LEAD-FREE BRASS ALLOY FOR HOT WORKING

Abstract

Provided is a lead-free brass alloy for hot working provided with good hot-working properties and mechanical characteristics. A lead-free brass alloy for hot working, comprising: 28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder being copper and unavoidable impurities, the zinc equivalent being in a range of 40.0 to 43.0, and the area ratio of the κ phase after hot working being 20% or less.

Description

TECHNICAL FIELD

The present invention relates to a lead-free brass alloy for hot working, having excellent resistance to dezincification and resistance to erosion and corrosion, and having good hot-working properties and mechanical characteristics.

BACKGROUND ART

Bronze, brass, and other copper alloys have conventionally been used in faucet parts for water supply, water contact parts for general piping, and in various valves in order to make use the excellent material characteristics of such alloys. These copper alloys require good machinability for working a product, and therefore lead has generally been included to thereby impart the required machinability. For example, JIS H5120 CAC406, CAC407, and other bronze alloys, and JIS H3250 C3604, C3771, and other brass alloys having excellent machinability contain 1 to 6 wt % of lead.

However, lead evaporates in the alloy melting and casting process, elutes into drinking water when used as a water contact part, and has other drawbacks. There is a deepening awareness that lead is a toxic element that negatively affects the human body and environmental sanitation, and the content of lead has been strictly restricted in increasing fashion in recent years. Accordingly, there is a need to develop a free-cutting copper alloy that does not contain lead.

In view of the background described above, in silzin bronze alloys, a Cu—Zn—Si alloy in which free-cutting is achieved by adding silicon without the inclusion of lead has been proposed and used (see Patent Documents 1 and 2). Additionally, there has been proposed a Cu—Zn—Si—Sn alloy in which tin has been added in order to enhance the corrosion resistance of a Cu—Zn—Si alloy (see Patent Document 3). There have also been proposed a Cu—Zn—Si—Bi alloy in which bismuth has been added in order to further improve the machinability of a Cu—Zn—Si alloy (see Patent Document 4), and a Cu—Zn—Si—Sn—Bi alloy (see Patent Document 5) in which tin has been added to the Cu—Zn—Si—Bi alloy in order to improve corrosion resistance. These alloys have excellent mechanical characteristics and dezincification resistance, and excellent machinability in the case that bismuth has been added, and alloys in which Bi has not be added are provided with excellent hot workability. In the case that bismuth is added to a Cu—Zn—Si alloy, there is an additional advantage in that the alloy can be used as scrap to be dissolved into raw materials.

PRIOR ART DOCUMENTS

Patent Documents

• [Patent Document 1] Japanese Patent No. 3917304
• [Patent Document 2] Japanese Laid-open Patent Application No. 2001-64742
• [Patent Document 3] Japanese Laid-open Patent Application No. 2002-12927
• [Patent Document 4] Japanese Laid-open Patent Application No. 2009-7657
• [Patent Document 5] Japanese Patent Application No. 2010-84231

DISCLOSURE OF THE INVENTION

Problems that the Invention is to Solve

The alloys disclosed in the above-noted documents can be said to have the main object of removing lead toxicity. Therefore, the most important issue in terms of performance is to maintain free-cutting characteristics without the inclusion of lead, and to a certain extent machinability has been ensured.

However, in the case that the alloy does not contain bismuth, machinability is improved by the silicon compound, but the improvement may be insufficient in some cases, and currently, a certain amount of bismuth must be added in order to improve machinability. Also, it is preferred the alloy contain bismuth from the viewpoint of use as scrap.

However, a bismuth-containing lead-free brass alloy can be hot worked in mold working in which there is little deformation, but in the case of molding work with a considerable amount of deformation, forge cracking or other defects readily occur unless the addition amount of bismuth and the forging conditions are rigorously controlled. It is known that hot forging a brass alloy has different conditions in which cracking occurs in a product depending on the working temperature. There are upper and lower limits to the working temperature at which working can be performed without cracks forming, and heating and forging must be carried out in this temperature region (hereinafter referred to as working temperature range). For example, the working temperature must be increased for the alloy in Patent Document 5, which contains about 0.7 wt % of bismuth, and since the working temperature range is very narrow, temperature management is difficult, and there is also a problem in terms of the amount of energy used. Patent Document 3 describes an alloy in which it is effective to add silicon as the element for improving hot forging characteristics, but in the embodiments, there is no data provided in relation to the hot-working characteristics in the case that bismuth has been added. The only evaluation is that the working temperature is on the single level of 750° C., and the working temperature range is unclear.

The inventors carried out studies and found that the working temperature range becomes very narrow when bismuth is included in a Cu—Zn—Si—Sn alloy. Therefore, problems readily occur in operations because the forging conditions must be rigorously controlled in order to subject this alloy to molding work that involves a considerable amount of deformation. In other words, broadening the working temperature range is a first issue and is important in order to apply the excellent corrosion resistance and machinability of this alloy to a large number of components.

Also, tin is added in order to increase dezincification and erosion and corrosion resistance, and elongation of a Cu—Zn—Si—Sn—Bi alloy is readily reduced. The κ phase and γ phase of this alloy precipitate and, depending on the precipitation conditions, the mechanical properties are readily degraded. Furthermore, these precipitation conditions are readily affected by the heat history or the like during manufacture, and it is important to accurately ascertain and suitably control the relationship between the configuration of the structure and the mechanical properties. In other words, a second issue is controlling the mechanical properties, more particularly, the elongation of the Cu—Zn—Si—Sn—Bi alloy.

The present invention was devised in order to solve the above-described problems, it being an object thereof to provide a lead-free brass alloy for hot working provided with good hot-working properties and mechanical characteristics.

Means for Solving the Problems

The main points of the present invention are described below.

A first aspect of the present invention relates to a lead-free brass alloy for hot working, characterized in comprising: 28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder being copper and unavoidable impurities, the zinc equivalent being in a range of 40.0 to 43.0, and the area ratio of the κ phase after hot working being 20% or less.

The present invention also relates to the lead-free brass alloy for hot working according to the first aspect, characterized in that elongation is 10% or more.

Effects of the Invention

The present invention is configured in the manner described above, and is therefore a lead-free brass alloy for hot working, provided with good hot-working properties and mechanical characteristics. In other words, adding 28.0 to 35.0 wt % zinc makes it possible to obtain good hot-working properties. In similar fashion to zinc, silicon is an essential element for obtaining good hot-working properties and the addition of 0.5 to 2.0 wt % is effective. Tin contributes to improvement in dezincification decay and resistance to erosion and corrosion decay. Bismuth is added in order to improve machinability. The zinc equivalent is determined by the balance among zinc, silicon, and other elements, and is a parameter for maintained a balance between hot-working properties and mechanical characteristics in particular. A zinc equivalent of 40.0 to 43.0 simultaneously satisfies the two characteristics. Also, the area ratio of the κ phase after hot working is 20% or less, whereby good mechanical characteristics are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a description of the zinc equivalent;

FIG. 2 is a chart showing the chemical components of samples used in the hot-working test;

FIG. 3 is a descriptive view showing the shape of the test piece in the hot-working test;

FIG. 4 is a chart showing the forging test results;

FIG. 5 is a graph showing the relationship between the Si addition amount and the working temperature range;

FIG. 6 is a graph showing the relationship between the Zn equivalent and the working temperature range;

FIG. 7 is a chart showing the chemical components of samples used in the tensile test;

FIG. 8 is a chart showing the test results of the tensile test;

FIG. 9 is a graph showing the relationship between the Si addition amount and the mechanical characteristics in a low Zn equivalent;

FIG. 10 is a graph showing the relationship between the Si addition amount and the mechanical characteristics in a high Zn equivalent;

FIG. 11 is a chart showing the chemical components of samples in which the relationship between the Si addition amount, the area ratio of the κ phase, and the elongation has been studied;

FIG. 12 is a chart showing the relationship between the Si addition amount, the area ratio of the κ phase, and the elongation;

FIG. 13 is a graph showing the relationship between the Si addition amount and the area ratio of the κ phase;

FIG. 14 is a graph showing the relationship between the area ratio of the κ phase and the elongation;

FIG. 15 is a chart showing the chemical components of samples used in the erosion and corrosion test, and the dezincification decay test;

FIG. 16 is a descriptive view showing the shape of the test piece in the erosion and corrosion test;

FIG. 17 is a chart showing the test conditions;

FIG. 18 is a chart showing the test results;

FIG. 19 is a chart showing the test results of the dezincification decay test;

FIG. 20 is a chart showing the chemical components of samples used in the machinability test;

FIG. 21 is a chart showing the test conditions;

FIG. 22 is a chart showing the test results; and

FIG. 23 is a photograph showing an example of the photographed microstructure.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention are briefly described below while indicating the effects of the present invention.

In order to obtain good resistance to dezincification and resistance to erosion and corrosion, and to provide good hot-working properties and mechanical characteristics, the present invention provides a lead-free brass alloy for hot working comprising: 28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder being copper and unavoidable impurities, wherein the zinc equivalent is in a range of 40.0 to 43.0.

The component composition as described above in the present invention, the reasons for specifying the mechanical characteristics, and the effects of the present invention will be briefly described below.

Zinc (Zn)

Zinc dissolves in the matrix of a Cu—Zn—Si copper alloy, and has the effect of increasing mechanical strength. Zinc also reduces the melting point of the alloy, increases the fluidity of the molten alloy, and enhances casting characteristics. Zinc also has the effect of improving hot working, and in order to obtain these effects, zinc must be added in the amount of 28.0 wt % or more due to the relationship between the later-described silicon addition amount and the zinc equivalent.

However, when the amount of zinc exceeds 35.0 wt %, the hot-working properties are conversely degraded due to the relationship between the later-described silicon addition amount and the zinc equivalent. Also, the mechanical characteristics of the alloy are liable to be degraded due to precipitation of a hard phase that is greater than necessary. Due to such reasons, the zinc content is set to 28.0 to 35.0 wt %.

Silicon (Si)

Silicon works as a deoxidizer during dissolution, enhances the fluidity of the molten alloy, and improves casting characteristics. A portion dissolves in the matrix and improves mechanical strength, and a portion works with zinc to cause the emergence of a hard phase that functions as a chip breaker during cutting work and improve machinability.

Furthermore, as a result of thoroughgoing research, the present inventors discovered the following, which dramatically improves the working temperature range (a value obtained by subtracting the lower limit from the upper limit of the working temperature in which hot forging can be carried out without the occurrence of cracking) of a Cu—Zn—Sn—Si alloy in the case that bismuth is included.

In the heating stage during hot working, bismuth has a property of readily aggregating at the grain boundary, and this is thought to be the cause of inhibiting hot-working properties. However, the addition of a suitable amount of silicon prevents bismuth aggregation and is effective in preventing forging cracks. In order to obtain these effects, silicon must be added in the amount of 0.5 wt % or more. When the content exceeds 2.0 wt %, hot-working properties are degraded even when the zinc equivalent has been kept at an optimal level, and the mechanical characteristics of the alloy are liable to be degraded due to the emergence of a hard phase that is greater than necessary. Due to such reasons, the silicon content is set to 0.5 to 2.0 wt %.

Tin (Sn)

Tin is effective for enhancing dezincification resistance and resistance to erosion and corrosion. Tin is particularly effective in improving erosion and corrosion properties, and in order to obtain these effects, tin must be added in the amount of 0.5 wt % or more. On the other hand, when the content exceeds 1.5 wt %, mechanical characteristics are liable to be degraded. Due to such reasons, the tin content is set to 0.5 to 1.5 wt %.

Bismuth (Bi)

A bismuth content less than 0.5 wt % can be considered to have little effect for improving machinability, but machinability is improved in accordance with the addition amount by adding 0.5 wt % or more. However, the addition of a large amount is not preferred in that it causes degradation in hot-working properties. A large amount not only causes degradation in hot-working properties, but also causes degradation in the mechanical characteristics, hence the upper limit is set to 1.5 wt %.

Lead (Pb)

The lead content is set to 0.10 wt % or less, and it is thereby possible to essentially avoid evaporation in the dissolution and casting processes of the alloy, as well as lead poisoning in the human body and/or environmental hygiene due to elution into drinking water or the like when the alloy is used as a water contact component. Due to such reasons, the lead content is limited to 0.10 wt % or less.

Copper (Cu)

Copper is an element that reduces sensitivity to dezincification decay and improves corrosion resistance and mechanical characteristics, but in the alloy of the present invention, the copper content is determined as the remainder due to the balance between the zinc content and silicon content. The effective content is 59.0 to 71.0 wt %.

Zinc Equivalent

The zinc equivalent is an important parameter for maintaining a broad working temperature range in the alloy of the present invention. As described above, a suitable addition of silicon makes it possible to maintain a broad working temperature range, but control is insufficient using silicon alone, and using the zinc equivalent computed by the balance between silicon, zinc, and the like to achieve limited control makes it possible to more reliably maintain a broad working temperature range. The present inventors carried out research and found that setting the zinc equivalent in the alloy of the present invention to 40.0 or more provides a working temperature range that is broad enough to satisfy industrial requirements. However, a zinc equivalent exceeding 43.0 is liable to lead to degradation in the mechanical characteristics. In view of this background, the zinc equivalent is set to 40.0 to 43.0.

The zinc equivalent is obtained using the Guillet formula (zinc equivalent=100×(B+Σtq)/(A+B+Σtq)), and the zinc equivalent of Bi is calculated using a factor of 1 (see FIG. 1).

κ Phase Quantitative Ratio or Heat Treatment

The addition of the elements described above and the use of hot working makes it possible to demonstrate the excellent function of the alloy of the present invention, but depending on the cooling speed and/or the processing rate during hot working, elongation may be slightly insufficient. In order to improve the elongation of the alloy of the present invention, the metal structure must be controlled, and setting the area ratio of the κ phase in the alloy of the present invention to 20% or less makes it possible to ensure elongation. Therefore, the area ratio of the κ phase is set to 20% or less. The method for controlling the structure is not particularly limited, and may be controlled using a hot-working method, heat treatment, or the like.

Embodiments

Specific embodiments of the present invention will be described below with reference to the drawings.

The alloy according to the present invention (the alloy of the present invention) and a comparative alloy were used as samples and tested in the manner described below.

1) Hot-Working Test

The chemical components of the samples used in the hot-working test are shown in FIG. 2. A molten alloy melted in a Siliconit furnace for test dissolution and prepared with the chemical components shown in FIG. 2 was cast in a mold having an outside diameter of 88 mm and a length of 120 mm, and then machined to an outside diameter of 78 mm and a length of 90 mm. Machined billets were extruded to a diameter of 22 mm, and the resulting rods were worked into a test piece shape such as that shown in FIG. 3. The working temperature was varied and these test pieces were forged with a processing rate of 80%. As used herein, the processing rate is calculated using the following formula.

Processing rate=100×(sample height prior to forging−sample height after forging)/sample height prior to forging

The test pieces (samples) after forging were observed macroscopically, the lower limit was subtracted from the upper limit of the working temperature at which forging can be carried out without the occurrence of cracking, and this was used to define the working temperature range and make evaluations. The heating time in all tests was 20 minutes. The working temperature range of each sample is shown in FIGS. 4 to 6.

(a) Effectiveness of the Silicon Addition

The effectiveness of adding silicon to the alloy of the present invention is shown in FIG. 5. In the case that silicon is not added, the working temperature range is narrow, but it is apparent that the working temperature range increases in accompaniment with the addition of silicon. The effect of these additions is to produce a satisfactory working temperature range with the addition of 0.5 wt % or more. However, when the addition amount exceeds 2.0 wt %, the working temperature range tends to be conversely reduced, and it was found that an effective silicon content is 0.5 to 2.0 wt %.

(b) Effectiveness of the Zinc Equivalent

Next, the effectiveness of the zinc equivalent is shown in FIG. 6. It was found that the zinc equivalent must be 40.0 to 43.0 in order to adequately maintain the working temperature range in the alloy of the present invention, and it was confirmed that the zinc equivalent must be controlled, as appropriate, in accordance with the effect of increasing the working temperature range by the silicon addition described above.

2) Tensile Test of the Hot-Working Material

The chemical components of the sample materials used in the tensile test are shown in FIG. 7. A molten alloy was cast in a mold having a diameter of 45 mm and a length of 100 mm, and was then machined into billets having a diameter of 40 mm and a length of 75 mm. The billets were subsequently heated to 650 to 750° C. and extruded to a diameter of 10 mm, then machined into test pieces in accordance with JIS Z2201 14A, and subjected to a tensile test using a universal testing machine. The results are shown in FIGS. 8 to 10.

When the effect of the silicon addition amount is considered, there is a noted tendency for the elongation to be reduced in accordance with the silicon addition amount, and this is particularly dramatically in the case that the zinc equivalent is high. It is apparent that the tensile strength tends to be temporarily reduced when the silicon content is near 1.0 wt % with the zinc equivalent near 40.6, and when the silicon content is near 2.0 wt % with the zinc equivalent near 42.5, but thereafter the tensile strength increases.

3) Metal Structure and Mechanical Characteristics

The alloy of the present invention has excellent hot-working properties as described above, and it is important to suitably control the Si addition amount and the zinc equivalent. However, elongation tends to be readily reduced when the zinc equivalent is high, and controlling the structure also becomes an issue.

The alloy of the present invention mainly has κ-phase and α-phase constituent structures, and between these two, the structure was observed with focus on the effect that the quantitative ratio of the κ phase has on the mechanical characteristics. Five locations were photographed using an optical microscope to obtain images at 500× magnification using the samples used in the tensile test described above. The quantitative ratio of the κ phase was measured using image processing software (an example of the photographs taken is shown in FIG. 23). These results are shown in FIGS. 11 to 14. The inventors found the following facts from these structural observations. The elongation of the alloy of the present invention was found to have a very strong correlation with the area ratio of the κ phase, and when elongation is to be increased, the area ratio of the κ phase must be kept low.

The relationship between the area ratio of the κ phase and the silicon addition amount increases in accordance with the silicon addition amount (see FIG. 13). In terms of the relationship between the area ratio of the κ phase and the silicon addition amount, elongation is 10% or more when the area ratio of the κ phase is 20% or less (see FIG. 14). Therefore, the area ratio of the κ phase in the alloy of the present invention must be 20% or less.

4) Corrosion Decay Test

(a) Erosion and Corrosion Test

The chemical components of the sample materials used in the erosion and corrosion test are shown in FIG. 15. A molten alloy melted in a Siliconit furnace for test dissolution and prepared with the chemical components shown in FIG. 15 was cast in a mold having a diameter of 40 mm and a length of 100 mm, and was then worked into a test piece shape such as that shown in FIG. 16. Testing was carried out with the test conditions of FIG. 17 using these test pieces. The test results are shown in FIG. 18. It was found from these results that the alloy of the present invention was slightly inferior to CAC406, but was considerably better than free-cutting brass.

(b) Dezincification Decay Test

The same samples as those used in the erosion and corrosion test were used. The test was carried out in accordance with ISO 6509. The test results are shown in FIG. 19. Good results were obtained with the alloy of the present invention in that the maximum decay depth was 100 μm or less for all samples.

5) Machinability Test

The chemical components of the sample materials used in the erosion and corrosion test are shown in FIG. 20. A molten alloy melted in a Siliconit furnace for test dissolution and prepared with the chemical components shown in FIG. 20 was cast in a JIS H5120 E mold, the outside diameter of the test pieces was worked using the cutting conditions shown in FIG. 21, and the cutting resistance of the test pieces was measured. The test results are shown in FIG. 22. In comparison with lead-containing bronze and lead-containing brass, the alloy of the present invention has higher resistance, but is on the same level as that of lead-free bronze.

In view of the above, it was confirmed that the lead-free brass alloy for hot working has good hot-working properties and mechanical characteristics, the lead-free brass alloy for hot working, comprising: 28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder being copper and unavoidable impurities, wherein the zinc equivalent is in a range of 40.0 to 43.0.

Claims

1. A lead-free brass alloy for hot working, characterized in comprising: 28.0 to 35.0 wt % zinc, 0.5 to 2.0 wt % silicon, 0.5 to 1.5 wt % tin, 0.5 to 1.5 wt % bismuth, 0.10 wt % or less lead, and the remainder being copper and unavoidable impurities, the zinc equivalent being in a range of 40.0 to 43.0, and the area ratio of the κ phase after hot working being 20% or less.

2. The lead-free brass alloy for hot working according to claim 1, characterized in that elongation is 10% or more.