 10. Furthermore, the bending moment also decreases with increasing P content. The chip shape in samples No.8 to No. 11 cheap.

[0065] The ß-grain size is smaller in samples No. 8 to No. 11 than in samples No. 6 and No. 7. The ß-grain size tends to decrease with increasing P content. On the other hand, samples No. 8 to No. 11 have a ß-grain size that is larger than the ß-grain size of samples No. 1 to No. 4. In particular, in samples No. 8 to No. 11, the horizontal ß-grain size is more than a factor of 1.25, usually even more than a factor of 2.5, larger than the vertical ß-grain size, whereas in samples No. 1 to No. 4, the horizontal ß-grain size is at most a factor of 1.25 larger than the vertical ß-grain size.

[0066] Sample No. 12 shows, due to the phosphorus content of 0.276 wt.%, a globular ß-grain with a vertical ß-grain size of 605 pm and a horizontal ß-grain size of 856 pm. However, sample No. 12 has a volume fraction of the ß-phase of only 22% and a low Si content of 0.277 wt.%, which increases the bending moment acting during machining and the condition

[0067] 92, 7249-0, 473254 [Beta]-80.6378 [Si]-142.65 [P]+279.309 [Si] [P] < 40 is not met: The value of f calculated from the Si content, P content and the volume fraction of the ß-phase is 42.0 and thus agrees well with the measured bending moment of 42.5 Nm. The comparison of sample no. 12 with sample no. 4, which has a similar composition, shows that a low Si content must be compensated by a higher Zn content so that the cast material has a sufficient volume fraction of ß-phase and thus favorable machinability properties. It is therefore necessary to select the alloy composition and process control such that the above-mentioned condition is met through the interaction of these variables and parameters. It is not sufficient to consider the aforementioned variables and parameters individually.

[0068] Samples No. 1 to No. 4 are characterized by grains with a topology without a preferred direction, i.e., globular grains, and a maximum ß-grain size of 800 pm, while in samples No. 8 to No. 11, the ß-grain size is generally larger. Furthermore, in samples No. 8, No. 10, and No. 11, the ß-grain size is significantly larger in the thickness direction ("horizontal") than in the width direction ("vertical"). Therefore, the grain formation in samples No. 8, No. 10, and No. 11, as in samples No. 6 and No. 7, is described as columnar and rated as unfavorable. The reason for the favorable topology and grain size in samples No. 1 to No. 4 is the phosphorus content of at least 0.24 wt.% in combination with the specially selected cooling rate in the temperature range between 550 °C and 350 °C during solidification after casting. The favorable topology and grain size in samples No. 1 to No.4 results in good mold filling during casting and good polishability of these samples.

Claims

Patent claims Casting material made of a copper-zinc alloy with the following composition in weight-%: Cu: 58.0 to 66.0%, Si: 0.15 to 1.2%, P: 0.20 to 0.38%, Sn: optional up to 0.5%, AI: optional up to 0.05%, Fe: optional up to 0.3%, Ni: optional up to 0.3%, Pb: optional up to 0.25%, Bi: optional up to 0.1%, Te, Se, In optionally up to 0.1% each B: optionally up to 0.01%, Balance Zn and unavoidable impurities, with the proportion of unavoidable impurities being less than 0.2 wt.%, wherein the alloy has a structure of a-phase, ß-phase and phosphide particles, and the proportion of the ß-phase in the sum of a-phase and ß-phase is at least 20 vol.% and at most 70 vol.%, wherein silicon is present in both the a-phase and the ß-phase, wherein in an area of ​​21000 pm 220 to 300 phosphide particles with an equivalent diameter of 0.5 to 1 pm, 30 to 120 phosphide particles with an equivalent diameter of 1 to 2 pm and 20 to 100 phosphide particles with an equivalent diameter of 2 to 5 pm are present, and the proportion of the ß-phase as well as the proportions of Si and P are chosen so that the condition 92, 7249-0, 473254 [Beta]-80.6378 [Si]-142.65 [P]+279.309-[Si] [P] < 40 is met, where [Beta] denotes the proportion of the ß-phase in vol. %, [Si] the proportion of silicon in weight % and [P] the proportion of phosphorus in weight %. Cast material made of a copper-zinc alloy according to claim 1, characterized in that the Pb content is at least 0.02 wt. %. Cast material made of a copper-zinc alloy according to claim 1 or 2, characterized in that the ratio of the weight fractions of P and the sum of Fe and Ni is more than 2.

0. Casting material made of a copper-zinc alloy according to one of claims 1 to 3, characterized in that the total proportions of Fe and Ni amount to at most 0.1 wt.%. Casting material made of a copper-zinc alloy according to one of claims 1 to 4, characterized in that the P content is at least 0.26 wt.% and at most 0.33 wt.%. Casting material made of a copper-zinc alloy according to one of claims 1 to 5, characterized in that the Si content is at least 0.50 wt% and at most 1.0 wt%.

7. A method for producing a cast product, the method comprising the following steps: a) melting a copper-zinc alloy having a composition according to one of claims 1 to 6, b) casting a cast product with subsequent cooling of the Cast product, wherein in a temperature range of 550 to 350 °C, the cooling rate is at least 20 °C per minute and at most 60 °C per minute.

8. Cast part made of a cast material according to one of claims 1 to 6.

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